SQL Server Select line where jobCategoryKey EXISTS once for one candidateKey I'm blocked with an SQL Query that I have to made on SQL Server 2014.

Here is my table TR_Candidate_To_Jobs_Category.



CREATE TABLE [dbo].[TR_Candidate_To_Jobs_Category]
       ( [candidateKey] [int] NOT NULL,
         [jobCategoryKey] [int] NOT NULL,
          CONSTRAINT [PK_TR_Candidate_To_Jobs_Category] PRIMARY KEY CLUSTERED
            ( [candidateKey] ASC, [jobCategoryKey] ASC )
            WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
       ) ON [PRIMARY]

INSERT [dbo].[TR_Candidate_To_Jobs_Category] ([candidateKey], [jobCategoryKey]) VALUES (2, 223)
INSERT [dbo].[TR_Candidate_To_Jobs_Category] ([candidateKey], [jobCategoryKey]) VALUES (2, 1245)
INSERT [dbo].[TR_Candidate_To_Jobs_Category] ([candidateKey], [jobCategoryKey]) VALUES (2, 225)
INSERT [dbo].[TR_Candidate_To_Jobs_Category] ([candidateKey], [jobCategoryKey]) VALUES (5, 137)
INSERT [dbo].[TR_Candidate_To_Jobs_Category] ([candidateKey], [jobCategoryKey]) VALUES (5, 142)
INSERT [dbo].[TR_Candidate_To_Jobs_Category] ([candidateKey], [jobCategoryKey]) VALUES (5, 143)
INSERT [dbo].[TR_Candidate_To_Jobs_Category] ([candidateKey], [jobCategoryKey]) VALUES (6, 1245)
INSERT [dbo].[TR_Candidate_To_Jobs_Category] ([candidateKey], [jobCategoryKey]) VALUES (6, 223)
INSERT [dbo].[TR_Candidate_To_Jobs_Category] ([candidateKey], [jobCategoryKey]) VALUES (6, 225)


I have to find all the candidates which have the jobCategoryKey = 1245 and only this one. So they may not have one or more other jobCategoryKey or zero jobCategoryKey.

I tried with HAVING, GROUP BY and so on, but I didn't find the right solution.
All the proposed solutions using COUNT may prove to be inefficient as the aggregates will most likely need to be computed for the entire set, unless the optimizer will be able to push the predicate down. There is a more natural solution which IMHO is much clearer, and typically more efficient. It involves wording the question in negative form: 


  "Show me all candidates for category 1245, who have no other
  association with any other category".


This translates to SQL immediately:

SELECT *
FROM Candidates_to_Jobs AS C1
WHERE jobCategoryKey = 1245
      AND 
      NOT EXISTS (
                  SELECT NULL
                  FROM Candidates_to_Jobs AS C2
                  WHERE C1.CandidateKey = C2.CandidateKey
                        AND C2.jobCategoryKey &lt;&gt; 1245
                 );