Convert subquery to join query

Question

this is the working MySQL query, is it possible to convert this into single flat join query to increase performance ? Rather than using sub-query below. Thanks

SELECT * from catAtt ca LEFT JOIN att a ON ca.attId = a.attId
WHERE ca.catId = 53 AND a.attCatId = 
(
SELECT DISTINCT eav.catId FROM ent LEFT JOIN eav ON ent.entId = eav.entId 
WHERE ent.key = 'somekey'
)

Answer 1

I would start by simplifying the query. Both your left join s are really inner joins because of the where clauses. Also, the select distinct should not be needed in the subquery.

select ca.*, att.*
from catAtt ca join
     att a 
     on ca.attId = a.attId 
where ca.catId = 53 and
      a.attCatId = (select eav.catId 
                    from ent join
                         eav 
                         on ent.entId = eav.entId 
                    where ent.key = 'somekey'
                   );

This suggests the use of indexes: catAtt(catId, attId) , att(attId, attCatId) , ent(key, entId) , and eav(entId, catId) .

You can also move the subquery in the where to the from . I don't think that will have much impact on performance. It is not correlated, so it is run once, and returns a single value.

Answer 2

SELECT * from catAtt ca 
LEFT JOIN att a ON ca.attId = a.attId
INNER JOIN 
(
  SELECT DISTINCT eav.catId FROM ent 
  LEFT JOIN eav ON ent.entId = eav.entId 
  WHERE ent.key = 'somekey'
)Z ON a.attCatId = Z.catId 
WHERE ca.catId = 53 

Try above code.

Hope this will help you.

Answer 3

I ve tried three type of queries on sql server.

SELECT * from catAtt ca JOIN att a ON ca.attId = a.attId
WHERE ca.catId = 53 AND a.attCatId = 
(
SELECT DISTINCT eav.catId FROM ent LEFT JOIN eav ON ent.entId = eav.entId 
WHERE ent.key = 'somekey'
)

SELECT * from catAtt ca 
JOIN att a ON ca.attId = a.attId
JOIN 
(
  SELECT DISTINCT eav.catId FROM ent 
  LEFT JOIN eav ON ent.entId = eav.entId 
  WHERE ent.key = 'somekey'
)Z ON a.attCatId = Z.catId 
WHERE ca.catId = 53 

SELECT * from catAtt ca JOIN att a ON ca.attId = a.attId
WHERE ca.catId = 53 AND 
EXISTS 
   (
    SELECT 1 FROM ent LEFT JOIN eav ON ent.entId = eav.entId 
    WHERE ent.key = 'somekey' AND a.attCatId = eav.catId
   )

And the query cost for every query is the same 33%.

Guess the DataBase is smarter than we are.