1 CREATE OR REPLACE FUNCTION f_week_to_date(a_week varchar2) RETURN CHAR IS  
 2   v_first_date   char(10);  
 3   v_date_of_week number(1);  
 4   
 5 BEGIN  
 6   select to_char(to_date(substr(a_week, 1, 4) || '0101', 'yyyymmdd'), 'D')  
 7     into v_date_of_week  
 8     from dual;  
 9   v_date_of_week := v_date_of_week - 1;  
10   if v_date_of_week <= 4 then  
11     select TO_CHAR(TO_DATE(SUBSTR(a_week, 1, 4) || '0101', 'yyyymmdd') +  
12                    SUBSTR(a_week, 5, 2) * 7 - 7 - v_date_of_week + 1,  
13                    'yyyy-mm-dd')  
14       into v_first_date  
15       from dual;  
16   else  
17     select TO_CHAR(TO_DATE(SUBSTR(a_week, 1, 4) || '0101', 'yyyymmdd') +  
18                    SUBSTR(a_week, 5, 2) * 7  - v_date_of_week + 1,  
19                    'yyyy-mm-dd')  
20       into v_first_date  
21       from dual;  
22   end if;  
23   
24   return v_first_date;  
25   
26 END;