Hi all,

I'm just learning about relational algebra & I'm trying to figure out what the purpose of all the operations is. I have the answer for some of them but not all - can you help me fill the blanks please?

Here's what I have so far:

Select to select tuples that satisfy a given predicate
Project to pick/eliminate specified attributes, eg. to eliminate duplicate columns
Union to find tuples that appear in either or both of two relations
(use Projection first, to get required columns from each relation, then Union on the two resulting relations)
Difference to find tuples that are in one relation but are not in another
(use Projection first, to get the required columns from each relation, then Difference on the two resulting relations)
Intersect to find tuples that are in both relations
(is not a fundamental operation, but is a more convenient way to write r - (r - s) )
(use Projection first, to get the required columns from each relation, then Intersect on the two resulting relations)
Product to pair up rows from two relations (to find what?)
Join to combine information from two or more relations so data from the relations can be used to extract information
Natural Join (what for?)
Theta Join (what for?)
Equi-Join (what for?)
Outer Join (what for?)
Semi-Join (what for?)
Divide (what for?)

I hope this isn't too big an ask for the forum; I've looked everywhere but I can't formulate an answer - please help!!

Thanks,

Wannabe

Are you not atually learning on a course with a tutor to ask??

Yes it is a University course but it's by correspondence & I can't get the tutor to reply to my e-mails.