join查询优化

更新使用过滤条件中包括自身的表

此方法不能无法在mysql中使用

UPDATE user1
SET over='齐天大圣'
WHERE user1.user_name IN (
SELECT b.user_name
FROM user1 a INNER JOIN user2 b ON a.user_name = b.user_name)
```
使用join语句优化
UPDATE user1 a JOIN (
SELECT b.user_name FROM user1 a INNER JOIN user2 b ON
a.user_name = b.user_name
)b ON a.user_name = b.user_name
SET a.over = '齐天大圣'

# 使用left join优化子查询
一般语句
```
SELECT a.user_name,a.over,
(SELECT over FROM user2 b
WHERE a.user_name=b.user_name) AS over2
FROM user1 a;
```
lef join 优化语句
```
SELECT a.user_name,a.over,b.over AS over2
FROM user1 a
LEFT JOIN user2 b ON
a.user_name = b.user_name
```