Name: Cngothic 
31st
07.08
sql语句:having子句与where子句用法与区别
having子句也是设定条件的语句与where有相似之处但也有区别。having子句在查询过程中慢于聚合语句(sum,min,max,avg,count).而where子句在查询过程中则快于聚合语句(sum,min,max,avg,count)。
简单说来：
where子句：
- select sum(num) as rmb from order where id>10
- //只有先查询出id大于10的记录才能进行聚合语句
having子句:
- select reportsto as manager, count(*) as reports from employees
- group by reportsto having count(*) > 4