WHERE

The WHERE clause provides filters to apply to the output of executing the FROM portion of a query.

Any expression that returns a boolean value can be used in the WHERE clause.

Simple filtering using a literal:

SELECT *
FROM cities
WHERE state_abbr = 'OH';

More complex expressions, including subqueries can be used as well:

SELECT *
FROM cities c1
WHERE population >= (SELECT avg(population) FROM cities c2 WHERE c1.state = c2.state)