将NOT NULL属性设置为MySQL中的现有列
要将NOTNULL属性设置为现有列，请使用ALTERTABLE命令。让我们首先创建一个表-
create table DemoTable1949 ( UserId int, UserName varchar(20) );
这是将NOTNULL属性设置为现有列的查询-
alter table DemoTable1949 modify UserName varchar(20) not null; Records: 0 Duplicates: 0 Warnings: 0
让我们检查表的描述-
desc DemoTable1949;
这将产生以下输出-
+----------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+-------+ | UserId | int(11) | YES | | NULL | | | UserName | varchar(20) | NO | | NULL | | +----------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
使用插入命令在表中插入一些记录-
insert into DemoTable1949 values(101,NULL); ERROR 1048 (23000): Column 'UserName' cannot be null insert into DemoTable1949 values(101,'Chris'); insert into DemoTable1949 values(102,'Bob');
使用select语句显示表中的所有记录-
select * from DemoTable1949;
这将产生以下输出-
+--------+----------+ | UserId | UserName | +--------+----------+ | 101 | Chris | | 102 | Bob | +--------+----------+ 2 rows in set (0.00 sec)