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Showing content from https://neon.com/postgresql/postgresql-tutorial/postgresql-primary-key below:

PostgreSQL Primary Key

Summary: in this tutorial, you will learn about the PostgreSQL primary key and how to manage primary key constraints effectively.

A primary key is a column or a group of columns used to uniquely identify a row in a table. The column that participates in the primary key is known as the primary key column.

A table can have zero or one primary key. It cannot have more than 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.

Technically, a primary key constraint is the combination of a not-null constraint and a UNIQUE constraint.

Typically, you define primary for a table when creating it:

CREATE TABLE table_name (
  column_1 data_type PRIMARY KEY,
  column_2 data_type,
  …
);

In this syntax, you define the primary key as the column constraint of the primary key column.

If the primary key consists of more than one column, you can define it using the table constraint:

CREATE TABLE table_name (
  column_1 data_type,
  column_2 data_type,
  column_3 data_type,
  …
  PRIMARY KEY(column_1, column2, ...)
);

To add a primary key to an existing table, you the ALTER TABLE ... ADD PRIMARY KEY statement:

ALTER TABLE table_name
ADD PRIMARY KEY (column_1, column_2, ...);

If you don’t explicitly specify the name for the primary key constraint, PostgreSQL will assign a default name to the primary key constraint.

By default, PostgreSQL uses the format table-name_pkey as the default name for the primary key constraint.

To assign a name for the primary key, you can use the CONSTRAINT clause as follows:

CONSTRAINT constraint_name
PRIMARY KEY(column_1, column_2,...);

Let’s explore some examples of using the PostgreSQL primary key.

1) Creating a table with a primary key that consists of one column

The following statement creates a table with a primary key that consists of one column:

CREATE TABLE orders(
  order_id SERIAL PRIMARY KEY,
  customer_id VARCHAR(255) NOT NULL,
  order_date DATE NOT NULL
);

In this example, we create the orders with the order_id as the primary key.

We define the order_id column with the type SERIAL so that PostgreSQL will generate a unique integer (1, 2, 3, and so on) when you insert a new row into the table without providing the value for the order_id column.

This ensures the value in the order_id is unique for every row in the table.

2) Creating a table with a primary key that consists of two columns

The following example shows how to define a primary key that consists of two columns order_id and item_no:

CREATE TABLE order_items(
  order_id INT,
  item_no SERIAL,
  item_description VARCHAR NOT NULL,
  quantity INTEGER NOT NULL,
  price DEC(10, 2),
  PRIMARY KEY (order_id, item_no)
);

First, create a table called products without defining any primary key.

CREATE TABLE products (
  product_id INT,
  name VARCHAR(255) NOT NULL,
  description TEXT,
  price DEC(10, 2) NOT NULL
);

Second, add a primary key constraint to the products table using the ALTER TABLE ... ADD PRIMARY KEY statement:

ALTER TABLE products
ADD PRIMARY KEY (product_id);

First, create a new table called vendors that does not have a primary key:

CREATE TABLE vendors (
  name VARCHAR(255)
);

Second, insert some rows into the vendors table:

INSERT INTO vendors (name)
VALUES
  ('Microsoft'),
  ('IBM'),
  ('Apple'),
  ('Samsung')
RETURNING *;

Output:

name
-----------
 Microsoft
 IBM
 Apple
 Samsung
(4 rows)

Third, add a primary key named vendor_id into the vendors table with the type SERIAL:

ALTER TABLE vendors
ADD COLUMN vendor_id SERIAL PRIMARY KEY;

Finally, verify the vendor_id column:

SELECT
  vendor_id,
  name
FROM
  vendors;

Output:

vendor_id |   name
-----------+-----------
         1 | Microsoft
         2 | IBM
         3 | Apple
         4 | Samsung
(4 rows)

To remove a primary key from a table, you use the following ALTER TABLE statement:

ALTER TABLE table_name
DROP CONSTRAINT primary_key_constraint;

In this syntax:

First, specify the name of the table from which you want to remove the primary key.
Second, provide the primary key constraint to drop.

Let’s take an example of removing the primary key constraint from the vendors table using psql.

First, display the structure of the vendors table using the \d command:

\d vendors

Output:

Table "public.vendors"
  Column   |          Type          | Collation | Nullable |                  Default
-----------+------------------------+-----------+----------+--------------------------------------------
 name      | character varying(255) |           |          |
 vendor_id | integer                |           | not null | nextval('vendors_vendor_id_seq'::regclass)
Indexes:
    "vendors_pkey" PRIMARY KEY, btree (vendor_id)

The output indicates that the primary key constraint is vendors_pkey.

Second, drop the primary key from the vendors table using the ALTER TABLE ... DROP CONSTRAINT statement:

ALTER TABLE vendors
DROP CONSTRAINT vendors_pkey;

Output:

ALTER TABLE

The statement removes only the primary key constraint but does not remove the vendor_id column:

SELECT vendor_id, name
FROM vendors;

Output:

vendor_id |   name
-----------+-----------
         1 | Microsoft
         2 | IBM
         3 | Apple
         4 | Samsung
(4 rows)

Use the PRIMARY KEY constraint to define a primary key for a table when creating the table.
Use the ALTER TABLE ... ADD PRIMARY KEY statement to add a primary key to a table.
Use the ALTER TABLE ... DROP CONSTRAINT statement to drop a primary key from a table.

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