Отсортируй по Client.
Если так не подходит, то
SELECT * FROM Temp_Rel_Proc, где

Temp_Rel_Proc процедура:

CREATE PROCEDURE TEMP_REL_PROC
RETURNS (
CLIENT INTEGER,
DAT INTEGER,
SUM_R INTEGER,
SUM_U INTEGER)
AS
DECLARE VARIABLE Temp_Client INTEGER;
BEGIN
FOR SELECT DISTINCT( Client ) FROM Temp_Tbl
INTO :Temp_Client
DO BEGIN
Client = Temp_Client;
Dat = NULL;
Sum_r = NULL;
Sum_u = NULL;
SUSPEND;
Client = NULL;
FOR SELECT Dat, Sum_r, Sum_u FROM Temp_Tbl
WHERE Client = :temp_Client
INTO :Dat, :Sum_R, :Sum_u
DO
SUSPEND;
END
END

Будет выглядеть так:

CLIENT DAT SUM_R SUM_U
27
34 45 0
30
11 22 22
32 34 34