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Practice

UNIQUE and DEFAULT Constraints

SQL lets you define constraints on columns to enforce data rules. Two common ones are:

  • UNIQUE: ensures all values in a column are distinct
  • DEFAULT: assigns an automatic value if none is provided

UNIQUE Constraint​

The UNIQUE constraint ensures no two rows in a column have the same value.

UNIQUE constraint on email
CREATE TABLE clients (
  id INT PRIMARY KEY,
  email TEXT UNIQUE,
  name TEXT,
  status TEXT DEFAULT 'active'
);

In this table, the email column is UNIQUE, meaning only one client can use alex@example.com.


DEFAULT Constraint​

The DEFAULT constraint sets a value automatically when none is given.

Create table with DEFAULT constraint
CREATE TABLE clients (
  id INT PRIMARY KEY,
  email TEXT UNIQUE,
  name TEXT,
  status TEXT DEFAULT 'active'
);

Here, status will default to 'active' unless you provide something else.


INSERT INTO Example​

The query below inserts a new client named Laura Adams.

Insert using default value
INSERT INTO clients (id, email, name)
VALUES (6, 'laura.adams@newdomain.com', 'Laura Adams');

If you don't specify a status, the default value 'active' will be used.

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