Executing prepared SQL in function without loop returning a table - sql

Question

I have this source here:
CREATE OR REPLACE FUNCTION SWITCHTEST (CHOICE VARCHAR(10))
RETURNS TABLE ( R_COL1 VARCHAR(1024) ,R_COL2 VARCHAR(1024) )
LANGUAGE SQL
MODIFIES SQL DATA
BEGIN
DECLARE SQLSTATE CHAR(5);
DECLARE SELECT1 VARCHAR(1024);
DECLARE L_COL1 VARCHAR(1024);
DECLARE L_COL2 VARCHAR(1024);
SET SELECT1 = 'SELECT TEST, DESCR FROM TESTTAB FETCH FIRST 10 ROWS ONLY';
SET SELECT2 = 'SELECT DESCR, COLOUR FROM TESTTAB FETCH FIRST 10 ROWS ONLY';
IF CHOICE = 'FIRST' THEN
PREPARE S1 FROM SELECT1;
ELSEIF CHOICE = 'SECOND' THEN
PREPARE S1 FROM SELECT2;
ELSE
END IF;
RETURN EXEC(S1);
END#
Calling it like
SELECT * FROM TABLE (SWITCHTEST('FIRST')) #
It should just execute the SQL in the S1 prepared statement and I don't want to use a loop in the function.
I am running DB2 Windows 10.5
Any ideas on how to fix this?
I know that the EXEC(S1) is wrong but I can't find anything on the IBM page that shows how to make this work.
Thank you for your help.
Viking

Answer 1

Using intermediate CREATED GLOBAL TEMPORARY TABLE.
--#SET TERMINATOR #
CREATE GLOBAL TEMPORARY TABLE SWITCHTEST ( R_COL1 VARCHAR(1024) ,R_COL2 VARCHAR(1024)) ON COMMIT PRESERVE ROWS NOT LOGGED#
CREATE OR REPLACE FUNCTION SWITCHTEST (CHOICE VARCHAR(10))
RETURNS TABLE (R_COL1 VARCHAR(1024), R_COL2 VARCHAR(1024) )
LANGUAGE SQL
MODIFIES SQL DATA
BEGIN ATOMIC
DELETE FROM SWITCHTEST;
IF CHOICE='FIRST' THEN
INSERT INTO SWITCHTEST SELECT TEST, DESCR FROM TESTTAB FETCH FIRST 10 ROWS ONLY;
ELSEIF CHOICE='SECOND' THEN
INSERT INTO SWITCHTEST SELECT DESCR, COLOUR FROM TESTTAB FETCH FIRST 10 ROWS ONLY;
END IF;
RETURN SELECT R_COL1, R_COL2 FROM SWITCHTEST;
END#

Answer 2

Not sure if it fits your needs, but you can use a case expression to choose columns for the result table. I don't think fetch first is supported in functions, but you can add that in the call:
CREATE OR REPLACE FUNCTION SWITCHTEST (CHOICE VARCHAR(10))
RETURNS TABLE ( R_COL1 VARCHAR(1024) ,R_COL2 VARCHAR(1024) )
LANGUAGE SQL
READS SQL DATA
RETURN
SELECT CASE CHOICE WHEN 'FIRST' THEN TEST ELSE DESCR END
, CASE CHOICE WHEN 'FIRST' THEN DESCR ELSE COLOUR END
FROM TESTTAB ;
SELECT * FROM TABLE (SWITCHTEST('FIRST'))
FETCH FIRST 10 ROWS ONLY;