SQL PRIMARY KEY

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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);

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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

sql primary key: and foreign key, auto increment, clustered, keys multiples, constraint, vs foreign key, vs unique, index, null, identity, What is the primary key in SQL?
What is primary key in SQL with example?
What is primary key and example?
Is a primary key required SQL?
SQL PRIMARY KEY - sql tutorial

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