Remove duplicate value when we use rollup
You can use a UNION:




SELECT open_bal, closing_bal, balance
FROM `billing_account`
UNION
SELECT NULL, NULL, sum(closing_bal)
FROM `billing_account`



open_bal | closing_bal |  balance
-------: | ----------: | -------:
 5005.00 |     3005.00 |  8005.00
 5004.00 |     3004.00 |  8004.00
 5003.00 |     3003.00 |  8003.00
 5002.00 |     3002.00 |  8002.00
 5001.00 |     3001.00 |  8001.00
  500.00 |      300.00 |   800.00
    null |        null | 15315.00



dbfiddle here