多个左连接的MySQL计数 - optomization

Question

我有一个查询通过使用LEFT JOIN和子查询从多个表中获取计数。这个想法是让activites成员曾参加过各种计数

的模式是这样的：

成员 PK member_id

table1的 PK tbl1_id FK member_id

表2 PK tbl2_id FK member_id

table3 PK tbl 3_id FK member_id

我的查询看起来是这样的：

SELECT t1.num1,t2.num2,t3.num3 
FROM member m 
LEFT JOIN 
(
    SELECT member_id,COUNT(*) as num1 
    FROM table1 
    GROUP BY member_id 
) t1 ON t1.member_id = m.member_id 
LEFT JOIN 
(
    SELECT member_id,COUNT(*) as num2 
    FROM table2 
    GROUP BY member_id 
) t2 ON t2.member_id = m.member_id 
LEFT JOIN 
(
    SELECT member_id,COUNT(*) as num3 
    FROM table3 
    GROUP BY member_id 
) t3 ON t3.member_id = m.member_id 
WHERE m.member_id = 27 

其中27是一个测试ID。实际的查询连接三个以上的表，并且在更改member_id的情况下多次运行查询。问题是这个查询运行速度很慢。我得到我需要的信息，但我想知道是否有人可以提出一种方法来优化这一点。任何建议非常感谢。非常感谢。

Answer 1

你应该重构你的查询。您可以通过重新排序查询收集数据的方式来做到这一点。怎么样？

• 应用WHERE子句第一
• 申请加入上次

这里是你的原始查询：

SELECT t1.num1,t2.num2,t3.num3 
FROM member m 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num1 
    FROM table1 
    GROUP BY member_id 
) t1 ON t1.member_id = m.member_id 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num2 
    FROM table2 
    GROUP BY member_id 
) t2 ON t2.member_id = m.member_id 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num3 
    FROM table3 
    GROUP BY member_id 
) t3 ON t3.member_id = m.member_id 
WHERE m.member_id = 27 

这里是你新的查询

SELECT 
    IFNULL(t1.num1,0) num1, 
    IFNULL(t1.num2,0) num2, 
    IFNULL(t1.num3,0) num3 
FROM 
(
    SELECT * FROM member m 
    WHERE member_id = 27 
) 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num1 
    FROM table1 
    WHERE member_id = 27 
    GROUP BY member_id 
) t1 ON t1.member_id = m.member_id 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num2 
    FROM table2 
    WHERE member_id = 27 
    GROUP BY member_id 
) t2 ON t2.member_id = m.member_id 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num3 
    FROM table3 
    WHERE member_id = 27 
    GROUP BY member_id 
) t3 ON t3.member_id = m.member_id 
; 

BTW我改为member m转换为SELECT * FROM member m WHERE member_id = 27以防您需要有关成员27的任何信息。我​​还为每个结果添加了IFNULL函数，以便在count为NULL的情况下生成0。

你需要绝对确保

• member_id是成员表
• member_id在表1，表2索引的主键，表3

试试看！ ！

Answer 2

不知道你的架构和你为指标做了什么，一个可能的方式，使这个速度是：

SELECT (select ifnull(count(*),0) from table1 where table1.member_id = m.id) as num1, 
(select ifnull(count(*),0) from table2 where table2.member_id = m.id) as num2, 
(select ifnull(count(*),0) from table3 where table3.member_id = m.id) as num3 
from member m 
WHERE m.member_id = 27 

现在，这是一个稍微高风险的建议，只是因为我什么都不知道关于您的数据库或其他什么运行，或瓶颈在哪里。

一般来说，最好是在您的查询中发布解释计划以获得更好的答案。

Answer 3

SELECT num1, num2, count(*) as num3 
FROM (
    SELECT member_id, num1, count(*) as num2 
    FROM (
    SELECT member_id, count(*) as num1 
    FROM member 
    LEFT JOIN table1 USING (member_id) 
    WHERE member_id = 27) as m1 
    LEFT JOIN table2 USING (member_id)) as m2 
LEFT JOIN table3 USING (member_id);