如何修改现有列的数据类型？
要修改现有列的数据类型，可以使用MODIFY。让我们首先创建一个表-
mysql> create table DemoTable ( ClientId varchar(100), ClientName varchar(100), ClientAge int, ClientProjectDeadline timestamp, ClientCountryName varchar(100), isMarried boolean, ClientNumber bigint );
检查表的描述-
mysql> desc DemoTable;
这将产生以下输出-
+-----------------------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-----------------------+--------------+------+-----+---------+-------+ | ClientId | varchar(100) | YES | | NULL | | | ClientName | varchar(100) | YES | | NULL | | | ClientAge | int(11) | YES | | NULL | | | ClientProjectDeadline | timestamp | YES | | NULL | | | ClientCountryName | varchar(100) | YES | | NULL | | | isMarried | tinyint(1) | YES | | NULL | | | ClientNumber | bigint(20) | YES | | NULL | | +-----------------------+--------------+------+-----+---------+-------+ 7 rows in set (0.34 sec)
现在，将column(ClientNumber)数据类型bigint更改为varchar(20)-
mysql> alter table DemoTable modify ClientNumber varchar(20); 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 | +-----------------------+--------------+------+-----+---------+-------+ | ClientId | varchar(100) | YES | | NULL | | | ClientName | varchar(100) | YES | | NULL | | | ClientAge | int(11) | YES | | NULL | | | ClientProjectDeadline | timestamp | YES | | NULL | | | ClientCountryName | varchar(100) | YES | | NULL | | | isMarried | tinyint(1) | YES | | NULL | | | ClientNumber | varchar(20) | YES | | NULL | | +-----------------------+--------------+------+-----+---------+-------+ 7 rows in set (0.00 sec)
查看上面的示例输出，数据类型已从bigint更改为varchar(20)。
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