select *
case 
 when A= 5  then ('YES' as B 'YES' as C 'YES' as D)
 when A= 6  then ('NO' as B 'NO' as C 'NO' as D)
end
from AAA

-- вариан с одним CASE
SELECT a, yes_no AS b, yes_no AS c, yes_no AS d
  FROM (
        select a, 
               case 
                 when A= 5  then 'YES'
                 when A= 6  then 'NO'
               end as yes_no
          from AAA
       ) just_alias;


-- пишем функции
CREATE OR REPLACE FUNCTION yesno(int, int, int)
RETURNS TEXT AS $$
  -- $1 - value to compare
  -- $2 - YES
  -- $3 - NO
  SELECT CASE WHEN $ 1  = $ 2  THEN 'YES'
              WHEN $ 1  = $ 3  THEN 'NO'
              ELSE 'UNKNOWN'
         END;
$$ LANGUAGE SQL STABLE;

-- проверяем
SELECT a, 
       yesno(a, 5 , 6 ) AS b,
       yesno(a, 5 , 6 ) AS c,
       yesno(a, 5 , 6 ) AS d
  FROM AAA;


-- конкретно твой вариант
CREATE OR REPLACE FUNCTION yesno_56(int)
RETURNS TEXT AS $$
  -- $1 - value to compare
  -- $2 - YES (5)
  -- $3 - NO (6)
  SELECT yesno($ 1 , 5 , 6 );
$$ LANGUAGE SQL STABLE;


-- проверяем
SELECT a, 
       yesno_56(a) AS b,
       yesno_56(a) AS c,
       yesno_56(a) AS d
  FROM AAA;

CREATE OR REPLACE FUNCTION yesno_new(int, int, int, text, text)
RETURNS TEXT AS $$
  -- $1 - value to compare
  -- $4 - yes text
  -- $5 - no text
  SELECT CASE WHEN $ 1  = $ 2  THEN $ 4 
              WHEN $ 1  = $ 3  THEN $ 5 
              ELSE 'UNKNOWN'
         END;
$$ LANGUAGE SQL STABLE;



SELECT a, 
       yesno_new(a,  5 ,  6 , 'да, да я 5!!!', 'почему я 6 ???') AS b,
       yesno_new(a,  7 ,  8 , 'счастливая 7', '8 рулез!') AS c,
       yesno_new(a,  112345 ,  2042342345 , 'много, однако', 'я таких чисел не знаю') AS D
  FROM AAA;