具有两个不同表的SQL条件INNER JOIN

Question

I have a following simplified sub-query that should be executed first:

(SELECT app.recordId, left(userid,1) userid, count(*)FROM
      app group by userid,recordId) y

Based on the result of this subquery, if userid='A' I would like to INNER JOIN with tableA , If userid='B' , I'd like to INNER JOIN with tableB .

I also want the result of both inner joins to appear. Is there a way so that I can do this without re-executing the subquery to minimize the execution time?

Answer 1

One approach is to use left join twice, probably with coalesce() :

SELECT y.*, COALESCE(a.col1, b.col1) as col1
FROM (SELECT app.recordId, left(userid, 1) as userid, count(*) as cnt
      FROM app
      GROUP BY recordId, left(userid, 1)
     ) y LEFT JOIN
     tableA a
     ON y.userId = 'A' and . . . LEFT JOIN
     tableB b
     ON y.userId = 'B' and . . .
WHERE y.userId IN ('A', 'B');

The . . . . . . is space for additional join conditions, which the question does not specify.

EDIT:

If you want to filter out rows that have no matches (ala an "inner join" approach):

WHERE y.userId IN ('A', 'B') and not (a.col is null and b.col is null)

where a.col and b.col are two columns used for the join condition.

Answer 2

You didn't say how you want the columns from the tables A and B to be handled. We also don't know how the tables are related. So all I can give you is an example.

Let's say you want a description that either comes from table A or B:

select agg.userid, a_and_b.description, agg.recordid, agg.rec_count
from 
(
  select left(userid,1) as userid, recordid, count(*) as rec_count
  from app
  group by left(userid,1), recordid
) agg
join
(
  select 'A' as userid, recordid, description from a
  union all
  select 'B' as userid, recordid, description from b
) a_and_b on a_and_b.userid = agg.userid and a_and_b.recordid = agg.recordid;