Filtering rows with WHERE — operators and conditions

WHERE Cuts Rows Before SELECT Runs

WHERE tells the database which rows to keep. Every row is tested against the condition — rows that don't match are dropped before the result is assembled.

-- Basic comparison
SELECT name, price FROM products WHERE price > 100;

-- Range shorthand
SELECT name FROM products WHERE price BETWEEN 50 AND 200;

-- Match a list of values
SELECT name FROM products WHERE category IN ('Electronics', 'Books');

-- Pattern match (% = any chars, _ = one char)
SELECT name FROM products WHERE name LIKE 'Air%';

NULL Is Not Zero or Empty String

NULL means unknown. Comparing NULL with = or != always returns false. Use IS NULL and IS NOT NULL instead:

-- Wrong: returns no rows even where discount is null
SELECT name FROM products WHERE discount = NULL;

-- Correct
SELECT name FROM products WHERE discount IS NULL;

Combining Conditions

Use AND, OR, and NOT to combine conditions. AND requires both sides to be true. OR requires either side. Wrap OR conditions in parentheses to avoid precedence bugs:

SELECT name FROM products
WHERE category = 'Electronics' AND (price < 500 OR on_sale = true);

Without the parentheses, AND binds tighter than OR and the logic changes.