MySQL 使用CASE表达式给行做标记

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:
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一.需求

把一列中的值(如EMP表中的JOB列)映射成一列“布尔”标记。

例如，希望如下返回:
±-------±---------±---------±-------±-----------±--------+
| ename | is_clerk | is_sales | is_mgr | is_analyst | is_prez |
±-------±---------±---------±-------±-----------±--------+
| KING | 0 | 0 | 0 | 0 | 1 |
| SCOTT | 0 | 0 | 0 | 1 | 0 |
| FORD | 0 | 0 | 0 | 1 | 0 |
| JONES | 0 | 0 | 1 | 0 | 0 |
| BLAKE | 0 | 0 | 1 | 0 | 0 |
| CLARK | 0 | 0 | 1 | 0 | 0 |
| ALLEN | 0 | 1 | 0 | 0 | 0 |
| WARD | 0 | 1 | 0 | 0 | 0 |
| MARTIN | 0 | 1 | 0 | 0 | 0 |
| TURNER | 0 | 1 | 0 | 0 | 0 |
| SMITH | 1 | 0 | 0 | 0 | 0 |
| ADAMS | 1 | 0 | 0 | 0 | 0 |
| JAMES | 1 | 0 | 0 | 0 | 0 |
| MILLER | 1 | 0 | 0 | 0 | 0 |
±-------±---------±---------±-------±-----------±--------+

这样的结果集可用于调试，它能够提供一个不同于其他典型结果集的数据视图。

二.解决方案

对每个雇员的JOB使用CASE表达式,并返回1或0表示他的JOB。
需要为每个可能的职位写一个CASE表达式，并创建一列:

select  ename,
        case when job = 'CLERK'
             then 1 else 0
        end as is_clerk,
        case when job = 'SALESMAN'
             then 1 else 0
        end as is_sales,
        case when job = 'MANAGER'
             then 1 else 0
        end as is_mgr,
        case when job = 'ANALYST'
             then 1 else 0
        end as is_analyst,
        case when job = 'PRESIDENT'
             then 1 else 0
        end as is_prez
  from  emp
 order  by 2,3,4,5,6;

测试记录:

mysql> select  ename,
    ->         case when job = 'CLERK'
    ->              then 1 else 0
    ->         end as is_clerk,
    ->         case when job = 'SALESMAN'
    ->              then 1 else 0
    ->         end as is_sales,
    ->         case when job = 'MANAGER'
    ->              then 1 else 0
    ->         end as is_mgr,
    ->         case when job = 'ANALYST'
    ->              then 1 else 0
    ->         end as is_analyst,
    ->         case when job = 'PRESIDENT'
    ->              then 1 else 0
    ->         end as is_prez
    ->   from  emp
    ->  order  by 2,3,4,5,6
    -> ;
+--------+----------+----------+--------+------------+---------+
| ename  | is_clerk | is_sales | is_mgr | is_analyst | is_prez |
+--------+----------+----------+--------+------------+---------+
| KING   |        0 |        0 |      0 |          0 |       1 |
| SCOTT  |        0 |        0 |      0 |          1 |       0 |
| FORD   |        0 |        0 |      0 |          1 |       0 |
| JONES  |        0 |        0 |      1 |          0 |       0 |
| BLAKE  |        0 |        0 |      1 |          0 |       0 |
| CLARK  |        0 |        0 |      1 |          0 |       0 |
| ALLEN  |        0 |        1 |      0 |          0 |       0 |
| WARD   |        0 |        1 |      0 |          0 |       0 |
| MARTIN |        0 |        1 |      0 |          0 |       0 |
| TURNER |        0 |        1 |      0 |          0 |       0 |
| SMITH  |        1 |        0 |      0 |          0 |       0 |
| ADAMS  |        1 |        0 |      0 |          0 |       0 |
| JAMES  |        1 |        0 |      0 |          0 |       0 |
| MILLER |        1 |        0 |      0 |          0 |       0 |
+--------+----------+----------+--------+------------+---------+
14 rows in set (0.00 sec)