16.统计出当前各个title类型对应的员工当前薪水对应的平均工资。

统计出当前各个title类型对应的员工当前薪水对应的平均工资。结果给出title以及平均工资avg。

CREATE TABLE `salaries` (
`emp_no` int(11) NOT NULL,
`salary` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`from_date`));
CREATE TABLE IF NOT EXISTS "titles" (
`emp_no` int(11) NOT NULL,
`title` varchar(50) NOT NULL,
`from_date` date NOT NULL,
`to_date` date DEFAULT NULL);

解析：group by要放在 where 后面

select title, avg(salary) as avg
from salaries as sa, titles as ti
where sa.emp_no = ti.emp_no
and sa.to_date = '9999-01-01'
and ti.to_date = '9999-01-01'
group by ti.title