MySQL 如何将NOT NULL约束添加到已创建的列

使用ALTER TABLE实现。让我们首先创建一个表 –

mysql> create table DemoTable
  -> (
  -> StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
  -> StudentName varchar(100)
  -> );
Query OK, 0 rows affected (0.86 sec)

让我们检查表的描述 –

mysql> desc DemoTable;

这将产生以下输出 –

+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| StudentId   | int(11)      | NO   | PRI | NULL    | auto_increment |
| StudentName | varchar(100) | YES  |     | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)

以下是将NOT NULL约束添加到最初未设置为NOT NULL的其他列“StudentName”的查询 –

mysql> alter table DemoTable modify StudentName varchar(100) NOT NULL;
Query OK, 0 rows affected (1.57 sec)
Records: 0 Duplicates: 0 Warnings: 0

让我们再次检查表的描述 –

mysql> desc DemoTable;

这将产生以下输出 –

+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| StudentId   | int(11)      | NO   | PRI | NULL    | auto_increment |
| StudentName | varchar(100) | NO   |     | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

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