Suppoe there is table called SOURCE, conatins column NAME and AGE.  The requirement is that to poulate records from SOURCE to the target tale TARGET. In TARGET table contain following columns NAME and CATEGORY. 

Following is the rule for the category selection
  
Age Group     Category 
---------------   ------------
0-17       ->           A
18-30     ->           B
31 - 50   ->           C
51-70     ->            D
71+        ->            E

Once the ETL jobs are done then as a  tester we can validate the data with the combination CASE and MINUS we can validate it as following

/* Source - Target */ 
/* Source*/ 
SELECT 
            NAME,
            (
              CASE 
                       WHEN AGE <18 THEN 'A' 
                       WHEN AGE  BETWEEN 18 AND 30 THEN 'B' 
                       WHEN  AGE BETWEEN 31 AND 50 THEN 'C' 
                       WHEN  AGE BETWEEN 51 AND 70 THEN 'D'              
                ELSE
                          'E' 
                END
              ) AS CATEGORY
    FROM 
             SOURCE     
 MINUS 
 /* Target */ 
      SELECT 
                NAME, 
                CATEGORY          
      FROM 
               TARGET 

Like this do a Target minus Source too. Idealy both the scripts (source -Target) & (Targe - Source) should return as values at all. If it returns any value means something wrong in the Code

Note:- Here i didn't include the source filtering part and Target filter part. Say if you want to take some filter condition that need to b applied in both Source part and target part
 
For Eg:- Time stamp.