将子查询转换为加入查询

Question

这是工作的MySQL查询，是否可以将此转换为单个平面连接查询以提高性能？而不是使用下面的子查询。由于

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

我会从简化查询开始。由于where子句，您的left join都是内连接。此外，子查询中不需要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' 
        ); 

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

您还可以将where中的子查询移动到from。我认为这不会对性能产生太大影响。它不相关，所以它运行一次，并返回一个值。

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 

试试上面的代码。

希望这会帮助你。

Answer 3

我试过三种查询在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 
    ) 

而且每个查询的查询成本都是相同的33％。

猜测DataBase比我们聪明。