SQL Left Outer Join 2 Tables with 2 columns and different conditions for each column

Question

I have a data table "DT" and a mapping table "MT" as follows:

DT table:

ComID - CountryCode
-------------------
1000    US
1001    US
1111    FR
1222    CN
1234    CN
1333    CN

MT table:

CountryCode - Region Name - ComID
----------------------------------
   US        North America   
   US            America    1001
   FR            Europe
   CN            CHINA      
   CN            ASIA       1222 
   CN            ASEAN      1333

I have a view which contains those 2 tables alongside with other mapping tables as well ( Data Table LEFT OUTER JOIN with other Mapping Tables )

What I want is to link the two table based on the country code, and for some exceptional companies I would like to use the ComID.

I have tried few queries but every one of them seems to not deliver the solution. Here is my try of achieving this:

    Select DT.*, MT.., ...    
    FROM   DT LEFT OUTER JOIN
           MT ON DT.CountryCode = MT.CountryCode AND DT .CC_COMPANY_CODE NOT IN ('1001', '1222', '1333') OR
    DT.ComID = MTComID AND DT.ComID IN ('1001', '1222', '1333')
    LEFT OUTER JOIN ... other tables ... LEFT OUTER JOIN ... other tables ...

The previous will return rows where both conditions applies:

CN -- CHINA -- 1234
CN -- ASIA -- 1222
CN -- ASEAN -- 1333

I have tried something like this but it will return a syntax error, the solution should be around this but I couldn't seem to find it.

Select DT.*, MT.., ...    
        FROM   DT LEFT OUTER JOIN
               MT ON DT.CountryCode = MT.CountryCode WHERE (DT .CC_COMPANY_CODE NOT IN ('1001', '1222', '1333')) OR
        ON DT.ComID = MTComID WHERE (DT.ComID IN ('1001', '1222', '1333'))
        LEFT OUTER JOIN ... other tables ...

Answer 1

This is one way to do it.

SELECT DT.CompID,DT.CountryCode,MT.RegionName
    FROM DT 
    JOIN MT 
        ON MT.CompID IS NOT NULL AND MT.CompID = DT.CompID
UNION
SELECT DT.CompID,DT.CountryCode,MT.RegionName
    FROM DT 
    JOIN MT 
        ON MT.CompID IS NULL AND MT.CountryCode = DT.CountryCode
        WHERE NOT EXISTS (SELECT TOP 1 1 FROM MT WHERE CompID = DT.CompID)

SQLFidder

Try not to hard-code the CompID as part of your logic as it will not be future proof.