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Summary: in this tutorial, you will learn about the primary key and how to use SQL primary key constraint to add a primary key to the table.
What is a Primary Key in SQL?
A primary key is a field or combination of fields that uniquely identifies a record. A primary key is a column or group of unique columns. In other words, every value for the primary key is unique.
The primary key that consists of two or more columns is also known as the composite primary key.
Rules for Primary Key
- Each table can have only one primary key.
- All values are unique and primary key values can uniquely identify each row.
- The system will not allow a row to be inserted with a primary key that already exists in the table.
- A primary key cannot be full.
If the primary key consists of only one column, you can define the use of PRIMARY KEY constraint as a column constraint:
CREATE TABLE table_name (
pk_column data_type PRIMARY KEY,
In case the primary key has two or more columns, you must use the PRIMARY KEY constraint as a table constraint:
CREATE TABLE table_name (
pk_column_2 data type,
PRIMARY KEY (pk_column_1, pk_column_2)
Each table can have only one primary key. Not all columns that participate in the primary key must be defined as NULL. SQL Server automatically sets the NOT NULL constraint for all primary key columns if the NOT column constraint is not specified for these columns.
Since the values in the course_id column are uniquely identified to the rows in the course table, the course_id column is the primary key of the course table.
Each table has one and only one primary key. The primary key does not accept NULL or duplicate values.
If the primary key contains two or more columns, the values may be duplicated in one column, but the combination of values from all columns in the primary key must be unique.
The primary key of the training table consists of the Employee_ID and Course_ID columns.
The values in the course_id column are duplicate, although the combination of values in employee_id and course_id is not.
To create a primary key in a new table
Here is the syntax to define the ID attribute as a primary key in a CUSTOMERS table.
CREATE TABLE CUSTOMERS( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR (25) , SALARY DECIMAL (18, 2), PRIMARY KEY (ID) );
To create a primary key constraint on the “ID” column when the CUSTOMERS table already exists, use the following syntax –
ALTER TABLE CUSTOMER ADD PRIMARY KEY (ID);
NOTE: If you use the ALTER TABLE statement to append a primary key, the primary key column (s) must already be declared to not contain the NULL value (when the table was first created).
Delete Primary Key
You can clear the primary key constraints from the table with the syntax given below.
ALTER TABLE table_name
DROP CONSTRAINT primary_key_constraint;
In this tutorial, we have introduced you to the primary key concept and shown you how to manage the primary key of a table including adding and removing the primary key.
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