如何在MySQL中为空行设置默认值？

要为空行设置默认值，请使用COALESCE（）的概念。让我们首先创建一个表 –

mysql> create table DemoTable1863
      (
      FirstName varchar(20)
      );
Query OK, 0 rows affected (0.00 sec)

使用插入命令在表中插入一些记录 –

mysql> insert into DemoTable1863 values('Chris');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1863 values(NULL);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1863 values('David');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1863 values(NULL);
Query OK, 1 row affected (0.00 sec)

使用select语句从表中显示所有记录 –

mysql> select * from DemoTable1863;

这将产生以下输出 –

+-----------+
| FirstName |
+-----------+
| Chris     |
| NULL      |
| David     |
| NULL      |
+-----------+
4 rows in set (0.00 sec)

以下是设置空行默认值的查询 –

mysql> select coalesce(FirstName,'UNKNOWN NAME') from DemoTable1863;

这将产生以下输出 –

+------------------------------------+
| coalesce(FirstName,'UNKNOWN NAME') |
+------------------------------------+
| Chris                              |
| UNKNOWN NAME                       |
| David                              |
| UNKNOWN NAME                       |
+------------------------------------+
4 rows in set (0.00 sec)

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