IF-ELSE-THEN:

SELECT field1, field2,
  CASE
    WHEN field1 > 0 THEN field2/field1
    ELSE 0
  END 
  AS field3
  FROM test;

field1 > 0 ? (field2 / field1 ) : 0   --> SQL

IF arg = 'a' THEN     --> plpgsql PL/SQL 
  RETURN 1;
ELSEIF a = 'b' THEN
  RETURN 5;
ELSE 
  RETURN 2;
END IF;


COALESCE  Like (OR ...) IN Lisp
COALESCE(a, b, c ...):
(or a b c ...)  -- Lisp, BUT must be same time
a || b || c     -- JavaScript

NULLIF =>  NULLIF(a, b):  
a == b ? null : a


SELECT COALESCE(NULLIF(y, ''),x), COALESCE(NULLIF(x,''),y),...




CREATE OR replace FUNCTION getf(arg character varying(255)) RETURNS int as $$
BEGIN
  if arg = 'a' THEN
    RETURN 1;
  else 
    RETURN 2;
  END if;
END; $$ LANGUAGE plpgsql  -- <= Note




CREATE OR REPLACE FUNCTION if_else(boolean, anyelement, anyelement)
 RETURNS anyelement LANGUAGE SQL AS
$func$
SELECT CASE WHEN $1 THEN $2 ELSE $3 END
$func$;