SQL加入Rails

Question

User.where(created_at: "2015-08-02").joins(:likes).count

What does this exactly mean in Rails? Specifically compared to User.where(created_at: "2015-08-02").count (without joins)

Please translate it to English. Thanks.

Answer 1

the first one joined 2 tables ( likes and users ) and count the entries in the users table which have an entry in the likes table. the other one count all users and don't care about entries in the likes table

so if you have 100 users, then both gives you count 100. if you have 100 users, and 90 users have a like, then your first gives you ccount of 90 and your second query gives you count of 100.