MySQL PRIMARY KEY Constraint
MySQL PRIMARY KEY Constraint
The PRIMARY KEY constraint uniquely identifies each record in a table.
Primary keys must contain UNIQUE values, and cannot contain NULL values.
A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).
PRIMARY KEY on CREATE TABLE
The following SQL creates a PRIMARY KEY on the "ID" column when the "Persons" table is created:
CREATE TABLE Persons
(
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
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:
CREATE TABLE Persons
(
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
CONSTRAINT PK_Person PRIMARY KEY (ID,LastName)
);Note: In the example above there is only ONE
PRIMARY KEY (PK_Person).
However, the VALUE of the primary key is made up of TWO COLUMNS (ID + LastName).
PRIMARY KEY on ALTER TABLE
To create a PRIMARY KEY constraint on the "ID" column when the table is already created, use the following SQL:
ALTER TABLE Persons
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:
ALTER TABLE Persons
ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);Note: If you use ALTER TABLE to add a primary key, the primary key column(s) must
have been declared to not contain NULL values (when the table was first created).
DROP a PRIMARY KEY Constraint
To drop a PRIMARY KEY constraint, use the following SQL:
ALTER TABLE Persons
DROP PRIMARY KEY;
