Mysql sets a unique field

Mysql sets a field to be unique. 1. When creating a table, add the uniqueness constraint.
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; 2. ADD the unique constraint alter table 't_ user' ADD unique ('username') to the created TABLE; mysql primary key index and unique index 1. the primary key must be a unique index, and the unique index is not necessarily a primary key; 2. A table can have multiple unique indexes, but only one primary key. 3. the primary key column does not allow null values, while the unique index column does. Author k45hifz