將子查詢轉換爲加入查詢

Question

這是工作的MySQL查詢，是否可以將此轉換爲單個平面連接查詢以提高性能？而不是使用下面的子查詢。由於

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

我會從簡化查詢開始。由於where子句，您的left join都是內連接。此外，子查詢中不需要select distinct。

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' 
        ); 

這表明使用索引：catAtt(catId, attId)，att(attId, attCatId)，ent(key, entId)和eav(entId, catId)。

您還可以將where中的子查詢移動到from。我認爲這不會對性能產生太大影響。它不相關，所以它運行一次，並返回一個值。

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 

試試上面的代碼。

希望這會幫助你。

Answer 3

我試過三種查詢在sql server上。

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 
    ) 

而且每個查詢的查詢成本都是相同的33％。

猜測DataBase比我們聰明。