Homework #3 FAQ
Do NOT wait until the last minute to do the programming assignments. If
everyone logs into Oracle tuesday night, Oracle will start to slow
down. This might not effect this assignment much, but later assignments
that are much more interactive (queries, etc.) will become frustrating if
Oracle slows down (a query that might normally take 30 seconds may take
Secondly, you want to take some extra time to make sure that the "filler"
data you generate for your database is valid -- make sure that you develop
unique keys as well as values that are duplicated in different tables so
that you can perform joins on the tables. If you don't take the time to do
this now, future assignments will not really be possible...
I have put my schema in a file, but when I ask Oracle to
read it, at the end I get a message of the
form ``input truncated to n characters''
Probably you have left the RETURN off the end of the last line.
Question: The homework says that Oracle isn't being backed up,
and I should save my data. How do I do that?
Answer: Oracle isn't being backed up, but all that means is
that if it crashes, some data may be lost. So don't store your only
copy of vital data there. When you load data using sqlldr, you have to
save the data in a file and the file gets loaded into Oracle. Save
that file in your leland account for the rest of the quarter. If you
are typing in "real" data, keep that real data. This is a good idea
anyway, since you may find it desirable eventually to "reload" your
data into Oracle (ie when you do assignment 4, which asks you to
modify the data in Oracle; to do this multiple times for debugging
purposes you may have to
reload the original data.)
What are the equivalents of group-by and aggregation in relational
There ain't none.
The questions we have asked you on HW3 can be answered in relational algebra
without grouping and aggregation.
The general idea is to first build a relation that is the cross-product of the
relation you want to aggregate with itself.
For instance, suppose we have Sells(bar,beer,price) and we want to
find the minimum price at which beer is sold.
We compute Sells TIMES Sells, and then select so the first of the
prices is less than the second price.
Now, in the second price column we have all the prices that are
We'll let you take it from there.
(Note: this technique works for MIN and MAX, but not for other aggregation
operators. However, it can be used to simulate SQL features like ALL,
ANY, and EXISTS, as well.)
Last modified: Tue Oct 19 16:36:27 PDT 1999