How to set a field unique in Mysql

This article is about how to set a field in Mysql is unique, the editor feels very practical, so share with you to learn, I hope you can learn something after reading this article, do not say much, follow the editor to have a look.

Mysql sets a field that is unique

1. Add uniqueness constraints when building tables

CREATE TABLE `t _ user` (

`Id` int (11) NOT NULL AUTO_INCREMENT

`username` varchar (18) NOT NULL unique

`password` varchar (18) NOT NULL

PRIMARY KEY (`Id`) www.2cto.com

) ENGINE=InnoDB AUTO_INCREMENT=1018 DEFAULT CHARSET=gbk

two。 Add uniqueness constraints to tables that have already been built

ALTER TABLE `t _ user` ADD unique (`username`)

Primary key index and unique index

1. The primary key must be the unique index, and the unique index is not necessarily the primary key.

two。 There can be multiple unique indexes in a table, but only one primary key

3. Null values are not allowed for primary key columns, while null values are allowed for unique index columns.

The above is the only way to set a certain field in Mysql, and the editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.