I did `SELECT` something as ' VARIABLE ', like this

```
SELECT  
    SUM(CASE WHEN BA_Idx = 503 THEN BIL_Debit ELSE 0 END) AS 'SUM_DEBIT_503',
    SUM(CASE WHEN BA_Idx = 504 THEN BIL_Debit ELSE 0 END) AS 'SUM_DEBIT_504'
FROM    
    Bill
WHERE   
    (@MEM_Idx = 0 OR MEM_Idx = @MEM_Idx) AND
    (LEFT(BIL_YYMMDD,7) = @BILM_YYMM) AND           
    BIL_IsValid = 'True'

SELECT @BILFC_TotalLaborCost = SUM_DEBIT_503 + SUM_DEBIT_504

```

I want to approach the VARIABLE(**'SUM\_DEBIT\_503', 'SUM\_DEBIT\_504'**) for calculating `@BILFC_TotalLaborCost`. 

How can I approach?