SQL Constraints

SQL Constraints

SQL constraints are rules applied to table columns to enforce data integrity and consistency. Constraints help prevent invalid data from being inserted into a table.

Why Constraints Matter

• Prevent missing required values
• Prevent duplicate data
• Ensure relationships between tables
• Enforce valid ranges or formats

Common SQL Constraints

• NOT NULL – ensures a column cannot have a NULL value
• UNIQUE – ensures all values in a column are different
• PRIMARY KEY – uniquely identifies each row
• FOREIGN KEY – links records between tables
• CHECK – ensures values satisfy a condition (support varies)
• DEFAULT – sets a default value when none is provided

Constraints in CREATE TABLE

You can define constraints when creating a table:

CREATE TABLE customers (
  id INT AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(100) NOT NULL,
  email VARCHAR(150) UNIQUE,
  status VARCHAR(20) DEFAULT 'active'
);

Constraints in ALTER TABLE

You can also add constraints to an existing table:

ALTER TABLE customers
ADD UNIQUE (email);

Foreign Key Example

CREATE TABLE orders (
  id INT AUTO_INCREMENT PRIMARY KEY,
  customer_id INT NOT NULL,
  order_date DATE NOT NULL,
  FOREIGN KEY (customer_id) REFERENCES customers(id)
);

Common Mistakes

• Not defining a primary key
• Forgetting indexes for foreign key columns (can hurt performance)
• Assuming CHECK works the same in every database system

Next Step

Continue with SQL NOT NULL to learn how to require values in a column.