WHERE Clause

The WHERE Clause

The WHERE clause filters records — it returns only the rows that meet a specified condition.

💡 WHERE is like a sieve for your data. It keeps the rows you want and discards the rest.

Basic Syntax

SELECT column1, column2
FROM table_name
WHERE condition;

WHERE with Numbers

SELECT first_name, last_name, salary
FROM employees
WHERE salary > 70000;

Comparison Operators

WHERE with Text

-- Exact match
SELECT * FROM customers WHERE city = 'New York';

-- Case-insensitive (database dependent)
SELECT * FROM customers WHERE LOWER(city) = 'new york';

WHERE with Dates

SELECT * FROM orders WHERE order_date >= '2024-01-01';
SELECT * FROM orders
WHERE order_date BETWEEN '2024-01-01' AND '2024-03-31';

WHERE with NULL

-- Check for NULL (cannot use = NULL)
SELECT * FROM customers WHERE email IS NULL;

-- Check for NOT NULL
SELECT * FROM customers WHERE email IS NOT NULL;

⚠️ NULL is not the same as 0 or empty string. Use IS NULL and IS NOT NULL to check for NULL values — never use = NULL.

✏️ Exercise: Find all employees who earn more than 60000, work in Engineering, and were hired after January 1, 2023

See Solution

SELECT first_name, last_name, salary
FROM employees
WHERE salary > 60000
  AND department = 'Engineering'
  AND hire_date > '2023-01-01';

✅ Key Takeaways

1. WHERE filters rows based on conditions
2. Use comparison operators for numbers, text, and dates
3. Use IS NULL / IS NOT NULL for NULL checks
4. Combine multiple conditions with AND and OR
5. WHERE is evaluated after FROM but before SELECT