Summary: in this tutorial, we will show you what the primary key is and how to manage PostgreSQL primary key constraints through SQL statements.

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A primary key is a column or a group of columns used to identify a row uniquely in a table.

You define primary keys through primary key constraints. Technically, a primary key constraint is the combination of a not-null constraint and a UNIQUE constraint.

A table can have one and only one primary key. It is a good practice to add a primary key to every table. When you add a primary key to a table, PostgreSQL creates a unique B-tree index on the column or a group of columns used to define the primary key.

Define primary key when creating the table

Normally, we add the primary key to a table when we define the table’s structure using CREATE TABLE statement.

The following statement creates a purchase order (PO) header table with the name po_headers.

The po_no is the primary key of the po_headers table, which uniquely identifies purchase order in the po_headers table.

In case the primary key consists of two or more columns, you define the primary key constraint as follows:

For example, the following statement creates the purchase order line items table whose primary key is a combination of purchase order number ( po_no) and line item number ( item_no).

If you don’t specify explicitly the name for primary key constraint, PostgreSQL will assign a default name to the primary key constraint. By default, PostgreSQL uses table-name_pkey as the default name for the primary key constraint. In this example, PostgreSQL creates the primary key constraint with the name po_items_pkey for the po_items table.

In case you want to specify the name of the primary key constraint, you use CONSTRAINT clause as follows:

Define primary key when changing the existing table structure

It is rare to define a primary key for existing table. In case you have to do it, you can use the ALTER TABLE statement to add a primary key constraint.

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The following statement creates a table named products without defining any primary key.

Suppose you want to add a primary key constraint to the products table, you can execute the following statement:

Postgres Auto Generate Uuid Primary Key

How to add an auto-incremented primary key to an existing table

Suppose, we have a vendors table that does not have any primary key.

And we add few rows to the vendors table using INSERT statement:

To verify the insert operation, we query data from the vendors table using the following SELECT statement:

Now, if we want to add a primary key named id into the vendors table and the id field is auto-incremented by one, we use the following statement:

Let’s check the vendors table again.

Remove primary key

Postgres Auto Increment Id Primary Key Pgadmin

To remove an existing primary key constraint, you also use the ALTER TABLE statement with the following syntax:

For example, to remove the primary key constraint of the products table, you use the following statement:

In this tutorial, you have learned how to add and remove primary key constraints using CREATE TABLE and ALTER TABLE statements.