MySQL 对不同组/分区进行聚集

备注:测试数据库版本为MySQL 8.0

如需要scott用户下建表及录入数据语句，可参考:
scott建表及录入数据sql脚本

一.需求

同时按不同维进行聚集。

例如，要返回这样的结果集:列出每个员工的名字、他所在的部门、该部门的员工数(包括他自己)、与他有同样职位的员工数(也包括他自己)以及emp表中的员工总数。

起结果集应该如下所示:
±-------±-------±-----------±----------±--------±------+
| ename | deptno | deptno_cnt | job | job_cnt | total |
±-------±-------±-----------±----------±--------±------+
| SCOTT | 20 | 5 | ANALYST | 2 | 14 |
| FORD | 20 | 5 | ANALYST | 2 | 14 |
| MILLER | 10 | 3 | CLERK | 4 | 14 |
| SMITH | 20 | 5 | CLERK | 4 | 14 |
| ADAMS | 20 | 5 | CLERK | 4 | 14 |
| JAMES | 30 | 6 | CLERK | 4 | 14 |
| CLARK | 10 | 3 | MANAGER | 3 | 14 |
| JONES | 20 | 5 | MANAGER | 3 | 14 |
| BLAKE | 30 | 6 | MANAGER | 3 | 14 |
| KING | 10 | 3 | PRESIDENT | 1 | 14 |
| ALLEN | 30 | 6 | SALESMAN | 4 | 14 |
| WARD | 30 | 6 | SALESMAN | 4 | 14 |
| MARTIN | 30 | 6 | SALESMAN | 4 | 14 |
| TURNER | 30 | 6 | SALESMAN | 4 | 14 |
±-------±-------±-----------±----------±--------±------+

二.解决方案

窗口函数使这个问题相当容易解决。如果不能使用窗口函数，也可以使用标量子查询。

select  ename,
        deptno,
        count(*) over w1 as 'deptno_cnt',
        job,
        count(*) over w2 as 'job_cnt',
        count(*) over w3 as 'total'
  from  emp
window w1 as (partition by deptno),
        w2 as (partition by job),
        w3 as ()

测试记录:

mysql> select  ename,
    ->         deptno,
    ->         count(*) over w1 as 'deptno_cnt',
    ->         job,
    ->         count(*) over w2 as 'job_cnt',
    ->         count(*) over w3 as 'total'
    ->   from  emp
    -> window w1 as (partition by deptno),
    ->         w2 as (partition by job),
    ->         w3 as ();
+--------+--------+------------+-----------+---------+-------+
| ename  | deptno | deptno_cnt | job       | job_cnt | total |
+--------+--------+------------+-----------+---------+-------+
| SCOTT  |     20 |          5 | ANALYST   |       2 |    14 |
| FORD   |     20 |          5 | ANALYST   |       2 |    14 |
| MILLER |     10 |          3 | CLERK     |       4 |    14 |
| SMITH  |     20 |          5 | CLERK     |       4 |    14 |
| ADAMS  |     20 |          5 | CLERK     |       4 |    14 |
| JAMES  |     30 |          6 | CLERK     |       4 |    14 |
| CLARK  |     10 |          3 | MANAGER   |       3 |    14 |
| JONES  |     20 |          5 | MANAGER   |       3 |    14 |
| BLAKE  |     30 |          6 | MANAGER   |       3 |    14 |
| KING   |     10 |          3 | PRESIDENT |       1 |    14 |
| ALLEN  |     30 |          6 | SALESMAN  |       4 |    14 |
| WARD   |     30 |          6 | SALESMAN  |       4 |    14 |
| MARTIN |     30 |          6 | SALESMAN  |       4 |    14 |
| TURNER |     30 |          6 | SALESMAN  |       4 |    14 |
+--------+--------+------------+-----------+---------+-------+
14 rows in set (0.00 sec)