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SQL Tutorial » SQL PRIMARY KEY

What is the primary key in SQL?

A primary key in the SQL table is a field in a table that uniquely identifies each row / record in a database.

Primary keys must contain unique values. A primary key column cannot have NULL values.

Syntax:

CREATE TABLE table_name(
ID_column INT NOT NULL,
NAME_column VARCHAR (20) NOT NULL,
ADDRESS_column CHAR (25),
PRIMARY KEY (ID)
);
Note: You would use these concepts while creating database tables.

To create a PRIMARY KEY constraint on the "ID_column" column when the table_name table already exists, use the following SQL syntax −

Syntax:

ALTER TABLE table_name
ADD PRIMARY KEY (ID_column);

 

SQL PRIMARY KEY on ALTER TABLE

To create a PRIMARY KEY constraint on the "ID_column" when sql table_name is already created, we use this SQL syntax

ALTER TABLE tabe_name
ADD PRIMARY KEY (ID_column);

To create a PRIMARY KEY constraint onmultiple columns when table_name table already exists, we use the following syntax.

ALTER TABLE table_name
ADD CONSTRAINT PK_table_name PRIMARY KEY (ID_column, NAME_column);

Related subjects:
How to create database phpmyadmin? SQL create table PHP select / view data Conect to MySQLi SQL alter table

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DROP (delete) PRIMARY KEY

You can remove(delete) primary key constraints from SQL table with the syntax shown below.:

ALTER TABLE table_name
DROP PRIMARY KEY;

 

SQL primary key auto increment

Auto-increment SQL allows a number to be generated automatically when a new record is inserted into yout table.

The following syntax SQL statement defines the "ID_column" column to be an auto-increment primary key field in the "table_name":

MySQL uses the AUTO_INCREMENT to perform an auto-increment feature.

ALTER TABLE Persons AUTO_INCREMENT=25;

By default AUTO_INCREMENT is starting from value 1



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What is primary key in SQL with example?
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SQL PRIMARY KEY - sql tutorial

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