MySQL 如何设置查询返回空值时为0？

你可以使用IFNULL()函数完成此操作。首先，让我们创建一个表 –

mysql> create table DemoTable
  -> (
  -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
  -> Value int
  -> );
Query OK, 0 rows affected (0.71 sec)

使用INSERT命令向表中插入一些记录 –

mysql> insert into DemoTable(Value) values(100);
Query OK, 1 row affected (0.18 sec)

mysql> insert into DemoTable(Value) values(140);
Query OK, 1 row affected (0.12 sec)

mysql> insert into DemoTable(Value) values(200);
Query OK, 1 row affected (0.26 sec)

mysql> insert into DemoTable(Value) values(450);
Query OK, 1 row affected (0.13 sec)

mysql> insert into DemoTable(Value) values(null);
Query OK, 1 row affected (0.15 sec)

mysql> insert into DemoTable(Value) values(90);
Query OK, 1 row affected (0.11 sec)

mysql> insert into DemoTable(Value) values(null);
Query OK, 1 row affected (0.10 sec)

使用SELECT语句显示表中的所有记录 –

mysql> select *from DemoTable;

阅读更多：MySQL 教程

输出

+----+-------+
| Id | Value |
+----+-------+
| 1  | 100   |
| 2  | 140   |
| 3  | 200   |
| 4  | 450   |
| 5  | NULL  |
| 6  | 90    |
| 7  | NULL  |
+----+-------+
7 rows in set (0.00 sec)

下面是在MySQL中查询返回空值时设置值为0的查询语句。

mysql> select ifnull(Value,0) AS Value from DemoTable;

输出

+-------+
| Value |
+-------+
| 100   |
| 140   |
| 200   |
| 450   |
| 0     |
| 90    |
| 0     |
+-------+
7 rows in set (0.00 sec)