Related
Is there a nice way in MySQL to replicate the SQL Server function ROW_NUMBER()?
For example:
SELECT
col1, col2,
ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY col3 DESC) AS intRow
FROM Table1
Then I could, for example, add a condition to limit intRow to 1 to get a single row with the highest col3 for each (col1, col2) pair.
There is no ranking functionality in MySQL. The closest you can get is to use a variable:
SELECT t.*,
#rownum := #rownum + 1 AS rank
FROM YOUR_TABLE t,
(SELECT #rownum := 0) r
so how would that work in my case? I'd need two variables, one for each of col1 and col2? Col2 would need resetting somehow when col1 changed..?
Yes. If it were Oracle, you could use the LEAD function to peak at the next value. Thankfully, Quassnoi covers the logic for what you need to implement in MySQL.
I want the row with the single highest col3 for each (col1, col2) pair.
That's a groupwise maximum, one of the most commonly-asked SQL questions (since it seems like it should be easy, but actually it kind of isn't).
I often plump for a null-self-join:
SELECT t0.col3
FROM table AS t0
LEFT JOIN table AS t1 ON t0.col1=t1.col1 AND t0.col2=t1.col2 AND t1.col3>t0.col3
WHERE t1.col1 IS NULL;
“Get the rows in the table for which no other row with matching col1,col2 has a higher col3.” (You will notice this and most other groupwise-maximum solutions will return multiple rows if more than one row has the same col1,col2,col3. If that's a problem you may need some post-processing.)
I always end up following this pattern. Given this table:
+------+------+
| i | j |
+------+------+
| 1 | 11 |
| 1 | 12 |
| 1 | 13 |
| 2 | 21 |
| 2 | 22 |
| 2 | 23 |
| 3 | 31 |
| 3 | 32 |
| 3 | 33 |
| 4 | 14 |
+------+------+
You can get this result:
+------+------+------------+
| i | j | row_number |
+------+------+------------+
| 1 | 11 | 1 |
| 1 | 12 | 2 |
| 1 | 13 | 3 |
| 2 | 21 | 1 |
| 2 | 22 | 2 |
| 2 | 23 | 3 |
| 3 | 31 | 1 |
| 3 | 32 | 2 |
| 3 | 33 | 3 |
| 4 | 14 | 1 |
+------+------+------------+
By running this query, which doesn't need any variable defined:
SELECT a.i, a.j, count(*) as row_number FROM test a
JOIN test b ON a.i = b.i AND a.j >= b.j
GROUP BY a.i, a.j
SELECT
#i:=#i+1 AS iterator,
t.*
FROM
tablename AS t,
(SELECT #i:=0) AS foo
From MySQL 8.0.0 and above you could natively use windowed functions.
1.4 What Is New in MySQL 8.0:
Window functions.
MySQL now supports window functions that, for each row from a query, perform a calculation using rows related to that row. These include functions such as RANK(), LAG(), and NTILE(). In addition, several existing aggregate functions now can be used as window functions; for example, SUM() and AVG().
ROW_NUMBER() over_clause :
Returns the number of the current row within its partition. Rows numbers range from 1 to the number of partition rows.
ORDER BY affects the order in which rows are numbered. Without ORDER BY, row numbering is indeterminate.
Demo:
CREATE TABLE Table1(
id INT AUTO_INCREMENT PRIMARY KEY, col1 INT,col2 INT, col3 TEXT);
INSERT INTO Table1(col1, col2, col3)
VALUES (1,1,'a'),(1,1,'b'),(1,1,'c'),
(2,1,'x'),(2,1,'y'),(2,2,'z');
SELECT
col1, col2,col3,
ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY col3 DESC) AS intRow
FROM Table1;
DBFiddle Demo
Check out this Article, it shows how to mimic SQL ROW_NUMBER() with a partition by in MySQL. I ran into this very same scenario in a WordPress Implementation. I needed ROW_NUMBER() and it wasn't there.
http://www.explodybits.com/2011/11/mysql-row-number/
The example in the article is using a single partition by field. To partition by additional fields you could do something like this:
SELECT #row_num := IF(#prev_value=concat_ws('',t.col1,t.col2),#row_num+1,1) AS RowNumber
,t.col1
,t.col2
,t.Col3
,t.col4
,#prev_value := concat_ws('',t.col1,t.col2)
FROM table1 t,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY t.col1,t.col2,t.col3,t.col4
Using concat_ws handles null's. I tested this against 3 fields using an int, date, and varchar. Hope this helps. Check out the article as it breaks this query down and explains it.
I would also vote for Mosty Mostacho's solution with minor modification to his query code:
SELECT a.i, a.j, (
SELECT count(*) from test b where a.j >= b.j AND a.i = b.i
) AS row_number FROM test a
Which will give the same result:
+------+------+------------+
| i | j | row_number |
+------+------+------------+
| 1 | 11 | 1 |
| 1 | 12 | 2 |
| 1 | 13 | 3 |
| 2 | 21 | 1 |
| 2 | 22 | 2 |
| 2 | 23 | 3 |
| 3 | 31 | 1 |
| 3 | 32 | 2 |
| 3 | 33 | 3 |
| 4 | 14 | 1 |
+------+------+------------+
for the table:
+------+------+
| i | j |
+------+------+
| 1 | 11 |
| 1 | 12 |
| 1 | 13 |
| 2 | 21 |
| 2 | 22 |
| 2 | 23 |
| 3 | 31 |
| 3 | 32 |
| 3 | 33 |
| 4 | 14 |
+------+------+
With the only difference that the query doesn't use JOIN and GROUP BY, relying on nested select instead.
I would define a function:
delimiter $$
DROP FUNCTION IF EXISTS `getFakeId`$$
CREATE FUNCTION `getFakeId`() RETURNS int(11)
DETERMINISTIC
begin
return if(#fakeId, #fakeId:=#fakeId+1, #fakeId:=1);
end$$
then I could do:
select getFakeId() as id, t.* from table t, (select #fakeId:=0) as t2;
Now you don't have a subquery, which you can't have in views.
query for row_number in mysql
set #row_number=0;
select (#row_number := #row_number +1) as num,id,name from sbs
There is no funtion like rownum, row_num() in MySQL but the way around is like below:
select
#s:=#s+1 serial_no,
tbl.*
from my_table tbl, (select #s:=0) as s;
Important: Please consider upgrading to MySQL 8+ and use the defined and documented ROW_NUMBER() function, and ditch old hacks tied to a feature limited ancient version of MySQL
Now here's one of those hacks:
The answers here that use in-query variables mostly/all seem to ignore the fact that the documentation says (paraphrase):
Don't rely on items in the SELECT list being evaluated in order from top to bottom. Don't assign variables in one SELECT item and use them in another one
As such, there's a risk they will churn out the wrong answer, because they typically do a
select
(row number variable that uses partition variable),
(assign partition variable)
If these are ever evaluated bottom up, the row number will stop working (no partitions)
So we need to use something with a guaranteed order of execution. Enter CASE WHEN:
SELECT
t.*,
#r := CASE
WHEN col = #prevcol THEN #r + 1
WHEN (#prevcol := col) = null THEN null
ELSE 1 END AS rn
FROM
t,
(SELECT #r := 0, #prevcol := null) x
ORDER BY col
As outline ld, order of assignment of prevcol is important - prevcol has to be compared to the current row's value before we assign it a value from the current row (otherwise it would be the current rows col value, not the previous row's col value).
Here's how this fits together:
The first WHEN is evaluated. If this row's col is the same as the previous row's col then #r is incremented and returned from the CASE. This return led values is stored in #r. It's a feature of MySQL that assignment returns the new value of what is assigned into #r into the result rows.
For the first row on the result set, #prevcol is null (it is initialised to null in the subquery) so this predicate is false. This first predicate also returns false every time col changes (current row is different to previous row). This causes the second WHEN to be evaluated.
The second WHEN predicate is always false, and it exists purely to assign a new value to #prevcol. Because this row's col is different to the previous row's col (we know this because if it were the same, the first WHEN would have been used), we have to assign the new value to keep it for testing next time. Because the assignment is made and then the result of the assignment is compared with null, and anything equated with null is false, this predicate is always false. But at least evaluating it did its job of keeping the value of col from this row, so it can be evaluated against the next row's col value
Because the second WHEN is false, it means in situations where the column we are partitioning by (col) has changed, it is the ELSE that gives a new value for #r, restarting the numbering from 1
We this get to a situation where this:
SELECT
t.*,
ROW_NUMBER() OVER(PARTITION BY pcol1, pcol2, ... pcolX ORDER BY ocol1, ocol2, ... ocolX) rn
FROM
t
Has the general form:
SELECT
t.*,
#r := CASE
WHEN col1 = #pcol1 AND col2 = #pcol2 AND ... AND colX = #pcolX THEN #r + 1
WHEN (#pcol1 := pcol1) = null OR (#pcol2 := col2) = null OR ... OR (#pcolX := colX) = null THEN null
ELSE 1
END AS rn
FROM
t,
(SELECT #r := 0, #pcol1 := null, #pcol2 := null, ..., #pcolX := null) x
ORDER BY pcol1, pcol2, ..., pcolX, ocol1, ocol2, ..., ocolX
Footnotes:
The p in pcol means "partition", the o in ocol means "order" - in the general form I dropped the "prev" from the variable name to reduce visual clutter
The brackets around (#pcolX := colX) = null are important. Without them you'll assign null to #pcolX and things stop working
It's a compromise that the result set has to be ordered by the partition columns too, for the previous column compare to work out. You can't thus have your rownumber ordered according to one column but your result set ordered to another You might be able to resolve this with subqueries but I believe the docs also state that subquery ordering may be ignored unless LIMIT is used and this could impact performance
I haven't delved into it beyond testing that the method works, but if there is a risk that the predicates in the second WHEN will be optimised away (anything compared to null is null/false so why bother running the assignment) and not executed, it also stops. This doesn't seem to happen in my experience but I'll gladly accept comments and propose solution if it could reasonably occur
It may be wise to cast the nulls that create #pcolX to the actual types of your columns, in the subquery that creates the #pcolX variables, viz: select #pcol1 := CAST(null as INT), #pcol2 := CAST(null as DATE)
The solution I found to work the best was using a subquery like this:
SELECT
col1, col2,
(
SELECT COUNT(*)
FROM Table1
WHERE col1 = t1.col1
AND col2 = t1.col2
AND col3 > t1.col3
) AS intRow
FROM Table1 t1
The PARTITION BY columns just get compared with '=' and separated by AND. The ORDER BY columns would be compared with '<' or '>', and separated by OR.
I've found this to be very flexible, even if it is a little bit costly.
The rownumber functionality can't be mimicked. You might get the results you expect, but you'll most likely get disappointed at some stage.
Here's what mysql documentation says:
For other statements, such as SELECT, you might get the results you expect, but this is not guaranteed. In the following statement, you might think that MySQL will evaluate #a first and then do an assignment second:
SELECT #a, #a:=#a+1, ...;
However, the order of evaluation for expressions involving user variables is undefined.
Regards,
Georgi.
MariaDB 10.2 is implementing "Window Functions", including RANK(), ROW_NUMBER() and several other things:
https://mariadb.com/kb/en/mariadb/window-functions/
Based on a talk at Percona Live this month, they are reasonably well optimized.
The syntax is identical to the code in the Question.
MySQL has supported the ROW_NUMBER() since version 8.0+.
If you use MySQL 8.0 or later, check it out ROW_NUMBER() function.
Otherwise, you have emulate ROW_NUMBER() function.
The row_number() is a ranking function that returns a sequential number of a row, starting from 1 for the first row.
for older version,
SELECT t.*,
#rowid := #rowid + 1 AS ROWID
FROM TABLE t,
(SELECT #rowid := 0) dummy;
This allows the same functionality that ROW_NUMBER() AND PARTITION BY provides to be achieved in MySQL
SELECT #row_num := IF(#prev_value=GENDER,#row_num+1,1) AS RowNumber
FirstName,
Age,
Gender,
#prev_value := GENDER
FROM Person,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY Gender, Age DESC
I don't see any simple answer covering the "PARTITION BY" part so here's mine :
SELECT
*
FROM (
select
CASE WHEN #partitionBy_1 = l THEN #row_number:=#row_number+1 ELSE #row_number:=1 END AS i
, #partitionBy_1:=l AS p
, t.*
from (
select #row_number:=0,#partitionBy_1:=null
) as x
cross join (
select 1 as n, 'a' as l
union all
select 1 as n, 'b' as l
union all
select 2 as n, 'b' as l
union all
select 2 as n, 'a' as l
union all
select 3 as n, 'a' as l
union all
select 3 as n, 'b' as l
) as t
ORDER BY l, n
) AS X
where i > 1
The ORDER BY clause must reflect your ROW_NUMBER need. Thus there's already a clear limitation: you can't have several ROW_NUMBER "emulation" of this form at the same time.
The order of the "computed column" matters. If you have mysql compute those column in another order, it might not work.
In this simple example I only put one but you can have several "PARTITION BY" parts
CASE WHEN #partitionBy_1 = part1 AND #partitionBy_2 = part2 [...] THEN #row_number:=#row_number+1 ELSE #row_number:=1 END AS i
, #partitionBy_1:=part1 AS P1
, #partitionBy_2:=part2 AS P2
[...]
FROM (
SELECT #row_number:=0,#partitionBy_1:=null,#partitionBy_2:=null[...]
) as x
This could also be a solution:
SET #row_number = 0;
SELECT
(#row_number:=#row_number + 1) AS num, firstName, lastName
FROM
employees
Solutions with cross join and comma won't work if your query has GROUP BY statement. For such cases you can use subselect:
SELECT (#row_number := #row_number + 1) AS rowNumber, res.*
FROM
(
SELECT SUM(r.amount)
FROM Results r
WHERE username = 1
GROUP BY r.amount
) res
CROSS JOIN (SELECT #row_number := 0) AS dummy
I think you can use DENSE_RANK() function here.
Example:
select `score`, DENSE_RANK() OVER( ORDER BY score desc ) as `rank` from Scores;
https://www.mysqltutorial.org/mysql-window-functions/mysql-dense_rank-function/
A bit late but may also help to someone who looks for answers...
Between rows/row_number example - recursive query that may be used in any SQL:
WITH data(row_num, some_val) AS
(
SELECT 1 row_num, 1 some_val FROM any_table --dual in Oracle
UNION ALL
SELECT row_num+1, some_val+row_num FROM data WHERE row_num < 20 -- any number
)
SELECT * FROM data
WHERE row_num BETWEEN 5 AND 10
/
ROW_NUM SOME_VAL
-------------------
5 11
6 16
7 22
8 29
9 37
10 46
Also a bit late but today I had the same need so I did search on Google and finally a simple general approach found here in Pinal Dave's article http://blog.sqlauthority.com/2014/03/09/mysql-reset-row-number-for-each-group-partition-by-row-number/
I wanted to focus on Paul's original question (that was my problem as well) so I summarize my solution as a working example.
Beacuse we want to partition over two column I would create a SET variable during the iteration to identify if a new group was started.
SELECT col1, col2, col3 FROM (
SELECT col1, col2, col3,
#n := CASE WHEN #v = MAKE_SET(3, col1, col2)
THEN #n + 1 -- if we are in the same group
ELSE 1 -- next group starts so we reset the counter
END AS row_number,
#v := MAKE_SET(3, col1, col2) -- we store the current value for next iteration
FROM Table1, (SELECT #n := 0, #v := NULL) r -- helper table for iteration with startup values
ORDER BY col1, col2, col3 DESC -- because we want the row with maximum value
) x WHERE row_number = 1 -- and here we select exactly the wanted row from each group
The 3 means at the first parameter of MAKE_SET that I want both value in the SET (3=1|2).
Of course if we do not have two or more columns constructing the groups we can eliminate the MAKE_SET operation. The construction is exactly the same. This is working for me as required. Many thanks to Pinal Dave for his clear demonstration.
This is not the most robust solution - but if you're just looking to create a partitioned rank on a field with only a few different values, it may not be unwieldily to use some case when logic with as many variables as you require.
Something like this has worked for me in the past:
SELECT t.*,
CASE WHEN <partition_field> = #rownum1 := #rownum1 + 1
WHEN <partition_field> = #rownum2 := #rownum2 + 1
...
END AS rank
FROM YOUR_TABLE t,
(SELECT #rownum1 := 0) r1, (SELECT #rownum2 := 0) r2
ORDER BY <rank_order_by_field>
;
Hope that makes sense / helps!
This Work perfectly for me to create RowNumber when we have more than one column. In this case two column.
SELECT #row_num := IF(#prev_value= concat(`Fk_Business_Unit_Code`,`NetIQ_Job_Code`), #row_num+1, 1) AS RowNumber,
`Fk_Business_Unit_Code`,
`NetIQ_Job_Code`,
`Supervisor_Name`,
#prev_value := concat(`Fk_Business_Unit_Code`,`NetIQ_Job_Code`)
FROM (SELECT DISTINCT `Fk_Business_Unit_Code`,`NetIQ_Job_Code`,`Supervisor_Name`
FROM Employee
ORDER BY `Fk_Business_Unit_Code`, `NetIQ_Job_Code`, `Supervisor_Name` DESC) z,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY `Fk_Business_Unit_Code`, `NetIQ_Job_Code`,`Supervisor_Name` DESC
MySQL Since version 8, supports ROW_NUMBER(), so you can use the below query as you would use in SQL Server
SELECT
col1, col2,
ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY col3 DESC) AS intRow
FROM Table1
I also tested it in Maria DB 10.4.21. It works there as well.
for the partioning over anothe column one way is that described by #abcdn. However, it has a low performance. I propose use this code which does not require joining a table with itself:
Considee the same table.
you can get paritioning like this:
set #row_num := 0;
set #j:= 0;
select IF(j= #j, #row_num := #row_num + 1, #row_num := 1) as row_num,
i, #j:= j as j
from tbl fh
order by j, i;
the reult would be like this :
The advantage is we do not need to join table with itself
I'm trying to create an aggregated function MEDIAN() in MySQL like MIN(), MAX(), AVG() which takes the input the column name or string that has concatenated values of the desired column.
I'm having trouble understanding the limitations of MySQL custom functions & would be really helpful if some can help me find out how this is done.
Example:
MySQL table has 2 columns (ID, num)
+----+-----+
| id | num |
+----+-----+
| 1 | 5 |
| 1 | 6 |
| 1 | 7 |
| 2 | 1 |
| 2 | 3 |
| 2 | 5 |
+----+-----+
SELECT id, MEDIAN(num) as median
FROM table
GROUP BY id;
OR
SELECT id, MEDIAN(GROUP_CONCAT(num SEPARATOR ',') as median
FROM table
GROUP BY id;
Expected Output is
+----+--------+
| id | median |
+----+--------+
| 1 | 6 |
| 2 | 3 |
+----+--------+
User defined aggregate stored functions were added in MariaDB-10.3.3
MySQL can do aggregate functions however not in SQL. They need a UDF (shared library implemenation)
EDIT: I am aware that this answer does not directly address the question, since the question is "how to create an aggregate median function in mySQL" and my answer specifically says how to do it without a UDF.
However, the accepted answer says that it is not possible in mySQL, so I gave a solution that would address the aggregate median ability without having to use a UDF, in case someone might want to calculate the aggregate medians anyway.
It is possible to do without a UDF, and I know of two ways to do it. The first uses two selects and a join, the first select to get the values and rankings, and the second select to get the counts, then joins them. The second uses json functions to get everything in one select. They are both a little lengthy, but they work and are reasonably fast.
SOLUTION #1 (two selects and a join, one to get counts, one to get rankings)
SELECT x.group_field,
avg(
if(
x.rank - y.vol/2 BETWEEN 0 AND 1,
value_field,
null
)
) as median
FROM (
SELECT group_field, value_field,
#r:= IF(#current=group_field, #r+1, 1) as rank,
#current:=group_field
FROM (
SELECT group_field, value_field
FROM table_name
ORDER BY group_field, value_field
) z, (SELECT #r:=0, #current:='') v
) x, (
SELECT group_field, count(*) as vol
FROM table_name
GROUP BY group_field
) y WHERE x.group_field = y.group_field
GROUP BY x.group_field;
SOLUTION #2 (uses a json object to store the counts and avoids the join)
SELECT group_field,
avg(
if(
rank - json_extract(#vols, path)/2 BETWEEN 0 AND 1,
value_field,
null
)
) as median
FROM (
SELECT group_field, value_field, path,
#rnk := if(#curr = group_field, #rnk+1, 1) as rank,
#vols := json_set(
#vols,
path,
coalesce(json_extract(#vols, path), 0) + 1
) as vols,
#curr := group_field
FROM (
SELECT p.group_field, p.value_field, concat('$.', p.group_field) as path
FROM table_name
JOIN (SELECT #curr:='', #rnk:=1, #vols:=json_object()) v
ORDER BY group_field, value_field DESC
) z
) y GROUP BY group_field;
Is there a nice way in MySQL to replicate the SQL Server function ROW_NUMBER()?
For example:
SELECT
col1, col2,
ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY col3 DESC) AS intRow
FROM Table1
Then I could, for example, add a condition to limit intRow to 1 to get a single row with the highest col3 for each (col1, col2) pair.
There is no ranking functionality in MySQL. The closest you can get is to use a variable:
SELECT t.*,
#rownum := #rownum + 1 AS rank
FROM YOUR_TABLE t,
(SELECT #rownum := 0) r
so how would that work in my case? I'd need two variables, one for each of col1 and col2? Col2 would need resetting somehow when col1 changed..?
Yes. If it were Oracle, you could use the LEAD function to peak at the next value. Thankfully, Quassnoi covers the logic for what you need to implement in MySQL.
I want the row with the single highest col3 for each (col1, col2) pair.
That's a groupwise maximum, one of the most commonly-asked SQL questions (since it seems like it should be easy, but actually it kind of isn't).
I often plump for a null-self-join:
SELECT t0.col3
FROM table AS t0
LEFT JOIN table AS t1 ON t0.col1=t1.col1 AND t0.col2=t1.col2 AND t1.col3>t0.col3
WHERE t1.col1 IS NULL;
“Get the rows in the table for which no other row with matching col1,col2 has a higher col3.” (You will notice this and most other groupwise-maximum solutions will return multiple rows if more than one row has the same col1,col2,col3. If that's a problem you may need some post-processing.)
I always end up following this pattern. Given this table:
+------+------+
| i | j |
+------+------+
| 1 | 11 |
| 1 | 12 |
| 1 | 13 |
| 2 | 21 |
| 2 | 22 |
| 2 | 23 |
| 3 | 31 |
| 3 | 32 |
| 3 | 33 |
| 4 | 14 |
+------+------+
You can get this result:
+------+------+------------+
| i | j | row_number |
+------+------+------------+
| 1 | 11 | 1 |
| 1 | 12 | 2 |
| 1 | 13 | 3 |
| 2 | 21 | 1 |
| 2 | 22 | 2 |
| 2 | 23 | 3 |
| 3 | 31 | 1 |
| 3 | 32 | 2 |
| 3 | 33 | 3 |
| 4 | 14 | 1 |
+------+------+------------+
By running this query, which doesn't need any variable defined:
SELECT a.i, a.j, count(*) as row_number FROM test a
JOIN test b ON a.i = b.i AND a.j >= b.j
GROUP BY a.i, a.j
SELECT
#i:=#i+1 AS iterator,
t.*
FROM
tablename AS t,
(SELECT #i:=0) AS foo
From MySQL 8.0.0 and above you could natively use windowed functions.
1.4 What Is New in MySQL 8.0:
Window functions.
MySQL now supports window functions that, for each row from a query, perform a calculation using rows related to that row. These include functions such as RANK(), LAG(), and NTILE(). In addition, several existing aggregate functions now can be used as window functions; for example, SUM() and AVG().
ROW_NUMBER() over_clause :
Returns the number of the current row within its partition. Rows numbers range from 1 to the number of partition rows.
ORDER BY affects the order in which rows are numbered. Without ORDER BY, row numbering is indeterminate.
Demo:
CREATE TABLE Table1(
id INT AUTO_INCREMENT PRIMARY KEY, col1 INT,col2 INT, col3 TEXT);
INSERT INTO Table1(col1, col2, col3)
VALUES (1,1,'a'),(1,1,'b'),(1,1,'c'),
(2,1,'x'),(2,1,'y'),(2,2,'z');
SELECT
col1, col2,col3,
ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY col3 DESC) AS intRow
FROM Table1;
DBFiddle Demo
Check out this Article, it shows how to mimic SQL ROW_NUMBER() with a partition by in MySQL. I ran into this very same scenario in a WordPress Implementation. I needed ROW_NUMBER() and it wasn't there.
http://www.explodybits.com/2011/11/mysql-row-number/
The example in the article is using a single partition by field. To partition by additional fields you could do something like this:
SELECT #row_num := IF(#prev_value=concat_ws('',t.col1,t.col2),#row_num+1,1) AS RowNumber
,t.col1
,t.col2
,t.Col3
,t.col4
,#prev_value := concat_ws('',t.col1,t.col2)
FROM table1 t,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY t.col1,t.col2,t.col3,t.col4
Using concat_ws handles null's. I tested this against 3 fields using an int, date, and varchar. Hope this helps. Check out the article as it breaks this query down and explains it.
I would also vote for Mosty Mostacho's solution with minor modification to his query code:
SELECT a.i, a.j, (
SELECT count(*) from test b where a.j >= b.j AND a.i = b.i
) AS row_number FROM test a
Which will give the same result:
+------+------+------------+
| i | j | row_number |
+------+------+------------+
| 1 | 11 | 1 |
| 1 | 12 | 2 |
| 1 | 13 | 3 |
| 2 | 21 | 1 |
| 2 | 22 | 2 |
| 2 | 23 | 3 |
| 3 | 31 | 1 |
| 3 | 32 | 2 |
| 3 | 33 | 3 |
| 4 | 14 | 1 |
+------+------+------------+
for the table:
+------+------+
| i | j |
+------+------+
| 1 | 11 |
| 1 | 12 |
| 1 | 13 |
| 2 | 21 |
| 2 | 22 |
| 2 | 23 |
| 3 | 31 |
| 3 | 32 |
| 3 | 33 |
| 4 | 14 |
+------+------+
With the only difference that the query doesn't use JOIN and GROUP BY, relying on nested select instead.
I would define a function:
delimiter $$
DROP FUNCTION IF EXISTS `getFakeId`$$
CREATE FUNCTION `getFakeId`() RETURNS int(11)
DETERMINISTIC
begin
return if(#fakeId, #fakeId:=#fakeId+1, #fakeId:=1);
end$$
then I could do:
select getFakeId() as id, t.* from table t, (select #fakeId:=0) as t2;
Now you don't have a subquery, which you can't have in views.
query for row_number in mysql
set #row_number=0;
select (#row_number := #row_number +1) as num,id,name from sbs
There is no funtion like rownum, row_num() in MySQL but the way around is like below:
select
#s:=#s+1 serial_no,
tbl.*
from my_table tbl, (select #s:=0) as s;
Important: Please consider upgrading to MySQL 8+ and use the defined and documented ROW_NUMBER() function, and ditch old hacks tied to a feature limited ancient version of MySQL
Now here's one of those hacks:
The answers here that use in-query variables mostly/all seem to ignore the fact that the documentation says (paraphrase):
Don't rely on items in the SELECT list being evaluated in order from top to bottom. Don't assign variables in one SELECT item and use them in another one
As such, there's a risk they will churn out the wrong answer, because they typically do a
select
(row number variable that uses partition variable),
(assign partition variable)
If these are ever evaluated bottom up, the row number will stop working (no partitions)
So we need to use something with a guaranteed order of execution. Enter CASE WHEN:
SELECT
t.*,
#r := CASE
WHEN col = #prevcol THEN #r + 1
WHEN (#prevcol := col) = null THEN null
ELSE 1 END AS rn
FROM
t,
(SELECT #r := 0, #prevcol := null) x
ORDER BY col
As outline ld, order of assignment of prevcol is important - prevcol has to be compared to the current row's value before we assign it a value from the current row (otherwise it would be the current rows col value, not the previous row's col value).
Here's how this fits together:
The first WHEN is evaluated. If this row's col is the same as the previous row's col then #r is incremented and returned from the CASE. This return led values is stored in #r. It's a feature of MySQL that assignment returns the new value of what is assigned into #r into the result rows.
For the first row on the result set, #prevcol is null (it is initialised to null in the subquery) so this predicate is false. This first predicate also returns false every time col changes (current row is different to previous row). This causes the second WHEN to be evaluated.
The second WHEN predicate is always false, and it exists purely to assign a new value to #prevcol. Because this row's col is different to the previous row's col (we know this because if it were the same, the first WHEN would have been used), we have to assign the new value to keep it for testing next time. Because the assignment is made and then the result of the assignment is compared with null, and anything equated with null is false, this predicate is always false. But at least evaluating it did its job of keeping the value of col from this row, so it can be evaluated against the next row's col value
Because the second WHEN is false, it means in situations where the column we are partitioning by (col) has changed, it is the ELSE that gives a new value for #r, restarting the numbering from 1
We this get to a situation where this:
SELECT
t.*,
ROW_NUMBER() OVER(PARTITION BY pcol1, pcol2, ... pcolX ORDER BY ocol1, ocol2, ... ocolX) rn
FROM
t
Has the general form:
SELECT
t.*,
#r := CASE
WHEN col1 = #pcol1 AND col2 = #pcol2 AND ... AND colX = #pcolX THEN #r + 1
WHEN (#pcol1 := pcol1) = null OR (#pcol2 := col2) = null OR ... OR (#pcolX := colX) = null THEN null
ELSE 1
END AS rn
FROM
t,
(SELECT #r := 0, #pcol1 := null, #pcol2 := null, ..., #pcolX := null) x
ORDER BY pcol1, pcol2, ..., pcolX, ocol1, ocol2, ..., ocolX
Footnotes:
The p in pcol means "partition", the o in ocol means "order" - in the general form I dropped the "prev" from the variable name to reduce visual clutter
The brackets around (#pcolX := colX) = null are important. Without them you'll assign null to #pcolX and things stop working
It's a compromise that the result set has to be ordered by the partition columns too, for the previous column compare to work out. You can't thus have your rownumber ordered according to one column but your result set ordered to another You might be able to resolve this with subqueries but I believe the docs also state that subquery ordering may be ignored unless LIMIT is used and this could impact performance
I haven't delved into it beyond testing that the method works, but if there is a risk that the predicates in the second WHEN will be optimised away (anything compared to null is null/false so why bother running the assignment) and not executed, it also stops. This doesn't seem to happen in my experience but I'll gladly accept comments and propose solution if it could reasonably occur
It may be wise to cast the nulls that create #pcolX to the actual types of your columns, in the subquery that creates the #pcolX variables, viz: select #pcol1 := CAST(null as INT), #pcol2 := CAST(null as DATE)
The solution I found to work the best was using a subquery like this:
SELECT
col1, col2,
(
SELECT COUNT(*)
FROM Table1
WHERE col1 = t1.col1
AND col2 = t1.col2
AND col3 > t1.col3
) AS intRow
FROM Table1 t1
The PARTITION BY columns just get compared with '=' and separated by AND. The ORDER BY columns would be compared with '<' or '>', and separated by OR.
I've found this to be very flexible, even if it is a little bit costly.
The rownumber functionality can't be mimicked. You might get the results you expect, but you'll most likely get disappointed at some stage.
Here's what mysql documentation says:
For other statements, such as SELECT, you might get the results you expect, but this is not guaranteed. In the following statement, you might think that MySQL will evaluate #a first and then do an assignment second:
SELECT #a, #a:=#a+1, ...;
However, the order of evaluation for expressions involving user variables is undefined.
Regards,
Georgi.
MariaDB 10.2 is implementing "Window Functions", including RANK(), ROW_NUMBER() and several other things:
https://mariadb.com/kb/en/mariadb/window-functions/
Based on a talk at Percona Live this month, they are reasonably well optimized.
The syntax is identical to the code in the Question.
MySQL has supported the ROW_NUMBER() since version 8.0+.
If you use MySQL 8.0 or later, check it out ROW_NUMBER() function.
Otherwise, you have emulate ROW_NUMBER() function.
The row_number() is a ranking function that returns a sequential number of a row, starting from 1 for the first row.
for older version,
SELECT t.*,
#rowid := #rowid + 1 AS ROWID
FROM TABLE t,
(SELECT #rowid := 0) dummy;
This allows the same functionality that ROW_NUMBER() AND PARTITION BY provides to be achieved in MySQL
SELECT #row_num := IF(#prev_value=GENDER,#row_num+1,1) AS RowNumber
FirstName,
Age,
Gender,
#prev_value := GENDER
FROM Person,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY Gender, Age DESC
I don't see any simple answer covering the "PARTITION BY" part so here's mine :
SELECT
*
FROM (
select
CASE WHEN #partitionBy_1 = l THEN #row_number:=#row_number+1 ELSE #row_number:=1 END AS i
, #partitionBy_1:=l AS p
, t.*
from (
select #row_number:=0,#partitionBy_1:=null
) as x
cross join (
select 1 as n, 'a' as l
union all
select 1 as n, 'b' as l
union all
select 2 as n, 'b' as l
union all
select 2 as n, 'a' as l
union all
select 3 as n, 'a' as l
union all
select 3 as n, 'b' as l
) as t
ORDER BY l, n
) AS X
where i > 1
The ORDER BY clause must reflect your ROW_NUMBER need. Thus there's already a clear limitation: you can't have several ROW_NUMBER "emulation" of this form at the same time.
The order of the "computed column" matters. If you have mysql compute those column in another order, it might not work.
In this simple example I only put one but you can have several "PARTITION BY" parts
CASE WHEN #partitionBy_1 = part1 AND #partitionBy_2 = part2 [...] THEN #row_number:=#row_number+1 ELSE #row_number:=1 END AS i
, #partitionBy_1:=part1 AS P1
, #partitionBy_2:=part2 AS P2
[...]
FROM (
SELECT #row_number:=0,#partitionBy_1:=null,#partitionBy_2:=null[...]
) as x
This could also be a solution:
SET #row_number = 0;
SELECT
(#row_number:=#row_number + 1) AS num, firstName, lastName
FROM
employees
Solutions with cross join and comma won't work if your query has GROUP BY statement. For such cases you can use subselect:
SELECT (#row_number := #row_number + 1) AS rowNumber, res.*
FROM
(
SELECT SUM(r.amount)
FROM Results r
WHERE username = 1
GROUP BY r.amount
) res
CROSS JOIN (SELECT #row_number := 0) AS dummy
I think you can use DENSE_RANK() function here.
Example:
select `score`, DENSE_RANK() OVER( ORDER BY score desc ) as `rank` from Scores;
https://www.mysqltutorial.org/mysql-window-functions/mysql-dense_rank-function/
A bit late but may also help to someone who looks for answers...
Between rows/row_number example - recursive query that may be used in any SQL:
WITH data(row_num, some_val) AS
(
SELECT 1 row_num, 1 some_val FROM any_table --dual in Oracle
UNION ALL
SELECT row_num+1, some_val+row_num FROM data WHERE row_num < 20 -- any number
)
SELECT * FROM data
WHERE row_num BETWEEN 5 AND 10
/
ROW_NUM SOME_VAL
-------------------
5 11
6 16
7 22
8 29
9 37
10 46
Also a bit late but today I had the same need so I did search on Google and finally a simple general approach found here in Pinal Dave's article http://blog.sqlauthority.com/2014/03/09/mysql-reset-row-number-for-each-group-partition-by-row-number/
I wanted to focus on Paul's original question (that was my problem as well) so I summarize my solution as a working example.
Beacuse we want to partition over two column I would create a SET variable during the iteration to identify if a new group was started.
SELECT col1, col2, col3 FROM (
SELECT col1, col2, col3,
#n := CASE WHEN #v = MAKE_SET(3, col1, col2)
THEN #n + 1 -- if we are in the same group
ELSE 1 -- next group starts so we reset the counter
END AS row_number,
#v := MAKE_SET(3, col1, col2) -- we store the current value for next iteration
FROM Table1, (SELECT #n := 0, #v := NULL) r -- helper table for iteration with startup values
ORDER BY col1, col2, col3 DESC -- because we want the row with maximum value
) x WHERE row_number = 1 -- and here we select exactly the wanted row from each group
The 3 means at the first parameter of MAKE_SET that I want both value in the SET (3=1|2).
Of course if we do not have two or more columns constructing the groups we can eliminate the MAKE_SET operation. The construction is exactly the same. This is working for me as required. Many thanks to Pinal Dave for his clear demonstration.
This is not the most robust solution - but if you're just looking to create a partitioned rank on a field with only a few different values, it may not be unwieldily to use some case when logic with as many variables as you require.
Something like this has worked for me in the past:
SELECT t.*,
CASE WHEN <partition_field> = #rownum1 := #rownum1 + 1
WHEN <partition_field> = #rownum2 := #rownum2 + 1
...
END AS rank
FROM YOUR_TABLE t,
(SELECT #rownum1 := 0) r1, (SELECT #rownum2 := 0) r2
ORDER BY <rank_order_by_field>
;
Hope that makes sense / helps!
This Work perfectly for me to create RowNumber when we have more than one column. In this case two column.
SELECT #row_num := IF(#prev_value= concat(`Fk_Business_Unit_Code`,`NetIQ_Job_Code`), #row_num+1, 1) AS RowNumber,
`Fk_Business_Unit_Code`,
`NetIQ_Job_Code`,
`Supervisor_Name`,
#prev_value := concat(`Fk_Business_Unit_Code`,`NetIQ_Job_Code`)
FROM (SELECT DISTINCT `Fk_Business_Unit_Code`,`NetIQ_Job_Code`,`Supervisor_Name`
FROM Employee
ORDER BY `Fk_Business_Unit_Code`, `NetIQ_Job_Code`, `Supervisor_Name` DESC) z,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY `Fk_Business_Unit_Code`, `NetIQ_Job_Code`,`Supervisor_Name` DESC
MySQL Since version 8, supports ROW_NUMBER(), so you can use the below query as you would use in SQL Server
SELECT
col1, col2,
ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY col3 DESC) AS intRow
FROM Table1
I also tested it in Maria DB 10.4.21. It works there as well.
for the partioning over anothe column one way is that described by #abcdn. However, it has a low performance. I propose use this code which does not require joining a table with itself:
Considee the same table.
you can get paritioning like this:
set #row_num := 0;
set #j:= 0;
select IF(j= #j, #row_num := #row_num + 1, #row_num := 1) as row_num,
i, #j:= j as j
from tbl fh
order by j, i;
the reult would be like this :
The advantage is we do not need to join table with itself
Is there a nice way in MySQL to replicate the SQL Server function ROW_NUMBER()?
For example:
SELECT
col1, col2,
ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY col3 DESC) AS intRow
FROM Table1
Then I could, for example, add a condition to limit intRow to 1 to get a single row with the highest col3 for each (col1, col2) pair.
There is no ranking functionality in MySQL. The closest you can get is to use a variable:
SELECT t.*,
#rownum := #rownum + 1 AS rank
FROM YOUR_TABLE t,
(SELECT #rownum := 0) r
so how would that work in my case? I'd need two variables, one for each of col1 and col2? Col2 would need resetting somehow when col1 changed..?
Yes. If it were Oracle, you could use the LEAD function to peak at the next value. Thankfully, Quassnoi covers the logic for what you need to implement in MySQL.
I want the row with the single highest col3 for each (col1, col2) pair.
That's a groupwise maximum, one of the most commonly-asked SQL questions (since it seems like it should be easy, but actually it kind of isn't).
I often plump for a null-self-join:
SELECT t0.col3
FROM table AS t0
LEFT JOIN table AS t1 ON t0.col1=t1.col1 AND t0.col2=t1.col2 AND t1.col3>t0.col3
WHERE t1.col1 IS NULL;
“Get the rows in the table for which no other row with matching col1,col2 has a higher col3.” (You will notice this and most other groupwise-maximum solutions will return multiple rows if more than one row has the same col1,col2,col3. If that's a problem you may need some post-processing.)
I always end up following this pattern. Given this table:
+------+------+
| i | j |
+------+------+
| 1 | 11 |
| 1 | 12 |
| 1 | 13 |
| 2 | 21 |
| 2 | 22 |
| 2 | 23 |
| 3 | 31 |
| 3 | 32 |
| 3 | 33 |
| 4 | 14 |
+------+------+
You can get this result:
+------+------+------------+
| i | j | row_number |
+------+------+------------+
| 1 | 11 | 1 |
| 1 | 12 | 2 |
| 1 | 13 | 3 |
| 2 | 21 | 1 |
| 2 | 22 | 2 |
| 2 | 23 | 3 |
| 3 | 31 | 1 |
| 3 | 32 | 2 |
| 3 | 33 | 3 |
| 4 | 14 | 1 |
+------+------+------------+
By running this query, which doesn't need any variable defined:
SELECT a.i, a.j, count(*) as row_number FROM test a
JOIN test b ON a.i = b.i AND a.j >= b.j
GROUP BY a.i, a.j
SELECT
#i:=#i+1 AS iterator,
t.*
FROM
tablename AS t,
(SELECT #i:=0) AS foo
From MySQL 8.0.0 and above you could natively use windowed functions.
1.4 What Is New in MySQL 8.0:
Window functions.
MySQL now supports window functions that, for each row from a query, perform a calculation using rows related to that row. These include functions such as RANK(), LAG(), and NTILE(). In addition, several existing aggregate functions now can be used as window functions; for example, SUM() and AVG().
ROW_NUMBER() over_clause :
Returns the number of the current row within its partition. Rows numbers range from 1 to the number of partition rows.
ORDER BY affects the order in which rows are numbered. Without ORDER BY, row numbering is indeterminate.
Demo:
CREATE TABLE Table1(
id INT AUTO_INCREMENT PRIMARY KEY, col1 INT,col2 INT, col3 TEXT);
INSERT INTO Table1(col1, col2, col3)
VALUES (1,1,'a'),(1,1,'b'),(1,1,'c'),
(2,1,'x'),(2,1,'y'),(2,2,'z');
SELECT
col1, col2,col3,
ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY col3 DESC) AS intRow
FROM Table1;
DBFiddle Demo
Check out this Article, it shows how to mimic SQL ROW_NUMBER() with a partition by in MySQL. I ran into this very same scenario in a WordPress Implementation. I needed ROW_NUMBER() and it wasn't there.
http://www.explodybits.com/2011/11/mysql-row-number/
The example in the article is using a single partition by field. To partition by additional fields you could do something like this:
SELECT #row_num := IF(#prev_value=concat_ws('',t.col1,t.col2),#row_num+1,1) AS RowNumber
,t.col1
,t.col2
,t.Col3
,t.col4
,#prev_value := concat_ws('',t.col1,t.col2)
FROM table1 t,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY t.col1,t.col2,t.col3,t.col4
Using concat_ws handles null's. I tested this against 3 fields using an int, date, and varchar. Hope this helps. Check out the article as it breaks this query down and explains it.
I would also vote for Mosty Mostacho's solution with minor modification to his query code:
SELECT a.i, a.j, (
SELECT count(*) from test b where a.j >= b.j AND a.i = b.i
) AS row_number FROM test a
Which will give the same result:
+------+------+------------+
| i | j | row_number |
+------+------+------------+
| 1 | 11 | 1 |
| 1 | 12 | 2 |
| 1 | 13 | 3 |
| 2 | 21 | 1 |
| 2 | 22 | 2 |
| 2 | 23 | 3 |
| 3 | 31 | 1 |
| 3 | 32 | 2 |
| 3 | 33 | 3 |
| 4 | 14 | 1 |
+------+------+------------+
for the table:
+------+------+
| i | j |
+------+------+
| 1 | 11 |
| 1 | 12 |
| 1 | 13 |
| 2 | 21 |
| 2 | 22 |
| 2 | 23 |
| 3 | 31 |
| 3 | 32 |
| 3 | 33 |
| 4 | 14 |
+------+------+
With the only difference that the query doesn't use JOIN and GROUP BY, relying on nested select instead.
I would define a function:
delimiter $$
DROP FUNCTION IF EXISTS `getFakeId`$$
CREATE FUNCTION `getFakeId`() RETURNS int(11)
DETERMINISTIC
begin
return if(#fakeId, #fakeId:=#fakeId+1, #fakeId:=1);
end$$
then I could do:
select getFakeId() as id, t.* from table t, (select #fakeId:=0) as t2;
Now you don't have a subquery, which you can't have in views.
query for row_number in mysql
set #row_number=0;
select (#row_number := #row_number +1) as num,id,name from sbs
There is no funtion like rownum, row_num() in MySQL but the way around is like below:
select
#s:=#s+1 serial_no,
tbl.*
from my_table tbl, (select #s:=0) as s;
Important: Please consider upgrading to MySQL 8+ and use the defined and documented ROW_NUMBER() function, and ditch old hacks tied to a feature limited ancient version of MySQL
Now here's one of those hacks:
The answers here that use in-query variables mostly/all seem to ignore the fact that the documentation says (paraphrase):
Don't rely on items in the SELECT list being evaluated in order from top to bottom. Don't assign variables in one SELECT item and use them in another one
As such, there's a risk they will churn out the wrong answer, because they typically do a
select
(row number variable that uses partition variable),
(assign partition variable)
If these are ever evaluated bottom up, the row number will stop working (no partitions)
So we need to use something with a guaranteed order of execution. Enter CASE WHEN:
SELECT
t.*,
#r := CASE
WHEN col = #prevcol THEN #r + 1
WHEN (#prevcol := col) = null THEN null
ELSE 1 END AS rn
FROM
t,
(SELECT #r := 0, #prevcol := null) x
ORDER BY col
As outline ld, order of assignment of prevcol is important - prevcol has to be compared to the current row's value before we assign it a value from the current row (otherwise it would be the current rows col value, not the previous row's col value).
Here's how this fits together:
The first WHEN is evaluated. If this row's col is the same as the previous row's col then #r is incremented and returned from the CASE. This return led values is stored in #r. It's a feature of MySQL that assignment returns the new value of what is assigned into #r into the result rows.
For the first row on the result set, #prevcol is null (it is initialised to null in the subquery) so this predicate is false. This first predicate also returns false every time col changes (current row is different to previous row). This causes the second WHEN to be evaluated.
The second WHEN predicate is always false, and it exists purely to assign a new value to #prevcol. Because this row's col is different to the previous row's col (we know this because if it were the same, the first WHEN would have been used), we have to assign the new value to keep it for testing next time. Because the assignment is made and then the result of the assignment is compared with null, and anything equated with null is false, this predicate is always false. But at least evaluating it did its job of keeping the value of col from this row, so it can be evaluated against the next row's col value
Because the second WHEN is false, it means in situations where the column we are partitioning by (col) has changed, it is the ELSE that gives a new value for #r, restarting the numbering from 1
We this get to a situation where this:
SELECT
t.*,
ROW_NUMBER() OVER(PARTITION BY pcol1, pcol2, ... pcolX ORDER BY ocol1, ocol2, ... ocolX) rn
FROM
t
Has the general form:
SELECT
t.*,
#r := CASE
WHEN col1 = #pcol1 AND col2 = #pcol2 AND ... AND colX = #pcolX THEN #r + 1
WHEN (#pcol1 := pcol1) = null OR (#pcol2 := col2) = null OR ... OR (#pcolX := colX) = null THEN null
ELSE 1
END AS rn
FROM
t,
(SELECT #r := 0, #pcol1 := null, #pcol2 := null, ..., #pcolX := null) x
ORDER BY pcol1, pcol2, ..., pcolX, ocol1, ocol2, ..., ocolX
Footnotes:
The p in pcol means "partition", the o in ocol means "order" - in the general form I dropped the "prev" from the variable name to reduce visual clutter
The brackets around (#pcolX := colX) = null are important. Without them you'll assign null to #pcolX and things stop working
It's a compromise that the result set has to be ordered by the partition columns too, for the previous column compare to work out. You can't thus have your rownumber ordered according to one column but your result set ordered to another You might be able to resolve this with subqueries but I believe the docs also state that subquery ordering may be ignored unless LIMIT is used and this could impact performance
I haven't delved into it beyond testing that the method works, but if there is a risk that the predicates in the second WHEN will be optimised away (anything compared to null is null/false so why bother running the assignment) and not executed, it also stops. This doesn't seem to happen in my experience but I'll gladly accept comments and propose solution if it could reasonably occur
It may be wise to cast the nulls that create #pcolX to the actual types of your columns, in the subquery that creates the #pcolX variables, viz: select #pcol1 := CAST(null as INT), #pcol2 := CAST(null as DATE)
The solution I found to work the best was using a subquery like this:
SELECT
col1, col2,
(
SELECT COUNT(*)
FROM Table1
WHERE col1 = t1.col1
AND col2 = t1.col2
AND col3 > t1.col3
) AS intRow
FROM Table1 t1
The PARTITION BY columns just get compared with '=' and separated by AND. The ORDER BY columns would be compared with '<' or '>', and separated by OR.
I've found this to be very flexible, even if it is a little bit costly.
The rownumber functionality can't be mimicked. You might get the results you expect, but you'll most likely get disappointed at some stage.
Here's what mysql documentation says:
For other statements, such as SELECT, you might get the results you expect, but this is not guaranteed. In the following statement, you might think that MySQL will evaluate #a first and then do an assignment second:
SELECT #a, #a:=#a+1, ...;
However, the order of evaluation for expressions involving user variables is undefined.
Regards,
Georgi.
MariaDB 10.2 is implementing "Window Functions", including RANK(), ROW_NUMBER() and several other things:
https://mariadb.com/kb/en/mariadb/window-functions/
Based on a talk at Percona Live this month, they are reasonably well optimized.
The syntax is identical to the code in the Question.
MySQL has supported the ROW_NUMBER() since version 8.0+.
If you use MySQL 8.0 or later, check it out ROW_NUMBER() function.
Otherwise, you have emulate ROW_NUMBER() function.
The row_number() is a ranking function that returns a sequential number of a row, starting from 1 for the first row.
for older version,
SELECT t.*,
#rowid := #rowid + 1 AS ROWID
FROM TABLE t,
(SELECT #rowid := 0) dummy;
This allows the same functionality that ROW_NUMBER() AND PARTITION BY provides to be achieved in MySQL
SELECT #row_num := IF(#prev_value=GENDER,#row_num+1,1) AS RowNumber
FirstName,
Age,
Gender,
#prev_value := GENDER
FROM Person,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY Gender, Age DESC
I don't see any simple answer covering the "PARTITION BY" part so here's mine :
SELECT
*
FROM (
select
CASE WHEN #partitionBy_1 = l THEN #row_number:=#row_number+1 ELSE #row_number:=1 END AS i
, #partitionBy_1:=l AS p
, t.*
from (
select #row_number:=0,#partitionBy_1:=null
) as x
cross join (
select 1 as n, 'a' as l
union all
select 1 as n, 'b' as l
union all
select 2 as n, 'b' as l
union all
select 2 as n, 'a' as l
union all
select 3 as n, 'a' as l
union all
select 3 as n, 'b' as l
) as t
ORDER BY l, n
) AS X
where i > 1
The ORDER BY clause must reflect your ROW_NUMBER need. Thus there's already a clear limitation: you can't have several ROW_NUMBER "emulation" of this form at the same time.
The order of the "computed column" matters. If you have mysql compute those column in another order, it might not work.
In this simple example I only put one but you can have several "PARTITION BY" parts
CASE WHEN #partitionBy_1 = part1 AND #partitionBy_2 = part2 [...] THEN #row_number:=#row_number+1 ELSE #row_number:=1 END AS i
, #partitionBy_1:=part1 AS P1
, #partitionBy_2:=part2 AS P2
[...]
FROM (
SELECT #row_number:=0,#partitionBy_1:=null,#partitionBy_2:=null[...]
) as x
This could also be a solution:
SET #row_number = 0;
SELECT
(#row_number:=#row_number + 1) AS num, firstName, lastName
FROM
employees
Solutions with cross join and comma won't work if your query has GROUP BY statement. For such cases you can use subselect:
SELECT (#row_number := #row_number + 1) AS rowNumber, res.*
FROM
(
SELECT SUM(r.amount)
FROM Results r
WHERE username = 1
GROUP BY r.amount
) res
CROSS JOIN (SELECT #row_number := 0) AS dummy
I think you can use DENSE_RANK() function here.
Example:
select `score`, DENSE_RANK() OVER( ORDER BY score desc ) as `rank` from Scores;
https://www.mysqltutorial.org/mysql-window-functions/mysql-dense_rank-function/
A bit late but may also help to someone who looks for answers...
Between rows/row_number example - recursive query that may be used in any SQL:
WITH data(row_num, some_val) AS
(
SELECT 1 row_num, 1 some_val FROM any_table --dual in Oracle
UNION ALL
SELECT row_num+1, some_val+row_num FROM data WHERE row_num < 20 -- any number
)
SELECT * FROM data
WHERE row_num BETWEEN 5 AND 10
/
ROW_NUM SOME_VAL
-------------------
5 11
6 16
7 22
8 29
9 37
10 46
Also a bit late but today I had the same need so I did search on Google and finally a simple general approach found here in Pinal Dave's article http://blog.sqlauthority.com/2014/03/09/mysql-reset-row-number-for-each-group-partition-by-row-number/
I wanted to focus on Paul's original question (that was my problem as well) so I summarize my solution as a working example.
Beacuse we want to partition over two column I would create a SET variable during the iteration to identify if a new group was started.
SELECT col1, col2, col3 FROM (
SELECT col1, col2, col3,
#n := CASE WHEN #v = MAKE_SET(3, col1, col2)
THEN #n + 1 -- if we are in the same group
ELSE 1 -- next group starts so we reset the counter
END AS row_number,
#v := MAKE_SET(3, col1, col2) -- we store the current value for next iteration
FROM Table1, (SELECT #n := 0, #v := NULL) r -- helper table for iteration with startup values
ORDER BY col1, col2, col3 DESC -- because we want the row with maximum value
) x WHERE row_number = 1 -- and here we select exactly the wanted row from each group
The 3 means at the first parameter of MAKE_SET that I want both value in the SET (3=1|2).
Of course if we do not have two or more columns constructing the groups we can eliminate the MAKE_SET operation. The construction is exactly the same. This is working for me as required. Many thanks to Pinal Dave for his clear demonstration.
This is not the most robust solution - but if you're just looking to create a partitioned rank on a field with only a few different values, it may not be unwieldily to use some case when logic with as many variables as you require.
Something like this has worked for me in the past:
SELECT t.*,
CASE WHEN <partition_field> = #rownum1 := #rownum1 + 1
WHEN <partition_field> = #rownum2 := #rownum2 + 1
...
END AS rank
FROM YOUR_TABLE t,
(SELECT #rownum1 := 0) r1, (SELECT #rownum2 := 0) r2
ORDER BY <rank_order_by_field>
;
Hope that makes sense / helps!
This Work perfectly for me to create RowNumber when we have more than one column. In this case two column.
SELECT #row_num := IF(#prev_value= concat(`Fk_Business_Unit_Code`,`NetIQ_Job_Code`), #row_num+1, 1) AS RowNumber,
`Fk_Business_Unit_Code`,
`NetIQ_Job_Code`,
`Supervisor_Name`,
#prev_value := concat(`Fk_Business_Unit_Code`,`NetIQ_Job_Code`)
FROM (SELECT DISTINCT `Fk_Business_Unit_Code`,`NetIQ_Job_Code`,`Supervisor_Name`
FROM Employee
ORDER BY `Fk_Business_Unit_Code`, `NetIQ_Job_Code`, `Supervisor_Name` DESC) z,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY `Fk_Business_Unit_Code`, `NetIQ_Job_Code`,`Supervisor_Name` DESC
MySQL Since version 8, supports ROW_NUMBER(), so you can use the below query as you would use in SQL Server
SELECT
col1, col2,
ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY col3 DESC) AS intRow
FROM Table1
I also tested it in Maria DB 10.4.21. It works there as well.
for the partioning over anothe column one way is that described by #abcdn. However, it has a low performance. I propose use this code which does not require joining a table with itself:
Considee the same table.
you can get paritioning like this:
set #row_num := 0;
set #j:= 0;
select IF(j= #j, #row_num := #row_num + 1, #row_num := 1) as row_num,
i, #j:= j as j
from tbl fh
order by j, i;
the reult would be like this :
The advantage is we do not need to join table with itself
Thank you stackoverflow community! I have learned SO much from you over the years and I've recently created an account. I hope the answer to this question isn't obviously somewhere already, but I am going to go crossed eyed if I read another post. Here's my problem:
I recently used a nested SELECT to get the highest score for each of my students from a table. I did so by a little trick another post taught me. I can't find the exact post I learned it from, but here is a snippet that's essentially the same trick. I imagine, if you are well versed in sql, it's nothing new to you:
SELECT id, authorId, answer, votes
FROM ( SELECT id, authorId, answer, votes
FROM answers
ORDER BY votes DESC) AS h
GROUP BY authorId
The ORDER BY ____ DESC makes the last value, the highest overwrite all previous, so you end up with only it...if I understand correctly. So, that was great and I tailored it to my needs. The only problem is, now, I'd like to add one more feature to it and I'm racking my brain cells over it. I'm hoping some generous person will just straighten me out. I want to get a complete list of students from my "rosters" table and if there is no score for a given student, in my "holder" table, I'd like it to display a "0". Here is what I have, and I don't know exactly how to tweak it to do just that:
SELECT *
FROM (
SELECT
holder.id,
#IFNULL(holder.score, 0) AS score,
holder.score AS score,
holder.total,
holder.student_id AS stu_id,
holder.date AS date,
users.firstname AS first,
users.lastname AS last,
users.stu_number AS stuno,
assignments.name AS test,
rosters.p_id,
preps.period AS period,
preps.user
FROM holder
JOIN rosters
ON rosters.stu_id = holder.student_id
JOIN users
ON users.id = holder.student_id
JOIN assignments
ON assignments.id = holder.t_id
JOIN preps
ON preps.id = rosters.p_id
WHERE holder.visible = 0
AND preps.user = 1
AND assignments.user = 1
AND holder.t_id = 1
AND preps.period = 2
ORDER BY score DESC
) x
GROUP BY stuno
ORDER BY last
You can see that line I commented out is one of my feeble attempts to get it to display a "0" if NULL, but it's not working. I get a complete list, but if the score isn't found for a student, that student isn't showing up in my list. Anyone have a solution/idea for me to try?
Am I overusing JOINs and making my life harder than it needs to be? I'm mostly self-taught, so I know I have some holes in the fundamentals. It hasn't stopped me from creating some crazy cool projects though...but every now and then I'm sure I'm causing myself some unnecessary grief.
///////////////////////////////////////////////
Here is what I have done with the answer below, so that it grabs info from my tables:
SELECT au.stu_id,
COALESCE(t.id, 0) as id,
COALESCE(t.score , 0) as score
FROM rosters au
LEFT JOIN (
SELECT *
FROM (
SELECT a.*,
#rownum := if(#prev_value = student_id,
#rownum + 1,
1) rn,
#prev_value := student_id as prev
FROM holder a,
(SELECT #rownum := 0, #prev_value := '') r
ORDER BY student_id, score DESC
) T
WHERE T.rn = 1) T
ON au.stu_id = T.student_id
So, this is working great, except it doesn't show students who don't have scores for a given test. If their score isn't found in the "holder" table, I'd like it to show up as a "0".
/////////////////
Wait a minute! I may have mispoke...I think it is working correctly. I'll need to tweak a few things and get back to you. By the way, thanks SO much for taking the time to help me!
Your first aproach only work because a bad design on MySQL.
The right aproach should be
SQL Fiddle Demo
SELECT a.id, a.authorId, a.answer, a.votes
FROM ( SELECT authorId,
MAX(votes) as votes
FROM answers
GROUP BY authorId ) AS h
JOIN answers a
ON a.authorId = h.authorId
AND a.votes = h.votes;
OUTPUT
| id | authorId | answer | votes |
|----|----------|--------|-------|
| 2 | a | x2 | 21 |
| 4 | b | x1 | 23 | ==>
| 5 | b | x2 | 23 | ==> duplicates max value are possible
But this have issue if several answer has same score. You need to include some logic to decide which one to show.
Also you can use variable to get the highest score.
SELECT *
FROM (
SELECT a.*,
#rownum := if(#prev_value = authorId,
#rownum + 1,
1) rn,
#prev_value := authorId as prev
FROM answers a,
(SELECT #rownum := 0, #prev_value := '') r
ORDER BY authorId, votes DESC
) T
WHERE T.rn = 1;
OUTPUT
| id | authorId | answer | votes | rn | prev |
|----|----------|--------|-------|----|------|
| 4 | b | x1 | 23 | 1 | b | => only one is show but would
| 2 | a | x2 | 21 | 1 | a | be random unless you specify some rule.
Now for your question you also need to use LEFT JOIN instead of JOIN to get the students without scores.
Something like this
SELECT au.authorId,
COALESCE(t.id, 0) as id,
COALESCE(t.answer , 0) as answer ,
COALESCE(t.votes , 0) as votes
FROM authors au
LEFT JOIN (
SELECT *
FROM (
SELECT a.*,
#rownum := if(#prev_value = authorId,
#rownum + 1,
1) rn,
#prev_value := authorId as prev
FROM answers a,
(SELECT #rownum := 0, #prev_value := '') r
ORDER BY authorId, votes DESC
) T
WHERE T.rn = 1) T
ON au.authorId = T.authorId
OUTPUT
| authorId | id | answer | votes |
|----------|----|--------|-------|
| a | 2 | x2 | 21 |
| b | 4 | x1 | 23 |
| c | 0 | 0 | 0 |