MySQL | Having子句的使用

错误示范
SELECT deptno FROM t_emp
WHERE AVG(sal) >=2000 -- 错在这里出现了聚合函数
GROUP BY deptno;

引入 HAVING 子句
SELECT deptno
FROM t_emp
GROUP BY deptno HAVING AVG(sal)>=2000;

SELECT deptno FROM t_emp
WHERE hiredate>="1982-01-01"
GROUP BY deptno HAVING COUNT(*) >= 2
ORDER BY deptno ASC;

# 查询每个部门中，1982年以后入职的员工超过2个人的部门编号
SELECT deptno
FROM t_emp
WHERE hiredate>="1982-01-01"
GROUP BY deptno HAVING COUNT(*)>=2;

# 查询每个部门中，1982年以后入职的员工超过2个人的部门编号
SELECT deptno
FROM t_emp
WHERE hiredate>="1982-01-01"
GROUP BY deptno HAVING COUNT(*)>=2 AND AVG(sal)>=2000;

错误示范
# 查询每个部门中，1982年以后入职的员工超过2个人的部门编号
SELECT deptno
FROM t_emp
WHERE hiredate>="1982-01-01"
GROUP BY deptno HAVING COUNT(*)>=2 AND sal>=AVG(sal);

SELECT deptno,COUNT(*) FROM t_emp
GROUP BY 1;

SELECT deptno,COUNT(*) FROM t_emp
GROUP BY 1 HAVING deptno IN(10,20);
-- 不建议这么写，浪费资源

SELECT deptno,COUNT(*) 
FROM t_emp
WHERE deptno IN(10,20)
GROUP BY 1;
-- 推荐这么写