PostgreSQL has a different syntax from MySQL for creating a function, but generally works the same way, as shown in the following query: 

CREATE FUNCTION hittinglevel(g_all SMALLINT)
 RETURNS VARCHAR(10)
 AS $hitlevel$
 DECLARE hitlevel varchar(10);
 BEGIN
 IF g_all BETWEEN 0 and 10 THEN
 SET hitlevel = 'barely any';
 ELSEIF g_all BETWEEN 11 and 50 THEN
 SET hitlevel = 'some';
 ELSEIF g_all BETWEEN 51 and 100 THEN
 SET hitlevel = 'many';
 ELSEIF g_all > 100 THEN
 SET hitlevel = 'tons';
 END IF;
 RETURN hitlevel;
 END;
 $hitlevel$
 LANGUAGE plpgsql;

You can call the function and drop the function the same way that you do in MySQL.