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Overview
constraint uniquely identifies each record in a database table.
constraint ensures unique values, and cannot contain NULL values (it is a combination of both a UNIQUE constraint and a NOT NULL constraint). A table can have only ONE PRIMARY KEY constraint. The primary key can either be a single column, or a combination of columns.
constraints in other tables (which enforces referential integrity between data in two tables).
on the "ID" column upon creation of the "Persons" table:
( ID int PRIMARY KEY, LastName varchar(255) NOT NULL, FirstName varchar(255),
);constraint on multiple columns, use the following SQL syntax:
( ID int, LastName varchar(255), FirstName varchar(255), Age int, PRIMARY KEY (ID, LastName)
);In the example above, the PRIMARY KEY
Formula
value is made up of two columns (ID + LastName).constraint on multiple columns, use the following SQL syntax:
( ID int, LastName varchar(255), FirstName varchar(255), Age int, CONSTRAINT PK_Person PRIMARY KEY (ID, LastName)
);In the example above, the PRIMARY KEY
Formula
is named "PK_Person", and the value is made up of two columns (ID + LastName).constraint on the "ID" column when the table already has been created, use the following SQL:
ADD PRIMARY KEY (ID);