I have this column: name and price. I don't really know how or why in mysql database there are few line that are double record exactly from the previous line.
how to select all records but show only one of the records if the record is double with a line in front or behind it?
For example I have this records:
id
name
price
1
book
5
2
lamp
7
3
lamp
7
4
book
5
5
book
5
the result I want is:
id
name
price
1
book
5
2
lamp
7
4
book
5
If you want to exclude rows that match the previous name, there are several ways like the following.
Case 1:
If you use MySQL8, you can use the LAG function.
SELECT t1.id,t1.name,t1.price FROM (
SELECT t2.id,t2.name,t2.price,
LAG(t2.name) OVER(ORDER BY t2.id) prev
FROM mytable t2
) t1
WHERE t1.prev IS NULL OR t1.name<>t1.prev
ORDER BY 1
Case 2:
If the ids are continuous without any steps, you will get the expected result by comparing name and the previous id by JOIN.
SELECT t1.id,t1.name,t1.price FROM mytable t1
LEFT JOIN mytable t2
ON t1.name=t2.name AND
t1.id=t2.id-1
WHERE t1.id=1 OR t2.id IS NOT NULL
ORDER BY 1
Case 3:
If the ids are not continuous, there is a way to get the maximum id that does not exceed the other id.
SELECT t1.id,t1.name,t1.price FROM mytable t1
LEFT JOIN mytable t2
ON t1.name=t2.name AND
t1.id=(SELECT MAX(t3.id) FROM mytable t3 WHERE t3.id<t2.id)
WHERE t1.id=1 OR t2.id IS NOT NULL
ORDER BY 1
DB Fiddle
Select distinct is not an option here as id column is always unique. I guess this will work for you:
select min(id), name, price from table_name group by name, price
Related
(This question is specific to MySQL 5.6, which does not include CTEs)
Let's say I have a table like this (actual table is made from a subquery with several joins and is a fair bit more complex):
ID NAME POWER ACCESS_LEVEL DEPTH
1 Odin poetry 1 0
1 Isis song 2 1
2 Enki water 1 0
2 Zeus storms 2 2
2 Thor hammer 2 3
I want to first group them up by ID (it's actually a double grouping by PRINCIPAL_TYPE, PRINCIPAL_ID if that matters), then select one row from each group, preferring the row with the highest ACCESS_LEVEL, and among rows with the same access, choosing the one with the lowest depth. No two rows will have the same (ID, DEPTH) so we don't need to worry beyond that point.
For example with the above table, we select:
ID NAME POWER ACCESS_LEVEL DEPTH
1 Isis song 2 1
2 Zeus storms 2 2
The groups are (Odin, Isis) and (Enki, Thor, Zeus). In the first group, we prefer Odin over Isis because Odin has a higher ACCESS_LEVEL. In the second group, we take Zeus because Zeus and Thor have higher ACCESS_LEVELs than Enki, and between those two, Zeus has the lower depth.
Worst case Ontario, I can do it at the application level, but doing it at the database level allows for using LIMIT and SORT BY to do paging, instead of fetching the whole result set.
Here is one method that uses a correlated subquery:
select t.*
from t
where (access_level, depth) = (select access_level, depth
from t t2
where t2.id = t.id
order by access_level desc, depth asc
limit 1
);
Could be you need some grouped table based on subquery
select *
my_table m2
inner join (
select id, t.level, min(depth) depth
from my_table m
inner join (
select id, max(ACCESS_LEVEL) level
from my_table
group by id ) t on t.id= m.id and t.level = m.access_level
group by id level ) t2 on t2.id = m2.id
and t2.depth = m2.depth and t2.level = m2.access_level
This may work.
select *
from your_table t1
where (t1.access_level, t1.depth) = (
select t2.access_level, min(t2.depth)
from your_table t2
where (t2.access_level, t2.id) = (
select max(t3.access_level), t3.id
from your_table t3
where t3.id = t1.id
)
)
However, I feel that Gordon's solution will probably lead to a better query plan.
I am trying to display all records from table1 even if the catid not existing in table2 (all employee in table2 should have all catid from table1 with 0 days if not exising in table2) with the following sql query but getting an error
Error Code: 1054. Unknown column 'catid' in 'group statement'
select empid,days from table2 union select catid from
table1 group by empid, catid;
table1:
catid
1
2
3
table2:
empid catid days (computed column count(*))
1000 1 8
1000 3 10
expected result:
empid catid days
1000 1 8
1000 2 0 <---catid 2 and days 0 if catid not existing in table2 for empid 1000
1000 3 10
That is not the function of the union statement. Union statement does a set like capability which merging two sets. What you are looking for a is a join with the table 1 where you do a count and group by catid. Your data model to achieve this output itself is grievously wrong ;)
select employeeid, catid, sum(days) from table1, table2 group by employeeid, catid;
You just need a LEFT JOIN:
Select tab2.empid, tab2.catid, ifnull(tab2.days, 0)
from tab2
left join tab1 on tab2.catid = tab1.catid
Please note : While doing a UNION the number and type of the columns present in the first select should be the same as the next Selects.
So you need to first make the select columns in sync first.
can you check this and add empid similarly.
SELECT TABLE1.CATID, IFNULL(TABLE2.DAYS,0) FROM table1 LEFT OUTER JOIN
table2 ON table1.catid = table2.catid
Please use LEFT JOIN with IFNULL.
Select table2.empid, table1.catid, IFNULL(table2.days, 0) from table2
LEFT JOIN table1 ON table2.catid = table1.catid;
I'm trying to figure out best way to take required rows from database.
Database table:
id user cat time
1 5 1 123
2 5 1 150
3 5 2 160
4 5 3 100
I want to take DISTINCT cat ... WHERE user=5 with MAX time value. How should I do that in efficient way?
You will want to use an aggregate function with a GROUP BY:
select user, cat, max(time) as Time
from yourtable
group by user, cat
See SQL Fiddle with Demo
If you want to include the id column, then you can use a subquery:
select t1.id,
t1.user,
t1.cat,
t1.time
from yourtable t1
inner join
(
select max(time) Time, user, cat
from yourtable
group by user, cat
) t2
on t1.time = t2.time
and t1.user = t2.user
and t1.cat = t2.cat
See SQL Fiddle with Demo. I used a subquery to be sure that the id value that is returned with each max(time) row is the correct id.
I have a MySQL table like this
id Name count
1 ABC 1
2 CDF 3
3 FGH 4
using simply select query I get the values as
1 ABC 1
2 CDF 3
3 FGH 4
How I can get the result like this
1 ABC 1
2 CDF 3
3 FGH 4
4 NULL 0
You can see Last row. When Records are finished an extra row in this format
last_id+1, Null ,0 should be added. You can see above. Even I have no such row in my original table. There may be N rows not fixed 3,4
The answer is very simple
select (select max(id) from mytable)+1 as id, NULL as Name, 0 as count union all select id,Name,count from mytable;
This looks a little messy but it should work.
SELECT a.id, b.name, coalesce(b.`count`) as `count`
FROM
(
SELECT 1 as ID
UNION
SELECT 2 as ID
UNION
SELECT 3 as ID
UNION
SELECT 4 as ID
) a LEFT JOIN table1 b
ON a.id = b.id
WHERE a.ID IN (1,2,3,4)
UPDATE 1
You could simply generate a table that have 1 column preferably with name (ID) that has records maybe up 10,000 or more. Then you could simply join it with your table that has the original record. For Example, assuming that you have a table named DummyRecord with 1 column and has 10,000 rows on it
SELECT a.id, b.name, coalesce(b.`count`) as `count`
FROM DummyRecord a LEFT JOIN table1 b
ON a.id = b.id
WHERE a.ID >= 1 AND
a.ID <= 4
that's it. Or if you want to have from 10 to 100, then you could use this condition
...
WHERE a.ID >= 10 AND
a.ID <= 100
To clarify this is how one can append an extra row to the result set
select * from table union select 123 as id,'abc' as name
results
id | name
------------
*** | ***
*** | ***
123 | abc
Simply use mysql ROLLUP.
SELECT * FROM your_table
GROUP BY Name WITH ROLLUP;
select
x.id,
t.name,
ifnull(t.count, 0) as count
from
(SELECT 1 AS id
-- Part of the query below, you will need to generate dynamically,
-- just as you would otherwise need to generate 'in (1,2,3,4)'
UNION ALL SELECT 2
UNION ALL SELECT 3
UNION ALL SELECT 4
UNION ALL SELECT 5
) x
LEFT JOIN YourTable t
ON t.id = x.id
If the id does not exist in the table you're selecting from, you'll need to LEFT JOIN against a list of every id you want returned - this way, it will return the null values for ones that don't exist and the true values for those that do.
I would suggest creating a numbers table that is a single-columned table filled with numbers:
CREATE TABLE `numbers` (
id int(11) unsigned NOT NULL
);
And then inserting a large amount of numbers, starting at 1 and going up to what you think the highest id you'll ever see plus a thousand or so. Maybe go from 1 to 1000000 to be on the safe side. Regardless, you just need to make sure it's more-than-high enough to cover any possible id you'll run into.
After that, your query can look like:
SELECT n.id, a.*
FROM
`numbers` n
LEFT JOIN table t
ON t.id = n.id
WHERE n.id IN (1,2,3,4);
This solution will allow for a dynamically growing list of ids without the need for a sub-query with a list of unions; though, the other solutions provided will equally work for a small known list too (and could also be dynamically generated).
I have to count how many times a number from table2 occurs between the number in range table2.a and table2.b
i.e. we wanna know how many times we have this : a < start < b
I ran the following query :
UPDATE table2
SET occurrence =
(SELECT COUNT(*) FROM table1 WHERE start BETWEEN table2.a AND table2.b);
table2
ID a b occurrence
1 1 10
2 1 20
3 1 25
4 2 30
table1
ID start col1 col2 col3
1 1
2 7
3 10
4 21
5 25
6 27
7 30
table2 as
3 indexes on a, b and occurrence
1567 rows (so we will SELECT COUNT(*) over table2 1567 times..)
ID column as PK
table1 as
1 index on start
42,000,000 rows
Column start was "ordered by column start"
ID column as PK
==> it took 2.5hours to do 2/3 of it. I need to speed this up... any suggestions ? :)
You could try to add the id column to the index on table 1:
CREATE INDEX start_index ON table1 (start,id);
And rewrite the query to
UPDATE table2
SET occurrence =
(SELECT COUNT(id) FROM table1 WHERE start BETWEEN table2.a AND table2.b);
This is called "covering index": http://www.simple-talk.com/sql/learn-sql-server/using-covering-indexes-to-improve-query-performance/
-> The whole query on table 1 can be served through the data in the index -> no additional page lookup for the actual record.
Use a stored procedure. Keep the result from COUNT in a local variable, then use it to run the UPDATE query.
I will do this
// use one expensive join
create table tmp
select table2.id, count(*) as occurrence
from table1
inner join table1
on table1.start between table2.a and table2.b
group by table1.id;
update table2, tmp
set table2.occurrence=tmp.occurrence
where table2.id=tmp.id;
I think count(*) makes the database read the data rows when in your case it only needs to read the index. Try:
UPDATE table2
SET occurrence =
(SELECT COUNT(1) FROM table1 WHERE start BETWEEN table2.a AND table2.b);