加快MySQL的內部查詢

Question

我目前有：

SELECT ads, id as mid, (SELECT IFNULL(SUM(amount),0) FROM trans WHERE paidout=0 AND user_id=mid) as amount FROM accounts

我怎麼能執行內部和查詢，然後再通過它放在一起以同樣的方式結餘？

Answer 1

子查詢將針對返回的每一行執行。如果您將其轉換爲LEFT JOIN，它將被執行一次，並且應該快得多。

SELECT 
    ads, 
    id AS mid, 
    COALESCE(amount, 0) AS amount 
FROM 
    accounts 
    LEFT JOIN (
    /* Subquery returns SUM(amount) per user_id */ 
    /* LEFT JOIN in case there are no matching records */ 
    SELECT user_id, SUM(amount) AS amount 
    FROM trans 
    WHERE paidout = 0 
    GROUP BY user_id 
) amt ON accounts.id = amt.user_id 

的SUM()可以在你的情況下，外部查詢進行爲好，應用GROUP BY超過所有其他列。

SELECT 
    accounts.ads, 
    accounts.id AS mid 
    COALESCE(SUM(amount), 0) AS amount 
FROM 
    accounts 
    LEFT JOIN trans ON accounts.id = trans.user_id 
WHERE trans.paidout = 0 
GROUP BY 
    accounts.ads, 
    mid 

Answer 2

像這樣的東西應該工作：

SELECT ads, accounts.id as mid, ifnull(sum(amount),0) as amount 
FROM accounts 
LEFT JOIN trans ON paidout=0 AND user_id=accounts.id 
GROUP BY accounts.id 

Answer 3

這將只有兩個選擇VS（1+（以量行NUM））選擇，因爲你今天有它。

select ads, mid, IFNULL(SUM(amount),0) as total 
from (
    SELECT a.ads, a.id as mid, amount 
    from accounts a 
    join trans t on t.user_id = a.id and t.paidout = 0 
) 
group by ads, mid