how to have if else in postgres query code example

Example 1: select if then postgresql

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

Example 2: postgres if else

DO $$
DECLARE
  a integer := 10;
  b integer := 20;
BEGIN 
  IF a > b THEN
	RAISE NOTICE 'a is greater than b';
  END IF;

  IF a < b THEN
	RAISE NOTICE 'a is less than b';
  END IF;

  IF a = b THEN
	RAISE NOTICE 'a is equal to b';
  END IF;
END $$;