SQL Primary Keys

SQL Primary Keys – In SQL, the primary key is a single field or a combination of several fields that uniquely define a record. None of the fields that are part of the primary key should contain the value NULL. The table can only contain one primary key.

You can use CREATE TABLE operator or ALTER TABLE operator to create primary key in SQL.

Create primary key (CREATE TABLE operator)

The primary key can be created when the CREATE TABLE operator is executed in SQL.

Syntax for creating a primary key using the CREATE TABLE operator in SQL

CREATE TABLE table_name_id
(
column1 datatype_id [ NULL | NOT NULL ],
column2 datatype_id [ NULL | NOT NULL ],
…
CONSTRAINT constraint_name_id PRIMARY KEY (pk_col1_id, pk_col2_id, ... pk_col_n_id)
);

Or :

CREATE TABLE table_name_id
(
column1 datatype_id CONSTRAINT constraint_name_id PRIMARY KEY,
column2 datatype_id [ NULL | NOT NULL ],
…
);

• table_name_id – The name of the table you want to create
• column1, column2_id – The columns that you want to create in the table
• constraint_name_id – Name of the primary key
• pk_col1_id, pk_col2_id, … pk_col_n_id –  Columns that make up the primary key

Example:
Let’s see how to create a primary key using the CREATE TABLE statement in SQL. We will start with a very simple one, where our primary key consists of only one column.
For example:

CREATE TABLE suppls
( suppl_id int NOT NULL,
suppl_name char(50) NOT NULL,
contact_name_id char(50),
CONSTRAINT suppls_pk PRIMARY KEY (suppl_id)
);

In this example, we created a primary key for the table called suppls_pk. It consists of only one column, the suppl_id column.

We could use an alternative syntax and create the same primary key in the following way.

CREATE TABLE suppls
( suppl_id int CONSTRAINT suppls_pk PRIMARY KEY,
suppl_name char(50) NOT NULL,
contact_name_id char(50)
);

If you create a primary key that consists of 2 or more columns, you are limited to using only the first syntax where the primary key is defined at the end of the CREATE TABLE statement.
For example:

CREATE TABLE contacts_id
( l_name VARCHAR(30) NOT NULL,
f_name VARCHAR(25) NOT NULL,
birthday DATE_id,
CONSTRAINT contacts_pk_id PRIMARY KEY (l_name, f_name)
);

This example creates a primary key with the name contacts_pk_id, which consists of a combination of l_name and f_name columns. Therefore, each combination of l_name and f_name must be unique in the contacts_pk_id table.

Create primary key (ALTER TABLE operator)

If your table already exists and you want to add your primary key later, you can use the ALTER TABLE operator to create your primary key.

Syntax for creating a primary key using the ALTER TABLE operator in SQL

ALTER TABLE tab_name
ADD CONSTRAINT constraint_name_id
PRIMARY KEY (col1, col2, ... col_n);

• tab_name – Table name to change. This is the table to which you want to add the primary key
• constraint_name_id – Name of the primary key
• col1, col2,… col_n – Columns that make up the primary key

Example:
Let’s see how to create a primary key using the ALTER TABLE operator in SQL. Let’s say that we already have a table created in our database. We could add the primary key to the suppliers table using the following ALTER TABLE operator.

ALTER TABLE suppls
ADD CONSTRAINT_pk_id
PRIMARY KEY (suppl_id);

In this example, we created a primary key for an existing table called sources_pk_id. It consists of the suppl_id column.

We could also create a primary key with more than one field, as shown below.

ALTER TABLE suppls
ADD CONSTRAINT_pk_id
PRIMARY KEY (suppl_id, suppl_name);

This example would create a primary key with the name suppl_pk, which consists of a combination of the columns suppl_id and suppl_name.

Delete primary key

In SQL you can delete a primary key using the ALTER TABLE operator.

Syntax to delete the primary key in SQL

ALTER TABLE table_name_id
DROP PRIMARY KEY;

• table_name_id – Table name to change. This is the table whose primary key you want to delete

Example:
Let’s see how to remove a primary key using the ALTER TABLE operator in SQL.

ALTER TABLE suppls
DROP PRIMARY KEY;

in this example, we removed the primary key from the table. We do not need to give the name of the primary key, since there can only be one primary key in the table.

Creating Primary and Foreign Keys in SQL Server 2012