How can we apply UNIQUE constraints on fields of existing MySQL table?-Mysql Tutorial-php.cn

我们如何对现有 MySQL 表的字段应用 UNIQUE 约束？

We can apply UNIQUE constraints to columns of existing MySQL tables with the help of ALTER TABLE statement.

Syntax

ALTER TABLE table_name MODIFY colum_name datatype UNIQUE; OR ALTER TABLE table_name ADD UNIQUE (colum_name);

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Example

Suppose we have a table named "Test4" and we want to add a UNIQUE constraint to the "Name" column, then we can use ALTER The TABLE command is used to complete the following -

mysql> DESCRIBE test4; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | ID | int(11) | YES | UNI | NULL | | | Name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.04 sec) mysql> ALTER TABLE test4 MODIFY Name Varchar(20) UNIQUE; Query OK, 0 rows affected (0.22 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> DESCRIBE test4; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | ID | int(11) | YES | UNI | NULL | | | Name | varchar(20) | YES | UNI | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.04 sec)

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As can be seen from the above result set, MySQL has added a UNIQUE constraint to the field "Name". We can also add UNIQUE constraints using the following query -

mysql> Alter table test4 add UNIQUE(name); Query OK, 0 rows affected (0.16 sec) Records: 0 Duplicates: 0 Warnings: 0

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