Complex SQL issue in DB2 - sql

Question

I have a table TABLE_CLIENT_BOOK which contains USER_ID and BOOK_CODE. This table shows which all books are with a particular USER_ID.
USER_ID BOOK_CODE
------------------------------
1 123
1 124
1 567
2 123
2 432
3 432
3 567
-------------------------------
I have another table TABLE_BOOK which contains details about each book.
BOOK_CODE DETAILS
----------------------------------
123 abcd
124 pqrs
432 xyzw
567 lmnop
568 efgh
----------------------------------
I want to write a query between the two tables which will spit out USER_ID and BOOK_CODE where BOOK_CODE column should have the id of all books from TABLE_BOOK which that user doesn't have. For e.g. user 1 doesn't have books 432 and 568, user 2 doesn't have 124, 567 and 568 and user 3 doesn't have 123, 124, 568.
So the result table of the query would be:
USER_ID BOOK_CODE
----------------------------
1 432
1 568
2 124
2 567
2 568
3 123
3 124
3 568
-----------------------------
This report is to advertise the books which user doesn't have.
How do I achieve this in SQL in DB2 9 ?
Thanks for reading!

Answer 1

Do a CROSS JOIN to get all user/book combinations. Use NOT EXISTS to exclude already existing combinations:
select distinct tcb.USER_ID, tb.BOOK_CODE
from TABLE_CLIENT_BOOK tcb
cross join TABLE_BOOK tb
where not exists (select * from TABLE_CLIENT_BOOK tcb2
where tcb2.USER_ID = tcb.USER_ID
and tcb2.BOOK_CODE = tb.BOOK_CODE)
order by tcb.USER_ID, tb.BOOK_CODE
Alternatively, EXCEPT:
select tcb.USER_ID, tb.BOOK_CODE
from TABLE_CLIENT_BOOK tcb
cross join TABLE_BOOK tb
EXCEPT
select USER_ID, BOOK_CODE
from TABLE_CLIENT_BOOK
order by tcb.USER_ID, tb.BOOK_CODE
No DISTINCT needed here. EXCEPT removes the duplicates.
Executes as:
SQL>select distinct tcb.USER_ID, tb.BOOK_CODE
SQL&from TABLE_CLIENT_BOOK tcb
SQL& cross join TABLE_BOOK tb
SQL&where not exists (select * from TABLE_CLIENT_BOOK tcb2
SQL& where tcb2.USER_ID = tcb.USER_ID
SQL& and tcb2.BOOK_CODE = tb.BOOK_CODE)
SQL&order by tcb.USER_ID, tb.BOOK_CODE;
USER_ID BOOK_CODE
=========== ===========
1 432
1 568
2 124
2 567
2 568
3 123
3 124
3 568
8 rows found
SQL>select tcb.USER_ID, tb.BOOK_CODE
SQL&from TABLE_CLIENT_BOOK tcb
SQL& cross join TABLE_BOOK tb
SQL&EXCEPT
SQL&select USER_ID, BOOK_CODE
SQL&from TABLE_CLIENT_BOOK
SQL&order by tcb.USER_ID, tb.BOOK_CODE;
USER_ID BOOK_CODE
=========== ===========
1 432
1 568
2 124
2 567
2 568
3 123
3 124
3 568
8 rows found