To make a date column null, use ALTER TABLE and MODIFY and set the date to NULL. Following is the syntax −
alter table yourTableName modify column yourColumnName date NULL;
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首先让我们创建一个表格。在这里，我们将列设置为NOT NULL −
mysql> create table DemoTable ( ShippingDate date NOT NULL ); Query OK, 0 rows affected (0.78 sec)
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Now, insert NULL value in the above table. An error would generate since we have set the column to be NOT NULL −
mysql> insert into DemoTable values(null); ERROR 1048 (23000) − Column 'ShippingDate' cannot be null
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Now, let us alter the table and allow NULL in the above table −
mysql> alter table DemoTable modify column ShippingDate date NULL; Query OK, 0 rows affected (1.81 sec) Records : 0 Duplicates : 0 Warnings : 0
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现在，再次尝试使用插入命令将NULL插入到上述表中。由于我们已经修改了表以接受NULL，所以不会生成错误−
mysql> insert into DemoTable values(null); Query OK, 1 row affected (1.21 sec
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Display all records from the table using select statement −
mysql> select *from DemoTable;
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这将产生以下输出 −
+--------------+ | ShippingDate | +--------------+ | NULL | +--------------+ 1 row in set (0.00 sec)
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