MYSQL Left Join COUNTS from multiple tables

Question

I want to add columns that represent counts from other tables.

I have 3 tables.

Messages

MessageID    User      Message            


        

Answer 1

select
  t.Topic,
  t.Title,
  count(distinct s.starID) as StarCount,
  count(distinct m.User) as UserCount,
  count(distinct m.messageID) as MessageCount
from
  Topics t
  left join Messages m ON m.Topic = t.Topic
  left join Stars_Given s ON s.Topic = t.Topic
group by
  t.Topic,
  t.Title

Sql Fiddle

Or, you can perform the aggregation in sub-queries, which will likely be more efficient if you have a substantial amount of data in the tables:

select
  t.Topic,
  t.Title,
  s.StarCount,
  m.UserCount,
  m.MessageCount
from
  Topics t
  left join (
    select 
      Topic, 
      count(distinct User) as UserCount,
      count(*) as MessageCount
    from Messages
    group by Topic
  ) m ON m.Topic = t.Topic
  left join (
    select
      Topic, 
      count(*) as StarCount
    from Stars_Given 
    group by Topic
  ) s ON s.Topic = t.Topic

Sql Fiddle