检索行数，如果没有行则返回0

假设

created

date

Postgres提供了简化

generate_series()

SELECt d.created, COUNT(s.id) AS ctFROM  (   SELECt generate_series(min(created)  , max(created), interval '1 day')::date AS created   FROM   signups   ) dLEFT   JOIN signups s USING (created)GROUP  BY 1ORDER  BY 1 DESC;

这会自动从您的表格中检索最短和最长的一天，并且每天之间提供一行。