AND, OR, NOT — combining conditions

You can combine multiple conditions in WHERE using AND, OR, and NOT.

AND — both conditions must be true

SELECT first_name, salary
FROM employees
WHERE department_id = 60 AND salary > 5000;

OR — at least one condition must be true

SELECT first_name, department_id
FROM employees
WHERE department_id = 60 OR department_id = 80;

NOT — invert a condition

SELECT first_name, department_id
FROM employees
WHERE NOT department_id = 90;

Operator precedence

NOT binds tightest, then AND, then OR. This can cause surprises:

-- WARNING: this means department = 60 AND (salary > 5000 OR department = 80)
WHERE department_id = 60 AND salary > 5000 OR department_id = 80

Use parentheses to make intent explicit:

WHERE (department_id = 60 OR department_id = 80)
  AND salary > 5000

Tips

• Always use parentheses when mixing AND and OR — it prevents logic bugs and aids readability.
• NOT can be used with IN, BETWEEN, LIKE, and EXISTS (covered in later lessons).