SQL & PL/SQL

i am trying to update the 2 colums of a table based on the cursor result set with key values are matching.



DECLARE

CURSOR update_master
IS
SELECT B.cust_num,c.cust_num,
B.cust_name,B.cust_bal
FROM
Table_one B
JOIN
Table_two C
ON B.cust_num= C.cust_num;

BEGIN

FOR master_rec IN update_master

LOOP

UPDATE Table_three
SET col1 = master_rec.cust_name,
Col2 = master_rec.cust_bal,
Where WHERE Cust_num = master.cust_num;

END LOOP;
COMMIT;
END;

The 3 tables are having around 50000 records individually............

It is taking too much of time to execute......more than 6 minutes......

how can we optimize it?? can we write this update statement directly? instead of writing it in procedure?