Constraints in SQL

From the SQL cheat sheet · Constraints & Keys · verified Jul 2026

Constraints

PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, CHECK, and DEFAULT.

sql

-- Primary key
CREATE TABLE users (
  id INT PRIMARY KEY,
  email VARCHAR(255) UNIQUE NOT NULL
);

-- Foreign key
CREATE TABLE orders (
  id INT PRIMARY KEY,
  user_id INT,
  FOREIGN KEY (user_id) REFERENCES users(id)
    ON DELETE CASCADE
);

-- Add constraint to existing table
ALTER TABLE users ADD CONSTRAINT age_check CHECK (age >= 0);

💡 ON DELETE CASCADE automatically deletes child rows — use carefully

⚡ Composite primary keys use multiple columns — common in junction/pivot tables

📌 RESTRICT is the default ON DELETE behavior — it prevents deleting referenced rows

🟢 Name your constraints (ADD CONSTRAINT name) — makes them easier to drop later

constraintsprimary-keyforeign-keyunique

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