query result based loop syntax in if condition - postgresql

Question

I am new to PGSQL and trying to start a loop in database function that iterates on the basis of query result as shown below. I am using 8.2 version.
CREATE OR REPLACE FUNCTION demo(text)
RETURNS SETOF activityhistoryview
LANGUAGE plpgsql STABLE
AS $_$
DECLARE
tilldate ALIAS for $1;
actrec revpro_500.activity%ROWTYPE;
BEGIN
IF tilldate != '' THEN
FOR actrec IN
SELECT activity.* from revpro_500.activity WHERE activity.householdid = 950
LOOP
ELSE
FOR actrec IN
SELECT activity.* from revpro_500.activity WHERE activity.householdid = 500
LOOP
END IF;
BEGIN
/* rest code goes here */
END
END LOOP;
RETURN;
END;$_$;
After executing above function, I am getting below error.
ERROR: syntax error at or near "ELSE"
What I am missing here?

Answer 1

You can not nest loop queries like that. Instead, first evaluate what you want to do with tilldate, then make a single loop query:
CREATE OR REPLACE FUNCTION demo(tilldate text) RETURNS SETOF activityhistoryview
LANGUAGE plpgsql STABLE AS $_$
DECLARE
actrec revpro_500.activity%ROWTYPE;
hhid integer;
BEGIN
IF tilldate != '' THEN
hhid = 950;
ELSE
hhid = 500;
END IF;
FOR actrec IN
SELECT * from revpro_500.activity WHERE householdid = hhid
LOOP
BEGIN -- Do you really need a transaction block? If not, remove BEGIN/END
-- rest code goes here
END
END LOOP;
RETURN;
END;$_$;

Answer 2

Like in most languages you can't have your control structures overlapping, so on the line before the ELSE you open a LOOP, but do not close it before the ELSE, so the ELSE doesn't have an attached IF paired to it.
You can put the IF/ELSE block inside the loop, or outside it, but not overlapping.
Example:
-- Good
LOOP
-- some computations
IF tilldate != '' THEN
EXIT; -- exit loop
ELSE
-- some computations
END IF;
END LOOP;
-- Good
IF tilldate != '' THEN
LOOP
-- some computations
END LOOP;
ELSE
LOOP
-- some computations
END LOOP;
END IF;
-- Bad
IF
LOOP
-- some computations
ELSE
-- some computations
END IF;
END LOOP;