Hi, > I saw an example of creating tables (see below). I > wonder what the primary key (user_name, role_name) in > the table user_roles means? Does it mean that both > user_name and role_name are the primary key of the > user_roles table? How does a table have two primary > keys? > > create table users ( > user_name varchar(15) not null primary key, > user_pass varchar(15) not null > ); > > create table user_roles ( > user_name varchar(15) not null, > role_name varchar(15) not null, > primary key (user_name, role_name) > );
A table cannot have two primary keys, only zero or one. > primary key (user_name, role_name) This primary key is a "compound" primary key - a constraint for multiple columns. This means that every combination of values needs to be unique. These are valid (user, role): martijn, admin caroline, admin caroline, poweruser As you can see, you, as a user, can have multiple roles. However, you cannot enter such a row twice: (invalid): caroline, poweruser caroline, poweruser With regards, Martijn Tonies Database Workbench - developer tool for InterBase, Firebird, MySQL & MS SQL Server. Upscene Productions http://www.upscene.com -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
