Data Types & Keywords Quick Reference

Pick types that match your data. Wrong types waste space or break math — storing scores as VARCHAR prevents AVG().

Data Types overview

Pick types that match your data. Wrong types waste space or break math — storing scores as VARCHAR prevents AVG().

Real-life example: Data types are choosing box sizes — letters go in envelopes, bricks need crates, not the other way around.

INT / BIGINT — whole numbers (student_id, count)
DECIMAL(p,s) — exact decimals (money, scores)
VARCHAR(n) — text up to n characters (names, cities)
DATE / DATETIME / TIMESTAMP — calendar and clock
BOOLEAN — true/false flags (is_active)

Well-typed school schema snippet

CREATE TABLE students (
  student_id INT PRIMARY KEY,
  full_name VARCHAR(100) NOT NULL,
  grade_level SMALLINT,
  gpa DECIMAL(3, 2),
  is_active BOOLEAN DEFAULT TRUE,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

Keywords quick reference

These words appear throughout SQL documentation and this course. They are reserved — avoid using them as table or column names.

Real-life example: Keywords are traffic signals — SELECT means go read, DELETE means stop and think twice.

DDL: CREATE, ALTER, DROP, TRUNCATE
DML: SELECT, INSERT, UPDATE, DELETE
DCL: GRANT, REVOKE (permissions)
Clauses: FROM, WHERE, GROUP BY, HAVING, ORDER BY, LIMIT
Joins: INNER, LEFT, RIGHT, FULL, ON, UNION
Logic: AND, OR, NOT, IN, BETWEEN, LIKE, EXISTS

One query using many keywords

SELECT c.course_name, AVG(e.score) AS avg_score
FROM enrollments e
INNER JOIN courses c ON e.course_id = c.course_id
WHERE e.score IS NOT NULL
GROUP BY c.course_name
HAVING AVG(e.score) > 75
ORDER BY avg_score DESC
LIMIT 10;