AND, OR & NOT

Real questions usually involve more than one condition: products in a category and under a price, orders that are pending or shipped. The logical operators AND, OR and NOT combine conditions — and parentheses make sure they combine the way you mean.

AND: all conditions must be true

SELECT name, category, price
FROM products
WHERE category = 'Electronics' AND price < 50;

OR: at least one must be true

SELECT id, customer_id, status
FROM orders
WHERE status = 'pending' OR status = 'shipped';

NOT: reverses a condition

SELECT name, category
FROM products
WHERE NOT category = 'Electronics';

NOT is most useful with other operators: NOT IN, NOT LIKE, NOT BETWEEN, NOT EXISTS and IS NOT NULL, all covered in later lessons.

Precedence: AND before OR

Just as multiplication happens before addition, AND is evaluated before OR. This query looks like "cheap books or toys" but isn't:

SELECT name, category, price
FROM products
WHERE category = 'Books' OR category = 'Toys' AND price < 20;

It means category = 'Books' OR (category = 'Toys' AND price < 20), so every book appears regardless of price. Parentheses fix it:

SELECT name, category, price
FROM products
WHERE (category = 'Books' OR category = 'Toys') AND price < 30;

Truth tables

SQL has a third truth value, unknown, which appears whenever NULL is compared. WHERE keeps only rows where the condition is true — unknown rows are dropped. Lesson 11 explores what this means in practice.

Several conditions

SELECT first_name, job_title, salary, hire_date
FROM employees
WHERE department_id = 2
  AND salary >= 68000
  AND hire_date < '2023-01-01';

Putting each condition on its own line keeps long filters readable.

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