Какой из двух запросов вернет больше строк?
-- Случай 1
SELECT
COUNT(DISTINCT first.counter_column)
FROM
first
LEFT JOIN
second ON first.join_key = second.join_key
WHERE
second.filter_column >= 5
-- Случай 2
SELECT
COUNT(DISTINCT first.counter_column)
FROM
first
LEFT JOIN
second ON first.join_key = second.join_key
and second.filter_column >= 5