sql case when的使用

目录

• 例子1
• 例子2
• 例子3

最近在sql使用中，发现 case when 的功能相当强大。

可以根据现有字段定义新的字段，可以对新字段进行排序等等。

下面简单举例说明。

用来测试的数据表内容如下：

mysql> select * from test_student;
+----+------+-----+--------+-------+
| id | name | age | sex    | score |
+----+------+-----+--------+-------+
| 19 | John |  18 | male   |    90 |
| 20 | Lily |  17 | female |    89 |
| 21 | Jim  |  17 | male   |    92 |
| 22 | Alex |  16 | male   |    70 |
| 23 | Bell |  19 | ''     |    68 |
+----+------+-----+--------+-------+
5 rows in set (0.00 sec)

例子1

按照性别查询，定义新字段性别：

mysql> select id, name, case sex when 'male' then '男' when 'female' then '女' else 'unknown' end as '性别' from test_student;
+----+------+---------+
| id | name | 性别    |
+----+------+---------+
| 19 | John | 男      |
| 20 | Lily | 女      |
| 21 | Jim  | 男      |
| 22 | Alex | 男      |
| 23 | Bell | unknown |
+----+------+---------+
5 rows in set (0.00 sec)

或者使用如下方式：

mysql> select id, name, case when sex = 'male' then '男' when sex = 'female' then '女' else 'unknown' end as '性别' from test_student;
+----+------+---------+
| id | name | 性别    |
+----+------+---------+
| 19 | John | 男      |
| 20 | Lily | 女      |
| 21 | Jim  | 男      |
| 22 | Alex | 男      |
| 23 | Bell | unknown |
+----+------+---------+
5 rows in set (0.00 sec)

例子2

下面的例子中，使用两个字段进行组合case when。

当sex为male，并且 score > 90，则加上标签smart boy。

当sex为female，并且 score > 90，则加上标签smart girl。

其他情况，则为unknown。

mysql> select id, name, age, sex, score, case when sex = 'male' and score > 90 then 'smart boy' when sex = 'female' and score > 90 then 'smart girl' else 'unknown' end as 'label' from test_student;
+----+------+-----+--------+-------+-----------+
| id | name | age | sex    | score | label     |
+----+------+-----+--------+-------+-----------+
| 19 | John |  18 | male   |    90 | unknown   |
| 20 | Lily |  17 | female |    89 | unknown   |
| 21 | Jim  |  17 | male   |    92 | smart boy |
| 22 | Alex |  16 | male   |    70 | unknown   |
| 23 | Bell |  19 | ''     |    68 | unknown   |
+----+------+-----+--------+-------+-----------+
5 rows in set (0.00 sec)

例子3

定义新字段，并按照新字段排序。

按照score区间进行划分:

• score > 90 等级1
• 80 < score <= 90 等级2
• 70 < score <= 80 等级3
• score <= 70 等级4

并按照等级进行排序。

mysql> select id,name,age, sex,score, case when score > 90 then 1 when score > 80 and score <= 90 then 2 when score > 70 and score <= 80 then 3 else 4 end as honor_level from test_student order by honor_level;
+----+------+-----+--------+-------+-------------+
| id | name | age | sex    | score | honor_level |
+----+------+-----+--------+-------+-------------+
| 21 | Jim  |  17 | male   |    92 |           1 |
| 19 | John |  18 | male   |    90 |           2 |
| 20 | Lily |  17 | female |    89 |           2 |
| 22 | Alex |  16 | male   |    70 |           4 |
| 23 | Bell |  19 | ''     |    68 |           4 |
+----+------+-----+--------+-------+-------------+
5 rows in set (0.00 sec)

再例如，

根据sex字段，定义新字段sex_int，并按照sex_int进行排序：

mysql> select *, case when sex = 'male' then 1 when sex = 'female' then 2 else 3 end as sex_int from test_student order by sex_int;
+----+------+-----+--------+-------+---------+
| id | name | age | sex    | score | sex_int |
+----+------+-----+--------+-------+---------+
| 19 | John |  18 | male   |    90 |       1 |
| 21 | Jim  |  17 | male   |    92 |       1 |
| 22 | Alex |  16 | male   |    70 |       1 |
| 20 | Lily |  17 | female |    89 |       2 |
| 23 | Bell |  19 | ''     |    68 |       3 |
+----+------+-----+--------+-------+---------+
5 rows in set (0.00 sec)