SQL · Quick Reference

Lesson 3 — Data Types & Constraints Cheat Sheet

SQL

In one line: Choose the right numeric type to balance precision and storage efficiency.

Key Ideas

1Numeric Types. Choose the right numeric type to balance precision and storage efficiency.

Code Examples

-- CHAR: fixed-length (padded with spaces)
country_code CHAR(2)        -- always exactly 2 chars: 'PK', 'US'

-- VARCHAR: variable-length (efficient)
name VARCHAR(100)           -- stores only actual length + 1 byte

-- TEXT: for long content (up to 6...

-- DATE: stores date only (YYYY-MM-DD)
birthday DATE               -- '2006-03-15'

-- DATETIME: date + time (YYYY-MM-DD HH:MM:SS)
appointment DATETIME        -- '2026-06-18 14:30:00'

-- TIMESTAMP: like DATETIME but auto-converts timezone
created_at...

CREATE TABLE orders (
  id          INT PRIMARY KEY AUTO_INCREMENT,
  customer_id INT NOT NULL,
  total       DECIMAL(10,2) NOT NULL CHECK (total >= 0),
  status      VARCHAR(20) DEFAULT 'pending',
  order_date  DATE NOT NULL,
  UNIQUE (customer_id...