text: SELECT
    C.C_CUSTKEY,
    C.C_NAME,
    SUM(L.L_EXTENDEDPRICE * (1 - L.L_DISCOUNT)) AS REVENUE,
    C.C_ACCTBAL,
    N.N_NAME,
    C.C_ADDRESS,
    C.C_PHONE,
    C.C_COMMENT
  FROM
    customer C,
    orders O,
    lineitem L,
    nation N
  WHERE
    C.C_CUSTKEY = O.O_CUSTKEY
    AND L.L_ORDERKEY = O.O_ORDERKEY
    AND O.O_ORDERDATE >= DATE '1994-03-01'
    AND O.O_ORDERDATE < DATE '1994-03-01' + INTERVAL '3' MONTH
    AND L.L_RETURNFLAG = 'R'
    AND C.C_NATIONKEY = N.N_NATIONKEY
  GROUP BY
    C.C_CUSTKEY,
    C.C_NAME,
    C.C_ACCTBAL,
    C.C_PHONE,
    N.N_NAME,
    C.C_ADDRESS,
    C.C_COMMENT
  ORDER BY
    REVENUE DESC
  LIMIT 20;
  