WHERE Clause

The WHERE clause is how you filter rows in a query. Without it, SELECT returns every row in the table. With WHERE, you can pick exactly the rows you want. It is like asking a specific question instead of reading the entire phone book.

Basic Comparison Operators

You can compare values using these operators:

-- Equal to
SELECT * FROM students WHERE first_name = 'Alice';

-- Not equal to
SELECT * FROM students WHERE first_name != 'Alice';

-- Greater than / Less than
SELECT * FROM students WHERE date_of_birth > '2000-01-01';
SELECT * FROM students WHERE date_of_birth < '1999-12-31';

-- Greater than or equal / Less than or equal
SELECT * FROM students WHERE id >= 10;

Combining Conditions

Use AND and OR to combine multiple conditions:

-- AND: both conditions must be true
SELECT * FROM students
WHERE date_of_birth > '2000-01-01'
AND enrollment_date > '2023-01-01';

-- OR: at least one condition must be true
SELECT * FROM students
WHERE first_name = 'Alice' OR first_name = 'Bob';

You can chain as many conditions as you need. Use parentheses to control the order of evaluation.

BETWEEN

A shorthand for range comparisons:

SELECT * FROM students
WHERE date_of_birth BETWEEN '2000-01-01' AND '2002-12-31';

This is equivalent to using >= AND <=. It works with dates, numbers, and text.

IN

Check if a value matches any value in a list:

SELECT * FROM students
WHERE first_name IN ('Alice', 'Bob', 'Charlie');

-- The opposite: NOT IN
SELECT * FROM students
WHERE first_name NOT IN ('Alice', 'Bob');

This is cleaner than writing a bunch of OR conditions.

LIKE Pattern Matching

Search for patterns in text:

-- Names starting with 'A'
SELECT * FROM students WHERE first_name LIKE 'A%';

-- Names ending with 'son'
SELECT * FROM students WHERE last_name LIKE '%son';

-- Names with 'li' anywhere
SELECT * FROM students WHERE first_name LIKE '%li%';

-- Exactly 5 characters
SELECT * FROM students WHERE first_name LIKE '_____';

The % matches any sequence of characters. The _ matches exactly one character.

IS NULL

Check for missing values:

-- Find students without an email
SELECT * FROM students WHERE email IS NULL;

-- Find students with an email
SELECT * FROM students WHERE email IS NOT NULL;

Never use = NULL or != NULL. NULL is not a value — it represents the absence of a value. Always use IS NULL or IS NOT NULL.

NOT

Negate any condition:

SELECT * FROM students
WHERE NOT first_name = 'Alice';

-- Same as:
SELECT * FROM students
WHERE first_name != 'Alice';