使用多个内部联接过滤一个表与其他表

Question

I have four tables, Photo, Event, News, Spot and Photo is the table i want to check for records with relations to other tables.

Photo has the following sructure:

id
rel_model -> one of "news", "spot" and "event"
rel_id    -> id of the related record in rel_model table 
...

Tables other than Photo are constantly updated and some of the records ar deleted. I want to filter the photos to get the records that are related to existing records on other tables.

I tried the following

select 
    count(*)
from
    Photo
        inner join Event ON (rel_id = Event.id and rel_model="event") 
        inner join News ON (rel_id = News.id and rel_model="news")
        inner join Spot ON (rel_id = Spot.id and rel_model="spot"); 

but I get 0 results where trying it with only one inner join works for checking against single table

select 
    count(*)
from
    Photo
        inner join Event ON (rel_id = Event.id and rel_model="event") ;

I need to add some and-or logic inbetween inner joins, bit could not figure out how.

How can I fetch the photos that still have unbroken relations to other tables?

Answer 1

you could use this query

select 
    count(*)
from Photo as P
where
    P.rel_model = "event" and P.rel_id in (select T.id from Event as T) or
    P.rel_model = "news" and P.rel_id in (select T.id from News as T) or
    P.rel_model = "spot" and P.rel_id in (select T.id from Spot as T)

If you want to change your query, you should use left outer join :

select 
    count(*)
from Photo as P
    left outer join Event ON (rel_id = Event.id and rel_model="event") 
    left outer join News ON (rel_id = News.id and rel_model="news")
    left outer join Spot ON (rel_id = Spot.id and rel_model="spot")
where News.id is not null or Spot.id is not null or Event.id is not null

Your query return null rows because you trying to join same row with all three tables, but you join condition is matched only one, so other two inner joins eliminate you row.

Answer 2

You can do this using outer joins. With the inner joins you are losing a row when rel_id fails to match any of the three (and presumably, it matches only one of them, so you lose all rows). Then, you need to count each one separately:

select count(Event.id) + count(News.id) + count(Spot.id)
from Photo p left join
     Event
     ON p.rel_id = Event.id and rel_model="event" left join
     News
     ON p.rel_id = News.id and rel_model="news" left join
     Spot
     ON p.rel_id = Spot.id and rel_model="spot";