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The PRIMARY KEY constraint uniquely identifies each record in a table
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).
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)
);
PRIMARY KEY
(PK_Person).
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:
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);
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).
To drop a PRIMARY KEY
constraint, use the following SQL:
ALTER TABLE Persons
DROP PRIMARY KEY;