Filtering with WHERE

Narrowing the Result Set

WHERE filters rows before you see them. Only rows for which the condition is true come back.

SELECT * FROM customers WHERE city = 'Mumbai';
SELECT * FROM customers WHERE id >= 100;
SELECT * FROM customers WHERE is_active = TRUE;

Comparison Operators

• =, <> or != for equality/inequality.
• >, <, >=, <= for ordering.
• Beware: comparing to NULL is special (see below).

Combining Conditions

SELECT * FROM customers
WHERE city = 'Mumbai' AND is_active = TRUE;

SELECT * FROM customers
WHERE city = 'Mumbai' OR city = 'Pune';

SELECT * FROM customers
WHERE is_active = TRUE AND (city IN ('Mumbai', 'Pune') OR created_at > '2024-01-01');

AND and OR combine conditions; use parentheses to control precedence the same way as math.

Operators That Read Well

SELECT * FROM customers WHERE id IN (1, 2, 3);
SELECT * FROM customers WHERE created_at BETWEEN '2024-01-01' AND '2024-12-31';
SELECT * FROM customers WHERE name LIKE 'A%';     -- starts with A
SELECT * FROM customers WHERE name LIKE '_iya';    -- any char then 'iya'
SELECT * FROM customers WHERE city IS NULL;          -- never use = NULL

Key Points

• WHERE filters rows before the results show.
• Combine conditions with AND / OR and parentheses.
• IN, BETWEEN, LIKE and IS NULL are everyday tools.
• NULL comparisons need IS NULL, never = NULL.