Considering these 2 tables:
Cd (cd_id, title, no_tracks, date, position, weeks)
Band (band_id, band_name)
The task is: List all the band names and cd titles in Relational Algebra.
I wrote it this way: Π band_name (Band) AND Π title (Cd), however I'm not sure if this is correct.
I was also thinking about Union but I'm unsure.
Yes, Union is the way to go, but popovits' answer isn't quite right. In the Relational Algebra, the two operands must be 'Union Compatible' -- that is have the same-named attributes (and those attributes at the same type) -- ref http://en.wikipedia.org/wiki/Relational_algebra, section 'Set operators'. So:
1) Rename the attributes so that they are the same name.
2) You might have to manipulate the values so that they are the same type. (For example: are fields title and band_name the same length?)
Something to note is that if you have a CD title same as a band_name, you'll only get one tuple. (And that scenario is quite likely with self-titled albums).
Note that the RA is more disciplined than SQL. So:
1) If the columns to a SQL UNION are different-named, SQL takes the names from the left operand.
2) SQL also takes the types per the left operand.
(You could get truncation if title is declared shorter than band_name.)
In SQL you can get repeats in the result for self-titled albums, by using UNION ALL http://en.wikipedia.org/wiki/Set_operations_(SQL). Repeats just aren't allowed in RA, because every relation must be a set.
This is indeed done with a union.
π title(Cd) ∪ π band_name(Band)
