CHECK constraint

Add a column rule with CHECK.

UNIQUE

With UNIQUE you ensure all values in a column are distinct. Duplicate input is blocked.

CREATE TABLE customers (
  id INT PRIMARY KEY,
  email VARCHAR(100) UNIQUE
);

Email is unique; two people cannot share an email.

The table customers is created.

UNIQUE allows NULL values (most databases allow multiple NULLs). PRIMARY KEY does not allow NULL. UNIQUE is ideal for secondary unique fields.

Create customers with id INT PRIMARY KEY and email VARCHAR(100) UNIQUE.

Show example solution

CREATE TABLE customers (
  id INT PRIMARY KEY,
  email VARCHAR(100) UNIQUE
)

Unique

CREATE TABLE customers (
  email VARCHAR(100) ____
);

Show answer

CREATE TABLE customers (
  email VARCHAR(100) UNIQUE
);

Next lesson: FOREIGN KEY