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The best SQL PRIMARY KEY constraint Tutorial In 2024, In this tutorial you can learn SQL PRIMARY KEY constraint,SQL PRIMARY KEY constraint CREATE TABLE when,SQL PRIMARY KEY constraint ALTER TABLE when,Revocation of PRIMARY KEY constraint,

SQL PRIMARY KEY constraint

SQL PRIMARY KEY constraint

PRIMARY KEY constraint uniquely identifies each record in a database table.

The primary key must contain unique values.

A primary key column can not contain NULL values.

Each table should have a primary key, and each table can have only one primary key.

SQL PRIMARY KEY constraint CREATE TABLE when

The following SQL when the "Persons" table is created to create a PRIMARY KEY constraint on the "P_Id" column:

MySQL:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

To name PRIMARY KEY constraints, and define a plurality of columns PRIMARY KEY constraint, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)

Note: In the example above, only one primary key PRIMARY KEY (pk_PersonID).However, the value pk_PersonID is made up of two columns (P_Id and LastName) thereof.

SQL PRIMARY KEY constraint ALTER TABLE when

When the table has been created, create the PRIMARY KEY constraint for a column in "P_Id", please use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD PRIMARY KEY (P_Id)

To name PRIMARY KEY constraints, and define a plurality of columns PRIMARY KEY constraint, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)

NOTE: If you use the ALTER TABLE statement to add a primary key, you must declare the primary key column does not contain a NULL value (first created in the table).

Revocation of PRIMARY KEY constraint

To drop a PRIMARY KEY constraint, use the following SQL:

MySQL:

ALTER TABLE Persons
DROP PRIMARY KEY

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT pk_PersonID

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SQL PRIMARY KEY constraint
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