Question

I am trying to join one record in a table to another using a derived table and am having a bit of trouble figuring out the correct query to do so. What I want to do is have a JOIN of a derived table to a query where the derived table uses where statements depending on data from the outer query that is being joined to. So here is the current code that I am working on:

SELECT a.viewerid, a.id, v.id AS entry, a.jobid, v.sourceid, v.cost, a.applicant
FROM a_views a,
JOIN (
    SELECT TOP 1 id, sourceid, cost FROM a_views vt
    WHERE vt.viewerid = a.viewerid
      AND vt.viewed_at <= a.viewed_at
      AND vt.referrer NOT LIKE '%' + vt.hostName + '%'
      ORDER BY viewed_at DESC
) v

The derived table is a query of the same table that the outer query uses, and viewerid is a FK to itself across the table where id is a unique auto-incrementing PK. I need to get the latest record in the a_views table where the viewer id’s match, the datestamp (viewed_at) is less than the outer datestamp and the referrer column doesn’t contain the hostName column.

Answer 1

Sounds like you need APPLY:

SELECT a.viewerid, a.id, v.id AS entry, a.jobid, v.sourceid, v.cost, a.applicant
FROM a_views a
    CROSS APPLY (
        SELECT TOP 1 id, sourceid, cost FROM a_views vt
        WHERE vt.viewerid = a.viewerid
          AND vt.viewed_at <= a.viewed_at
          AND vt.referrer NOT LIKE '%' + vt.hostName + '%'
          ORDER BY viewed_at DESC
    ) v

Since your query has JOIN I’ve gone for CROSS APPLY, but you may need OUTER APPLY depending on your exact requirements.