Postgresql Simple IF ELSE Statement - postgresql

Question

In MS SQL I can execute a statement like:
Declare #foo int = 1, #foo1 int = 42
IF #foo <> 0
BEGIN
SELECT #foo
END
ELSE
BEGIN
SELECT #foo, #foo1
END
Does anyone have any idea how to run this statement on postgresql?
EDIT: MS SQL Example like :
CREATE PROCEDURE dbo.spIFtest
#p1 int = 1,
#p2 int = 10,
#isFilter bit = 0
AS
BEGIN
IF #isFilter = 1
BEGIN
SELECT idx FROM rw.octest where idx between #p1 and #p2
END
ELSE
BEGIN
SELECT idx FROM rw.octest
END
END
GO

Answer 1

Using DO With caveats:
DO $$
DECLARE
foo integer := 1;
foo1 integer := 42;
BEGIN
IF foo <> 0 THEN
PERFORM foo;
ELSE
PERFORM foo, foo1;
END IF;
END;
$$
;
DO cannot return anything.
You can fake a return:
DO $$
DECLARE
foo integer := 0;
foo1 integer := 42;
BEGIN
IF foo <> 0 THEN
SELECT INTO foo 1;
RAISE NOTICE 'foo is %', foo;
ELSE
SELECT INTO foo, foo1 1, 42 ;
RAISE NOTICE 'foo is %, foo1 is %', foo, foo1;
END IF;
END;
$$
;
NOTICE: foo is 1, foo1 is 42
DO

Answer 2

In PostgreSQL DO Block can execute the queries but they can not return any value.
So the first part of your question is not possible directly in postgresql.
For second part of your question: In PostgreSQL you can use Function (which is very powerful and effective) like below:
create or replace function spiftest()
returns table(idx_ int)
as $$
declare
p1 int := 1;
p2 int := 10;
isfilter boolean := 0;
begin
if isfilter then
return query
SELECT idx FROM octest where idx between p1 and p2;
else
return query
SELECT idx FROM octest ;
end if;
end;
$$
language plpgsql
calling above function for result:
select * from spiftest()
You can write it with parameters also like below:
create or replace function spiftest(p1 int, p2 int, isfilter boolean)
returns table(idx_ int)
as $$
begin
if isfilter then
return query
SELECT idx FROM octest where idx between p1 and p2;
else
return query
SELECT idx FROM octest ;
end if;
end;
$$
language plpgsql
to call above function
select * from spiftest(1,10,'t')