别忘记having子句

做用：在 SQL 中增长 HAVING 子句缘由是，WHERE 关键字没法与统计函数一块儿使用。

以下查询功能可用having子句

select company, count(employee) cnt
from works t
where salary>5000
group by company
having cnt>1

与使用中间表SQL相比性能差异巨大：

select company
from (select company, count(employee) cnt from works where salary>5000 group by company) t
where cnt>1

使用MySQL Workbanch 的Explain结果：