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❮ SQL Keywords
constraint uniquely identifies each record in a table. A table can have only one primary key, which may consist of one single or of multiple fields.
The following SQL creates a PRIMARY KEY on the "ID" column when the "Persons" table is created:
( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, PRIMARY KEY (ID)
);Formula
SQL Server / Oracle / MS Access:( ID int NOT NULL PRIMARY KEY, LastName varchar(255) NOT NULL, FirstName varchar(255),
);To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:
Formula
MySQL / SQL Server / Oracle / MS Access:( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, CONSTRAINT PK_Person PRIMARY KEY (ID,LastName)
);In the example above there is only ONE PRIMARY KEY (PK_Person).
Formula
However, the VALUE of the primary key is made up of TWO COLUMNS (ID + LastName).To create a PRIMARY KEY constraint on the "ID" column when the table is already created, use the following SQL:
Formula
MySQL / SQL Server / Oracle / MS Access:ADD PRIMARY KEY (ID);To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:
Formula
MySQL / SQL Server / Oracle / MS Access:ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);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:
DROP PRIMARY KEY;Formula
SQL Server / Oracle / MS Access:DROP CONSTRAINT PK_Person;❮ SQL Keywords