Filtering Data with WHERE in MySQL

The WHERE clause filters rows so that only those matching a condition appear in your results. Without WHERE, a SELECT returns every row in the table. Learning to write precise filters is essential for working with real-world datasets that may contain millions of records.

Comparison Operators

MySQL supports the standard comparison operators: =, != (or <>),<, >, <=, and >=. Use them to compare column values against literals or other columns.

-- Employees earning more than 60000
SELECT first_name, salary
FROM employees
WHERE salary > 60000;

-- Orders placed on a specific date
SELECT order_id, total
FROM orders
WHERE order_date = '2025-01-15';

Combining Conditions with AND / OR

Chain multiple conditions with AND (all must be true) or OR (at least one must be true). Use parentheses to control evaluation order and avoid ambiguity.

SELECT *
FROM employees
WHERE department = 'Engineering'
  AND salary >= 70000;

SELECT *
FROM products
WHERE (category = 'Electronics' OR category = 'Books')
  AND price < 50;

Pattern Matching with LIKE

The LIKE operator matches string patterns. Use % to represent any sequence of characters and _ to represent exactly one character.

-- Names starting with 'J'
SELECT first_name
FROM employees
WHERE first_name LIKE 'J%';

-- Exactly 5-character codes
SELECT code
FROM products
WHERE code LIKE '_____';

The IN Operator

IN checks whether a value matches any item in a list. It is cleaner than writing multiple OR conditions and works with subqueries too.

SELECT *
FROM orders
WHERE status IN ('pending', 'processing', 'shipped');

BETWEEN for Ranges

BETWEEN is inclusive on both ends and works with numbers, dates, and strings. It is equivalent to using >= and <= together but reads more naturally.

SELECT *
FROM employees
WHERE hire_date BETWEEN '2024-01-01' AND '2024-12-31';

Handling NULL Values

NULL represents missing or unknown data. You cannot compare NULL with =; instead useIS NULL or IS NOT NULL. Forgetting this is one of the most common beginner mistakes.

SELECT *
FROM employees
WHERE manager_id IS NULL;