MySQL ifnull函式判斷欄位值為null時使用預設值
阿新 • • 發佈：2019-01-29
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。
假設有兩張表
name表：
| 1 2 3 4 5 6 7 8 9 10 |
mysql> select
* from name;
+------+-----------+
| id
| user |
+------+-----------+
| 1 | xiaoqiang |
| 2 | lisi |
| 3 | xiaowu |
| 4 | laoliu |+------+-----------+
4 rows in
set (0.00 sec)
|
phone表：
| 1 2 3 4 5 6 7 8 |
mysql> select
* from phone;
+------+-------+
| id
| phone |
+------+-------+
| 2 | 110 |
| 3 | 119 |
+------+-------+
2 rows in
set (0.00 sec)
|
聯表查詢結果：
| 1 2 3 4 5 6 7 8 9 10 |
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：
| 1 2 3 4 5 6 7 8 9 10 |
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
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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
| 1 2 3 4 5 6 7 8 9 10 |
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 |
+------+-----------+-------+
|