Oracle

Question

Why/How HAVING clause 可以在 GROUP BY

之前

select count(1) from tableA
having id >1
group by id
order by count(1)

我知道 HAVING 子句可以没有 GROUP BY 子句，

但是在定义 GROUP BY 时，为什么 HAVING 没有像 ORDER BY 子句那样被强制如此？

Derby processes a SelectExpression in the following order:

FROM clause

WHERE clause

GROUP BY (or implicit GROUP BY)

HAVING clause

ORDER BY clause

在所有文档中它出现在 GROUP BY:

之后

GROUP BY WORKDEPT 
HAVING MAX(SALARY) < (SELECT AVG(SALARY)
                         FROM EMPLOYEE
                         WHERE NOT WORKDEPT = EMP_COR.WORKDEPT)

编辑 Oreilly文章指出顺序不重要（但不说明原因）

The order of the GROUP BY clause and the HAVING clause in a SELECT statement is not important. We can specify the GROUP BY clause before the HAVING clause, or vice versa

我怀疑是因为 HAVING 可以没有 GROUP BY 子句然后命令不强制执行。

Answer 1

因为 having 子句作用于结果（过滤 select 的结果）

查询 select 可以仅基于 select 或者如果是聚合查询
结果在 GROUP BY 之后产生这样，从语义上讲，HAVING 被放置在查询 CLAUSE 的末尾 .. 就在表示子句 (ORDER BY)

之前

Oracle - 为什么 HAVING 子句可以在 GROUP BY 之前

Oracle - Why HAVING clause can be before GROUP BY

group-by

having