如何使用相同的子查詢的SELECT和WHERE性能目的條款

Question

，是它在SELECT列表可能重新WHERE子句中的子查詢，像這樣：

SELECT p.id, nItens FROM purchase AS p 
WHERE ((SELECT COUNT(*) AS n FROM itens WHERE purchase_id=p.id) AS nItens) > 1 

避免：

SELECT p.id, (SELECT COUNT(*) AS n FROM itens WHERE purchase_id=p.id) AS nItens 
FROM purchase AS p 
WHERE (SELECT COUNT(*) AS n FROM itens WHERE purchase_id=p.id) > 1 

Answer 1

在MySQL中，你可以使用列別名having子句中，而不是where條款。如果你有一個沒有group by的having子句 - 在MySQL中 - 那麼它的行爲就像是where。所以，你可以這樣做：

SELECT p.id, (SELECT COUNT(*) AS n FROM itens WHERE purchase_id=p.id) AS nItens 
FROM purchase AS p 
HAVING nItens > 1; 

但是，您的查詢可能是更好的寫法如下：

select p.id, count(*) as nItens 
from purchase p join 
    itens i 
    on p.id = i.purchase_id 
group by p.id 
having count(*) > 1; 

注意，如果沒有條件，你需要爲同一邏輯left outer join讓那些沒有購買匹配iten。因爲你的條件至少需要兩次這樣的匹配，所以你不需要左連接。

Answer 2

SELECT p.id, (SELECT COUNT(*) AS n FROM itens WHERE purchase_id=p.id) AS nItens 
FROM purchase AS p 
HAVING nItens > 1 

Answer 3

SELECT p.id 
    , COUNT(n.purchase_id) ttl 
    FROM purchase p 
    JOIN itens n 
    ON n.purchase_id = p.id 
GROUP 
    BY p.id 
HAVING ttl > 1