Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE OR REPLACE FUNCTION any_class(test TEXT, transpose TEXT, construction TEXT) 
RETURNS TEXT 
AS $$
    SELECT CASE
        WHEN test IN ('a', 'b') THEN 'result1'
        WHEN test IN ('c', 'd') THEN 'result2'
        WHEN test IN ('e', 'f') THEN 'result3'
        WHEN test IN ('g', 'h') THEN 'result4'
        WHEN test IN ('i', 'j') THEN 'result5'
        WHEN test IN ('k', 'l', 'm') THEN 'result6'
        WHEN test IN ('n') THEN 'result7'
        WHEN test IN ('o') THEN 'result8'
        WHEN test IN ('p') THEN 'result9'
        WHEN test IN ('q', 'r', 's', 't', 'u') THEN 'result10'
        ELSE CASE
            WHEN transpose IN ('a', 'b') THEN 'result1'
            WHEN transpose IN ('c', 'd') THEN 'result2'
            WHEN transpose IN ('e', 'f') THEN 'result3'
            WHEN transpose IN ('g', 'h') THEN 'result4'
            WHEN transpose IN ('i', 'j') THEN 'result5'
            WHEN transpose IN ('k', 'l', 'm') THEN 'result6'
            WHEN transpose IN ('n') THEN 'result7'
            WHEN transpose IN ('o') THEN 'result8'
            WHEN transpose IN ('p') THEN 'result9'
            WHEN transpose IN ('q', 'r', 's', 't', 'u') THEN 'result10'
            ELSE CASE
                WHEN construction IN ('a', 'b') THEN 'result1'
                WHEN construction IN ('c', 'd') THEN 'result2'
                WHEN construction IN ('e', 'f') THEN 'result3'
                WHEN construction IN ('g', 'h') THEN 'result4'
                WHEN construction IN ('i', 'j') THEN 'result5'
                WHEN construction IN ('k', 'l', 'm') THEN 'result6'
                WHEN construction IN ('n') THEN 'result7'
                WHEN construction IN ('o') THEN 'result8'
                WHEN construction IN ('p') THEN 'result9'
                WHEN construction IN ('q', 'r', 's', 't', 'u') THEN 'result10'
                END
            END
    END;
$$
LANGUAGE SQL
IMMUTABLE PARALLEL SAFE;
SELECT any_class('z', 'z', 'z')


any_class


null


CREATE OR REPLACE FUNCTION any_class(test TEXT, transpose TEXT, construction TEXT) 
RETURNS TEXT 
AS $any_class$
	
	DECLARE res text;
	BEGIN
	
	CREATE OR REPLACE FUNCTION one_class(val TEXT) 
	RETURNS text 
	AS $one_class$
		SELECT CASE
			WHEN val IN ('a', 'b') THEN 'result1'
			WHEN val IN ('c', 'd') THEN 'result2'
			WHEN val IN ('e', 'f') THEN 'result3'
			WHEN val IN ('g', 'h') THEN 'result4'
			WHEN val IN ('i', 'j') THEN 'result5'
			WHEN val IN ('k', 'l', 'm') THEN 'result6'
			WHEN val IN ('n') THEN 'result7'
			WHEN val IN ('o') THEN 'result8'
			WHEN val IN ('p') THEN 'result9'
			WHEN val IN ('q', 'r', 's', 't', 'u') THEN 'result10'
	  END;
	$one_class$ language sql;

	SELECT COALESCE(one_class(test), one_class(transpose), one_class(construction)) INTO res;
	RETURN res;

	END;
$any_class$ 
language plpgsql;
SELECT any_class('z', 'z', 'k')


any_class


result6