MySQL 创建纵向直方图

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

生成一个从底部向上延伸的直方图。

例如，采用纵向直方图显示每个部门的职员数，一个星号"*"表示一个员工。

返回结果集应该如:
±-----±-----±-----+
| d10 | d20 | d30 |
±-----±-----±-----+
| * | * | * |
| * | * | * |
| * | * | * |
| NULL | * | * |
| NULL | * | * |
| NULL | NULL | * |
±-----±-----±-----+

二.解决方案

select  max(deptno_10) as d10,
        max(deptno_20) as d20,
        max(deptno_30) as d30
  from  (
select  case when e.deptno = 10 then '*' else null end deptno_10,
        case when e.deptno = 20 then '*' else null end deptno_20,
        case when e.deptno = 30 then '*' else null end deptno_30,
        ( select count(*) from emp d
           where e.deptno = d.deptno and e.empno < d.empno ) as rnk
  from  emp e
        ) x
 group by rnk
order by 1 desc, 2 desc, 3 desc;

测试记录:

mysql> select  max(deptno_10) as d10,
    ->         max(deptno_20) as d20,
    ->         max(deptno_30) as d30
    ->   from  (
    -> select  case when e.deptno = 10 then '*' else null end deptno_10,
    ->         case when e.deptno = 20 then '*' else null end deptno_20,
    ->         case when e.deptno = 30 then '*' else null end deptno_30,
    ->         ( select count(*) from emp d
    ->            where e.deptno = d.deptno and e.empno < d.empno ) as rnk
    ->   from  emp e
    ->         ) x
    ->  group by rnk
    -> order by 1 desc, 2 desc, 3 desc;
+------+------+------+
| d10  | d20  | d30  |
+------+------+------+
| *    | *    | *    |
| *    | *    | *    |
| *    | *    | *    |
| NULL | *    | *    |
| NULL | *    | *    |
| NULL | NULL | *    |
+------+------+------+
6 rows in set (0.00 sec)