How can I select from in other select in MYSQL query?
Something like this
SET #row_number = 0;
SELECT a.num FROM
(SELECT
(#row_number:=#row_number + 1) AS num, id
FROM
main) as a where a.id=6
I want to know the number of records where id=6 if it's the first row, second row or third one
If your query has the filter where a.id = 6, then the row with id = 6 will always be the first row of the result set.
I am interpreting your question to mean: "if I sorted by id ascending, what row number is the row with id = 6 going to be on". If so, you can use a simple aggregation:
SELECT COUNT(*)
FROM main m
WHERE m.id <= 6;
Your query seems inspired by enumerating all the rows. You could do this version as well:
select m.*
from (select m.*, (#rn := #rn + 1) as rn
from main m cross join
(select #rn := 0) params
order by id
) m
where id = 6;
The first version should be more efficient, particularly with an index on id.
Related
There is a query in MySQL 5.7:
SELECT * FROM
(
SELECT (#rowNum:=#rowNum+1) AS rowNo,t.* FROM table_target t,(SELECT (#rowNum :=0)) AS b
WHERE p_d = '2020-11-08'
ORDER BY kills DESC
) t
WHERE t.uid= '8888'
Running this query, there is no exception but column B disappears and if using select b from in the outter query, it returns unknown column exception.
I have 2 questions:
Why the (SELECT (#rowNum :=0)) does not appear?
Is the (#rowNum:=#rowNum+1) equivelent to row_number() over () in Oracle? If so, how to understand it...
Thanks for your help in advance.
In addition, I just found if I put the (SELECT (#rowNum :=0) ) in the left:
...
SELECT (SELECT (#rowNum :=0) ) AS b, (#rowNum:=#rowNum+1) AS rowNo , t.* FROM table_target t
...
Then the row number column does not increase any more, why could this happen?
You have asked 3 questions here:
Question 1: Why the (SELECT (#rowNum :=0)) does not appear?
Answer: You have used (SELECT (#rowNum :=0)) as B as a table joining it but not calling it in column list after select. That's why it is not showing it in output. You have called it as (#rowNum:=#rowNum+1) which is showing the value after increment means starting from 1.
Question 2: Is the (#rowNum:=#rowNum+1) equivalent to row_number() over () in Oracle? If so, how to understand it
Answer: Yes, it is equivalent. MySql 8.0 and above also support this (known as window function). It works as:
At the time of initialization of the query (SELECT (#rowNum :=0)) variable #rowNum will be initialized with value 0.
When we are calling (#rowNum:=#rowNum+1) in select then it will add 1 in #rowNum and assign it to itself for every row returned by select query.
This is how it will print the row number.
Question 3: if I put the (SELECT (#rowNum :=0) ) in the left:
Answer: If you put the (SELECT (#rowNum :=0) ) as field list after select then it will initialize the value of #rownum to 0 in every row returned by select. This is why you will not get incremented value.
The column "disappears" because the value is NULL. MySQL does not guarantee the order of evaluation of expressions in the SELECT, the initialization might not work.
Second, you code does not do what you intend, even if that worked, because variables may not respect the ORDER BY. I think you intend:
select k.*
from (select (#rownum := #rownumn + 1) as rownum, k.*
from (select k.*
from kills k
where k.p_d = '2020-11-08'
order by kills desc
) k cross join
(select #rownum := 0) params
) k
where t.uid = '8888';
There are probably better ways to do what you want. But your question is specifically about variables and MySQL 5.7.
I am trying to fetch first 50% of records from a MySQL Table User. I know we can use limit or top for finding them but the total number of records are not fixed so hard coding the actual number in the limit or top doesn't gives me first 50% of records. How can I achieve this?
If you are running MySQL 8.0, you can use window functions for this: ntile() does exactly what you ask for. Assuming that your ordering column is id:
select *
from (select t.*, ntile(2) over(order by id) nt from mytable) t
where nt = 1
In earlier versions, one option is a user variable and a join with an aggregate query:
select *
from (
select t.*, #rn := #rn + 1 rn
fom (select * from mytable order by id) t
cross join (select #rn := 0) x
cross join (select count(*) cnt from mytable) c
) t
where rn <= cnt / 2
Mysql directly not supports this. You can try with two queries or use subqueries
Something like this.
find the count of total records/2
that value has to be applied in the limit clause.
SET #count = (SELECT COUNT(*)/2 FROM table);
SET #sql = CONCAT('SELECT * FROM table LIMIT ', #count);
SELECT * FROM table name LIMIT (select COUNT(*)/2 from table name);
Table A shows results that I have by running the following SQL in MySQL.
SELECT * FROM table
WHERE MATCH (title) AGAINST ('marka tv')
Table A
Table B shows results that I want to get. As you can see the groups are in round-robin order.
Table B
If I understand the question, you want to sort the output so the groups are in a round-robin fashion rather than ordered. You can do this by enumerating the values within each group and then using that information for sorting:
SELECT t.*
FROM (SELECT t.*,
(#rn := if(#g = groups, #rn + 1,
if(#g := groups, 1, 1)
)
) as rn
FROM table t CROSS JOIN
(SELECT #rn := 0, #g := '') params
WHERE MATCH (title) AGAINST ('marka tv')
ORDER BY groups
) t
ORDER BY rn, groups;
Consider a subquery in a derived table to calculate a group number to be sorted at final table:
SELECT f.*
FROM
(SELECT t1.* ,
(SELECT count(*)
FROM table t2
WHERE (t2.title <= t1.title)
AND (t1.groups = t2.groups)) AS groupNo
FROM table t1
WHERE MATCH (t1.title) AGAINST ('marka tv')
) AS f
ORDER BY groupNo, groups
I tried to excute an inner select query as followed:
SET #rank = 0;
SELECT #rank := #rank + 1 AS id,
(SELECT a.id, COUNT(*) FROM table_A a GROUP BY a.id) AS count
FROM table_e e, table_p p
WHERE id= '100';
but I got the error as belowed
“Operand should contain 1 column(s)”
I tested the sub-query on its own and it worked fine. What is wrong with it?
EDIT
In fact my count sub query looks like this:
(SELECT COUNT(*) FROM table_e e, table_p p a GROUP BY p.id) AS count
I´m trying to count how many items in table_e exist in table_p.
Problem is following part of your query, which selects two columns for one alias:
(SELECT a.id, COUNT(*) FROM table_A a GROUP BY a.id) AS count
Bases to the name of the alias, selecting a.id is probably not needed.
Say I have a table like ID, NAME, SCORE. Now normally, to get the rankings of the teams, I'd select all and order by. Sometimes though, I don't want to know all the rankings, just the ranking of one team. If I added a column RANK, is there any way for MySQL to automatically fill in those values for me based off of SCORE? (I believe MS Excel has this capability)
and if so, how does it handle ties?
thanks
You can calculate the rankings when you make your query:
SELECT * FROM (
SELECT teams.*, #rownum := #rownum + 1 AS rank
FROM teams, (SELECT #rownum := 0) T1
ORDER BY score DESC) T2
WHERE id = 1
It works by initializing a variable called rownum to 0 and then iterating over the rows in order of decreasing score. For each team the rownum is increased and the team is assigned a rank based on the current value of rownum. The outer select applies a where clause so that only one row is returned.
Here is an improved version that assigns the same rank to teams that have tied scores:
SELECT id, name, teams.score, rank FROM (
SELECT score, #rownum := #rownum + 1 AS rank
FROM (SELECT DISTINCT(score) FROM teams) T1, (SELECT #rownum := 0) T2
ORDER BY score DESC) T3
JOIN teams ON T3.score = teams.score
If this isn't fast enough for you, then use a trigger instead.
looks like I'm looking for a MySQL trigger
Get All Teams:
SELECT
s1.name,
s1.score,
COUNT(s2.name) + 1 AS rank
FROM scores s1
LEFT OUTER JOIN scores AS s2 ON (s1.score < s2.score)
GROUP BY s1.name
ORDER BY COUNT(s2.name)
Get One Team ('The Canucks'):
SELECT
s1.name,
s1.score,
COUNT(s2.name) + 1 AS rank
FROM scores s1
LEFT OUTER JOIN scores AS s2 ON (s1.score < s2.score)
GROUP BY s1.name
HAVING s1.name = 'The Canucks'
ORDER BY COUNT(s2.name)
The method shown in the above examples get the ranking dynamically (not filling a regular or temp table).
Note: both of these assume a given team only exists once in scores table for the rank value to be correct.