sql 语句 的一些优化小总结

1.用exists 代替 in

原理：exists 是存在一个即返回一个 而in是做全盘扫描得出所有条件内的数据

(高效)
select *From member where age>18 and exists (select 'x' from Person where member.name =Person.name='cyao')
(低效)
select *From member where age>18 and name in (select name From Person where name='cyao')

2.用exists 代替DISITNCT

（高效）
select name ,age from mamber as d where exists (select 'x' from person as e where e.name = d.name)
(低效)
select Distinct name ,age from mamber as d ,person as e where e.name = d.name

3.尽量避免在select 后面 加*

4.尽量避免空值查询 所有的字段尽量有设置默认值。例如num列默认值为0；判断由where num is null  -> where num=0

5.尽量使用数字型字段，例如status=1 激活  status=2 代表删除

6.避免频繁创建和删除临时表，以减少系统表资源的消耗。