MySQL 将结果集方向转为一列

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

把查询中返回的所有列转换为1列。
例如，返回deptno 10中所有员工的ename、job和sal，而且要把这3个值放到一列中。
为每个员工返回3行信息，而且在两个员工之间加一个空白行。

希望返回的结果集如下:
±----------+
| emps |
±----------+
| CLARK |
| MANAGER |
| 2450 |
| NULL |
| KING |
| PRESIDENT |
| 5000 |
| NULL |
| MILLER |
| CLERK |
| 1300 |
| NULL |
±----------+

二.解决方案

关键是用笛卡尔积为每个员工返回4行。这样就可以为每列产生一行，而且在两个员工之间加一个空白行。

select case rn
            when 1 then ename
            when 2 then job
            when 3 then cast(sal as char(4))
       end emps
  from (
 select  e.ename, e.job,e.sal,
         row_number() over w as 'rn'
   from  emp e,
         ( select * 
             from emp where job = 'CLERK') four_rows

  where e.deptno = 10
  window w as (partition by e.empno order by e.empno )
    ) x;

测试记录:

mysql> select case rn
    ->             when 1 then ename
    ->             when 2 then job
    ->             when 3 then cast(sal as char(4))
    ->        end emps
    ->   from (
    ->  select  e.ename, e.job,e.sal,
    ->          row_number() over w as 'rn'
    ->    from  emp e,
    ->          ( select *
    ->              from emp where job = 'CLERK') four_rows
    ->
    ->   where e.deptno = 10
    ->   window w as (partition by e.empno order by e.empno )
    ->     ) x;
+-----------+
| emps      |
+-----------+
| CLARK     |
| MANAGER   |
| 2450      |
| NULL      |
| KING      |
| PRESIDENT |
| 5000      |
| NULL      |
| MILLER    |
| CLERK     |
| 1300      |
| NULL      |
+-----------+
12 rows in set, 3 warnings (0.00 sec)