Firebird-concatenating fields that return from a case

SELECT DP.DATA AS DATA, S.DESCRICAO, COUNT(P.CODIGO) AS QNTD,
  (CASE
        WHEN (CAST(SUBSTRING(P.HORAFIM FROM 1 FOR 2) AS INTEGER) =  CAST(SUBSTRING(P.HORAINI FROM 1 FOR 2) AS INTEGER)) THEN
          (CAST(SUBSTRING(P.HORAFIM FROM 1 FOR 2) AS INTEGER) - CAST(SUBSTRING(P.HORAINI FROM 1 FOR 2) AS INTEGER))
    ELSE
      CAST(0 AS INTEGER)
  END) AS HORAS,
  (CASE
    WHEN (CAST(SUBSTRING(P.HORAFIM FROM 4 FOR 2) AS INTEGER) = CAST(SUBSTRING(P.HORAINI FROM 4 FOR 2) AS INTEGER)) THEN
     CAST(0 AS INTEGER)
    WHEN (CAST(SUBSTRING(P.HORAFIM FROM 4 FOR 2) AS INTEGER) > CAST(SUBSTRING(P.HORAINI FROM 4 FOR 2) AS INTEGER)) THEN
     (CAST(SUBSTRING(P.HORAFIM FROM 4 FOR 2) AS INTEGER) - CAST(SUBSTRING(P.HORAINI FROM 4 FOR 2) AS INTEGER))
    ELSE
     (CASE
       WHEN ( (CAST(SUBSTRING(P.HORAFIM FROM 1 FOR 2) AS INTEGER) >  CAST(SUBSTRING(P.HORAINI FROM 1 FOR 2) AS INTEGER))
              AND (CAST(SUBSTRING(P.HORAINI FROM 4 FOR 2) AS INTEGER) > CAST(SUBSTRING(P.HORAFIM FROM 4 FOR 2) AS INTEGER)) ) THEN
       ((CAST(SUBSTRING(P.HORAFIM FROM 4 FOR 2) AS INTEGER)+ 60) - CAST(SUBSTRING(P.HORAINI FROM 4 FOR 2) AS INTEGER))
     ELSE
       (CAST(SUBSTRING(P.HORAFIM FROM 4 FOR 2) AS INTEGER) - CAST(SUBSTRING(P.HORAINI FROM 4 FOR 2) AS INTEGER))
     END)
  END) AS MINUTOS,

  (CASE
   WHEN (CAST(SUBSTRING(P.HORAFIM FROM 7 FOR 2) AS INTEGER) = CAST(SUBSTRING(P.HORAINI FROM 7 FOR 2) AS INTEGER)) THEN
     CAST(0 AS INTEGER)
   WHEN (CAST(SUBSTRING(P.HORAFIM FROM 7 FOR 2) AS INTEGER) > CAST(SUBSTRING(P.HORAINI FROM 7 FOR 2) AS INTEGER)) THEN
     (CAST(SUBSTRING(P.HORAFIM FROM 7 FOR 2) AS INTEGER) - CAST(SUBSTRING(P.HORAINI FROM 7 FOR 2) AS INTEGER))
   ELSE
     (CAST(SUBSTRING(P.HORAINI FROM 7 FOR 2) AS INTEGER) - CAST(SUBSTRING(P.HORAFIM FROM 7 FOR 2) AS INTEGER))
  END) AS SEGUNDOS
FROM DET_PEDIDO DP
INNER JOIN PEDIDO P ON P.PEDIDO = DP.PEDIDO AND P.DATAINI = DP.DATA
INNER JOIN SERVICOS S ON S.CODSERV = DP.CODSERV
WHERE P.DATAINI between '02.01.2020' and '02.01.2020'

I have this query in SQL and I need to concatenate the hours, minutes and seconds field. How do I do this in firebird without using procedure? Also, I couldn't group the same information. Signature Type with econ value.

What I need is Q stay this way: 00:07:48.