MySQL ifnull函数判断字段值为null时使用默认值

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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)

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