Filter rows with WHERE

A WHERE clause keeps rows where its condition is true. False and unknown conditions are both excluded.

Suppose people contains:

name	active	age	email
Ada	true	36	ada@example.com
Lin	true	15	NULL
Sam	false	42	sam@example.com

This query returns only Ada:

SELECT name, email
FROM people
WHERE active = TRUE AND age >= 18;

SQL has three-valued logic: true, false, and unknown. A comparison with NULL normally produces unknown:

WHERE email = NULL

That condition does not find missing email addresses. Use:

WHERE email IS NULL

The same issue appears with inequality. email <> 'ada@example.com' returns Sam, but not Lin, because Lin’s comparison is unknown. Use email <> 'ada@example.com' OR email IS NULL when missing values belong in the result.

Parentheses make mixed conditions explicit:

WHERE active = TRUE
  AND (country = 'Italy' OR country = 'Denmark')

Without the parentheses, operator precedence can include inactive rows from one country.

Use IN for a set of values, BETWEEN for an inclusive range, and LIKE for a pattern when those forms express the requirement clearly. They do not remove the need to reason about NULL.

Your action is to predict which three sample rows match four conditions: age >= 18, email IS NULL, email <> 'ada@example.com', and active = TRUE OR age >= 18. Then run the queries.