PostgreSQL IF-THEN-ELSE control structure

Question

Why do I always get the following error from Postgres?

syntax error at or near "IF"

I read PostgreSQL: Documentation: 8.3: Control Structures. First I tried to execute a difficult query (with subquery), but then I tried to execute a simple one like this:

IF 2 <> 0 THEN select * from users; END IF;

The error is still the same. What am I doing wrong?

Answer 1

IF 2 <> 0 THEN select * from users; END IF;

You cannot use PL/pgSQL statements outside plpgsql functions. And if this fragment is from plpgsql function, then it is nonsense too. You cannot directly return result of query like T-SQL does.

CREATE OR REPLACE FUNCTION test(p int)
RETURNS SETOF users AS $$
BEGIN
  IF p = 1 THEN
    RETURN QUERY SELECT * FROM users;
  END IF;
  RETURN;
END;
$$ LANGUAGE plpgsql;

When you would get some result from function, you have to use RETURN statement - plpgsql knows only function, it doesn't support procedures - so unbounded SELECT has not sense.

Answer 2

You're not enclosing that PL/pgSQL control structure in an anonymous block or a PL/pgSQL function.

For the SQL version of this control structure see the docs for CASE.

Answer 3

You're not enclosing that PL/pgSQL. They need to be enclosed with anonymous code block. Example for your code:

DO $$ BEGIN

    IF 2 <> 0 THEN select * from users; END IF;

END$$;