MySQL内置函数-单行函数（流程控制函数）

if函数

mysql> select if(1>2,'yes','no');
+--------------------+
| if(1>2,'yes','no') |
+--------------------+
| no                 |
+--------------------+
1 row in set (0.00 sec)

mysql> select user,if(user='root','管理员','普通用户') from mysql.user;
+------------------+--------------------------------------------+
| user             | if(user='root','管理员','普通用户')        |
+------------------+--------------------------------------------+
| test             | 普通用户                                   |
| test1            | 普通用户                                   |
| root             | 管理员                                     |
| mysql.infoschema | 普通用户                                   |
| mysql.session    | 普通用户                                   |
| mysql.sys        | 普通用户                                   |
| root             | 管理员                                     |
| test             | 普通用户                                   |
+------------------+--------------------------------------------+
8 rows in set (0.01 sec)

case函数

等值判断

mysql> select * from t2;
+------+------+
| id   | comm |
+------+------+
|    1 | NULL |
|    2 |   15 |
|    3 |   18 |
|    4 |   25 |
|    5 | NULL |
+------+------+
5 rows in set (0.00 sec)

mysql> select case comm when 15 then 'low' when 18 then 'middle' else 'high' end from t2 where comm is not null;
+--------------------------------------------------------------------+
| case comm when 15 then 'low' when 18 then 'middle' else 'high' end |
+--------------------------------------------------------------------+
| low                                                                |
| middle                                                             |
| high                                                               |
+--------------------------------------------------------------------+
3 rows in set (0.00 sec)

表达式

mysql> select * from t2;
+------+------+
| id   | comm |
+------+------+
|    1 | NULL |
|    2 |   15 |
|    3 |   18 |
|    4 |   25 |
|    5 | NULL |
+------+------+
5 rows in set (0.00 sec)

## 控制不会参与比较，需要使用coalesce函数或ifnull函数进行转换才可以
mysql> select id,case when comm <10 then "low" when 10<comm<20 then "middle" else  'high' end from t2;
+------+------------------------------------------------------------------------------+
| id   | case when comm <10 then "low" when 10<comm<20 then "middle" else  'high' end |
+------+------------------------------------------------------------------------------+
|    1 | high                                                                         |
|    2 | middle                                                                       |
|    3 | middle                                                                       |
|    4 | middle                                                                       |
|    5 | high                                                                         |
+------+------------------------------------------------------------------------------+
5 rows in set (0.00 sec)

mysql> update t2 set comm =5 where id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> update t2 set comm =3 where id=5;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select id,case when comm <10 then "low" when 10<comm<20 then "middle" else  'high' end from t2;
+------+------------------------------------------------------------------------------+
| id   | case when comm <10 then "low" when 10<comm<20 then "middle" else  'high' end |
+------+------------------------------------------------------------------------------+
|    1 | low                                                                          |
|    2 | middle                                                                       |
|    3 | middle                                                                       |
|    4 | middle                                                                       |
|    5 | low                                                                          |
+------+------------------------------------------------------------------------------+
5 rows in set (0.00 sec)

ifnull函数 判断值是否为0

mysql> select ifnull(comm,0) from t2;
+----------------+
| ifnull(comm,0) |
+----------------+
|              0 |
|             15 |
|             18 |
|             25 |
|              0 |
+----------------+
5 rows in set (0.01 sec)