SQL & PL/SQL

Hello everyone,

I'm trying to write a query where I can update a pastdue_fees column in a book_trans table based on a difference between return_dte and due_dte columns.

I am using Oracle SQL. This is what I have so far for my decode function:

SQL> SELECT
2 DECODE(SIGN((return_dte - due_dte)*2),
3 '-1', '0',
4 '1', '12', 'Null')
5 FROM book_trans;

DECO
----
Null
12
Null
0

So the logic is that if the sign is -1, the value in return_dte column should be 0; if it's +1 then it's 12 and everything else is Null.

So now, I need to enter my decode function into the update statement to update the columns. However, I get error messages.

The logic should be:
UPDATE book_trans SET PastDue_fees = decode(expression)

I've given it a couple of different tries with the following results:

SQL> UPDATE book_trans
2 SET pastdue_fees = SELECT
3 DECODE(SIGN((return_dte - due_dte)*2),
4 '-1', '0',
5 '1', '12', 'Null')
6 FROM book_trans;
SET pastdue_fees = SELECT
*
ERROR at line 2:
ORA-00936: missing expression


SQL> UPDATE book_trans
2 SET pastdue_fees =
3 DECODE(SIGN((return_dte - due_dte)*2),
4 '-1', '0',
5 '1', '12', 'Null')
6 FROM book_trans;
FROM book_trans
*
ERROR at line 6:
ORA-00933: SQL command not properly ended

Any help or tips would be greatly appreciated as I've been taking SQL for about six weeks and not very proficient!

Thanks!