The PRIMARY KEY constraint uniquely identifies each record in a database table.
Primary keys must contain unique values.
A primary key column cannot contain NULL values.
Every table should have a primary key, and each table can have only ONE primary key.
The Below SQL creates a PRIMARY KEY on the "P_Id" column when the "Persons"
table is created:
SQL Server / Oracle / MS Access:
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on
multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
To create a PRIMARY KEY constraint on the "P_Id" column when the table is already
created, use the following SQL:
Note: If you use the ALTER TABLE statement to add a primary key, the primary key column(s)
must already have been declared to not contain NULL values (when the table was first created).
To drop a PRIMARY KEY constraint, use the following SQL:
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