在2个联接以及where或条件之后检查联接查询中的空值

Question

I have a linq query that is checking to see if a value exists, given criteria. It joins Permits , AccessControlTypes , and EntanceEnhancementHistories , all on their Guid FKs. It checks Permits of Type "Full" or "Limited" to see if they have any associated EntranceEnhancementHistories .

bool LimitedorFullEntranceEnhancementValue = (
     from p in context.Permits
     join a in context.AccessControlTypes on p.EntranceAccessControlTypeGUID equals a.GUID
     join e in context.EntranceEnhancementHistories on p.GUID equals e.PermitGUID
     where p.GUID.Equals(PermitGuid) 
       && ((a.Description.Equals("Full") || a.Description.Equals("Limited")) 
       && (e.GUID == null))
     select e).Any();
return LimitedorFullEntranceEnhancementValue;

This query is failing. It's returning false in the case that is does find description of full or limited, but does not find an entrancehancementvalue (it should be null ). It works properly without the check for a null entranceenhancementvalue , and I'm able to get it working with 2 queries, but I don't think this should be necessary.

Answer 1

You're using an "inner join" on p.GUID equals e.PermitGUID , which means you'll only see results where an e.PermitGUID exists. I think you want something more like this:

 (from p in context.Permits
 join a in context.AccessControlTypes on p.EntranceAccessControlTypeGUID equals a.GUID
 where p.GUID.Equals(PermitGuid) 
   && ((a.Description.Equals("Full") || a.Description.Equals("Limited")) 
   && !context.EntranceEnhancementHistories.Any(e => p.GUID == e.PermitGUID)
 select p).Any();

Answer 2

You need to use a LEFT OUTER JOIN if you expect to see rows where the Guid key is null:

(from p in context.Permits
 join a in context.AccessControlTypes on p.EntranceAccessControlTypeGUID equals a.GUID
 join t in context.EntranceEnhancementHistories on p.GUID equals e.PermitGUID into leftJoin
 join e in leftJoin on p.DefaultIfEmpty()
 where p.GUID.Equals(PermitGuid) 
   && ((a.Description.Equals("Full") || a.Description.Equals("Limited")) 
   && (e.GUID == null))
 select e).Any();

return LimitedorFullEntranceEnhancementValue;