MySQL ifnull函数判断字段值为null时使用默认值

在使用left join等联表查询时，常遇到某些字段为null，一般都在后台语言使用if a==null 判断做处理，其实MySQL本身也有一个ifnull函数可以处理。

假设有两张表

name表：

mysql> select * from name;
+------+-----------+
| id   | user      |
+------+-----------+
|    1 | xiaoqiang |
|    2 | lisi      |
|    3 | xiaowu    |
|    4 | laoliu    |
+------+-----------+
4 rows in set (0.00 sec)

phone表：

mysql> select * from phone;
+------+-------+
| id   | phone |
+------+-------+
|    2 | 110   |
|    3 | 119   |
+------+-------+
2 rows in set (0.00 sec)

联表查询结果：

mysql> select a.*,b.phone from name a left join phone b on a.id=b.id;
+------+-----------+-------+
| id   | user      | phone |
+------+-----------+-------+
|    1 | xiaoqiang | NULL  |
|    2 | lisi      | 110   |
|    3 | xiaowu    | 119   |
|    4 | laoliu    | NULL  |
+------+-----------+-------+
4 rows in set (0.01 sec)

其中id=1、id=4两行的phone值为null，我们可以使用ifnull函数判断它为null时给一个默认值：

mysql> select a.*,ifnull(b.phone,0) from name a left join phone b on a.id=b.id;  
+------+-----------+-------------------+
| id   | user      | ifnull(b.phone,0) |
+------+-----------+-------------------+
|    1 | xiaoqiang | 0                 |
|    2 | lisi      | 110               |
|    3 | xiaowu    | 119               |
|    4 | laoliu    | 0                 |
+------+-----------+-------------------+
4 rows in set (0.00 sec)

甚至也可以指定某一个字段值来填补

mysql> select a.*,ifnull(b.phone,a.id) from name a left join phone b on a.id=b.id;      
+------+-----------+----------------------+
| id   | user      | ifnull(b.phone,a.id) |
+------+-----------+----------------------+
|    1 | xiaoqiang | 1                    |
|    2 | lisi      | 110                  |
|    3 | xiaowu    | 119                  |
|    4 | laoliu    | 4                    |
+------+-----------+----------------------+
4 rows in set (0.00 sec)

当b.phone的值为null时，使用name表中的id字段来填补。但使用ifnull后列名看起来很不友好，可以用as还原

mysql> select a.*,ifnull(b.phone,a.id) as phone from name a left join phone b on a.id=b.id;  
+------+-----------+-------+
| id   | user      | phone |
+------+-----------+-------+
|    1 | xiaoqiang | 1     |
|    2 | lisi      | 110   |
|    3 | xiaowu    | 119   |
|    4 | laoliu    | 4     |
+------+-----------+-------+
4 rows in set (0.00 sec)