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LEFT JOIN subquery
Hi,
The following is a simplified version of a complex query i wrote.
SELECT table2.x, table2.y
FROM table1
LEFT JOIN (SELECT table3.x, max(table3.y)
FROM table3
GROUP BY table3.x) AS table2
ON table1.x = table2.x
and it works... but it is slower since the subquery fetches whole table again and again.
Is there any way to make this faster by adding a condition inside subquery...
I want to do something like this.
SELECT table2.x, table2.y
FROM table1
LEFT JOIN (SELECT table3.x, max(table3.y)
FROM table3
WHERE table3.x = table1.x <<<<<<------------- does not work // Unknown table 'table1' in where clause
GROUP BY table3.x) AS table2
ON table1.x = table2.x
Any alternative query is welcome.
Thanks
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Answers
Yes use of table1 in subquery is suported for example in oracle but in mysql won't work
SELECT table2.x, table2.y
FROM table1
LEFT JOIN (SELECT table3.x, max(table3.y)
FROM table3
inner join table1 using (x)
GROUP BY table3.x) AS table2
ON table1.x = table2.x
wouldn't this speedup yur query if table 3 is really big and use the same where clouse in sub query and outside it?
Same result will produce:
SELECT table3.x, max(table3.y)
FROM table3
inner join table1 using (x)
GROUP BY table3.x
but meabye you were looking for something like this:
SELECT table2.x, table2.y
FROM table1
LEFT JOIN (SELECT table3.x, max(table3.y)
FROM table3
WHERE table3.x IN (SELECT table1.x FROM table1)
GROUP BY table3.x) AS table2
ON table1.x = table2.x
or to not call queries on table1 twice you could create temporary table with records from table1 you want to use
create temporary temp_table1
SELECT table1.x FROM table1
WHERE..........
then just:
SELECT table2.x, table2.y
FROM temp_table1
LEFT JOIN (SELECT table3.x, max(table3.y)
FROM table3
WHERE table3.x IN (SELECT temp_table1.x FROM temp_table1)
GROUP BY table3.x) AS table2
ON temp_table1.x = table2.x
Will this be quicker enough?
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by: aneeshattingalPosted on 2006-03-21 at 23:08:20ID: 16254823
I dont find any erorrs there, check whether 'Table1' already exists on your database