別忘記having子句

做用：在 SQL 中增長 HAVING 子句緣由是，WHERE 關鍵字沒法與統計函數一塊兒使用。

以下查詢功能可用having子句

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

與使用中間表SQL相比性能差異巨大：

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結果：