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案例：写一个查询，计算市场部与工程部员工最高薪资的差额，输出结果为薪资值差异（Mysql 为例）
示例一：
1SELECT
2 abs(
3 SUM( t.s1 ) - SUM( t.s2 )
4 ) AS 薪差
5FROM
6 (
7 SELECT
8 CASE
9 d.department
10 WHEN '市场部' THEN
11 MAX( e.salary ) ELSE 0
12 END AS s1,
13 CASE
14 d.department
15 WHEN '工程部' THEN
16 MAX( e.salary ) ELSE 0
17 END AS s2
18 FROM
19 db_employee e
20 INNER JOIN db_dept d ON e.department_id = d.id
21 GROUP BY
22 e.department_id
23 ) t
示例二:
1select abs(sum(m)) as 薪差
2from (
3select max(salary) m from db_employee where department_id=4
4union
5select -max(salary) m from db_employee where department_id=1) as t
PS:
写法多样，以上仅以Mysql为例简单实现。
