mysql ifnull函式的使用
阿新 • • 發佈：2019-02-19
在使用left join等聯表查詢時，常遇到某些欄位為null，一般都在後臺語言使用if a==null 判斷做處理，其實MySQL本身也有一個ifnull函式可以處理。
name表：
phone表：mysql> select * from name; +------+-----------+ | id | user | +------+-----------+ | 1 | xiaoqiang | | 2 | lisi | | 3 | xiaowu | | 4 | laoliu | +------+-----------+ 4 rows in set (0.00 sec)
mysql> select * from phone;
+------+-------+
| id | phone |
+------+-------+
| 2 | 110 |
| 3 | 119 |
+------+-------+
2 rows in set (0.00 sec)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=4兩行的phone值為null，我們可以使用ifnull函式判斷它為null時給一個預設值：
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個欄位值來填補
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)當b.phone的值為null時，使用name表中的id欄位來填補。但使用ifnull後列名看起來很不友好，可以用as還原
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)