SQL : To get one of the duplicates by using a case statement

Question

I have a table in which there are two columns with duplicates.

id name  classname description 
-----------------------------
1  a     aa        aa:abcd  
2  a     Unknown   Unknown 
3  b     bb        unknown 
4  c     cc        abcd 

Now I have a select query where in I have to filter out all the duplicates and my description is shown as identifier, my result should be like this,

id name identifier
-----------------
1  a   aa
2  b   NULL 
3  c   NULL

where all the description having either without ':' as its char index should display as NULL or Unknown as Null.

I'm using the below select query to filter the duplicates in the 'name' column , but I'm unable to use the same query for description as I'm using case for obtaining result in order to trim my description 'aa: abcd' to aa

    select distinct 
           id,
           (select top 1 name 
             from table1 t 
            where t.name = t1.name 
            order by case t1.classname 
                        when 'Unknown Tag Class' then 0 
                        else 1 
                     end
            ) name,
            (case when charindex(':',Description)> 0
               then substring(Description,1,(charindex(':',Description)-1)) 
            end
            ) as Identifier
    from table1  t1

In the above query I want to modify the case statement of description so that i can filter duplicates and also trim the values like "aa:abcd" to "aa" and put them in identifier column.

Need help on this.


this is the query i am using 

IF  EXISTS (SELECT * FROM sys.objects     WHERE object_id = OBJECT_ID(N'[dbo].[EXEC_REP_TransposedTagAttributes]')
AND type in (N'U'))
BEGIN
   select distinct 
      [Att : 42674] as TagID
       ,Tagname
       ,isnull([Att : 14591],'-') as OriginatingContractor
       ,isnull([Att : 14594],'-') as System
      ,(case when charindex(':',TargetName)> 0 then 
    substring(TargetName,(charindex(':',TargetName)+1),len(TargetName)) 
   end) as SystemDescription
,(case when charindex(':',TagClassDescription)> 0 then 
    substring(TagClassDescription,1,(charindex(':',TagClassDescription)-1)) 
   end) as TagIdentifier
from EXEC_REP_TransposedTagAttributes t1
LEFT JOIN (SELECT SourceName, TargetName FROM EXEC_REP_Associations WHERE AssociationType = '3' and TargetClassName = 'SUB SYSTEM') b ON TagName = b.SourceName
where tagname='ZIH-210053' Order by [Att : 42674]
END
ELSE
 select 'Reporting Database is being refreshed, please wait.' as errMsg

and the result i am geting is

TagID Tagname OriginatingContractor System SystemDescription TagIdentifier

2609005 ZIH-210053 Hyundai Heavy Industries (Topsides) 210 Slugcatcher NULL 2609005 ZIH-210053 Hyundai Heavy Industries (Topsides) 210 Slugcatcher ZIH

there are also rows which have tag identifier as null and donot have duplicates

Answer 1

select id,name,
       CASE WHEN charindex(':',description)>0
         THEN LEFT(description,charindex(':',description)-1) 
         ELSE NULL
       END as identifier

from t as t1
where description like '%:%'
      or NOT EXISTS (select * from t where t.id<>t1.id and t.name=t1.name);

SQLFiddle demo