SQL Query to conditionally select a row from a table depending on join existence

Question

I have a query that joins 3 tables and I am trying to retrieve a conditional set of rows depending on whether the record exists in the third table.

If there is a match in the 3rd table then I just want the matching records from the 1st table. if there isn't a match from the 3rd table then I just want one record from the first table.

select
o.Object_ID,
reqCon.Connector_ID,
req.Object_ID as Requirement_ID

from t_object o

left join t_connector reqCon on reqCon.End_Object_ID = o.Object_ID and reqCon.Stereotype = 'deriveReqt'
left join t_object req on reqCon.Start_Object_ID = req.Object_ID and req.Stereotype = 'functionalRequirement'

This produces the following type of result, but the ones highlight with in Bold are the actual rows that are required.

Object_ID   Connector_ID    Requirement_ID
40936   43259
40936   43260
**40936 43299   38013**
40943   43264
40943   43265
**40943 43298   38014**
**44088 46245**
44088   46246   
**42669 44655**
42669   44656
**42670 44657**
42670   44658

Answer 1

One solution is union all :

select o.Object_ID, reqCon.Connector_ID, req.Object_ID as Requirement_ID
from t_object o join
     t_connector reqCon
     on reqCon.End_Object_ID = o.Object_ID and
        reqCon.Stereotype = 'deriveReqt' join
     t_object req
     on reqCon.Start_Object_ID = req.Object_ID and
        req.Stereotype = 'functionalRequirement'
union all
select o.Object_ID, min(reqCon.Connector_ID), null as Requirement_ID
from t_object o left join
     t_connector reqCon
     on reqCon.End_Object_ID = o.Object_ID and
        reqCon.Stereotype = 'deriveReqt' left join
     t_object req
     on reqCon.Start_Object_ID = req.Object_ID and
        req.Stereotype = 'functionalRequirement'
group by o.Object_Id
having sum(req.Object_Id is not null) = 0;

The first query brings in the matches. The second brings in the one row per object that has no match.