.. (לתיקייה המכילה) | ||
In question 2 - what should we answer if the expressions are equivalent? Should we prove it? | |
If the two expressions are equivalent, you should simply answer "Yes" or "Equivalent". Otherwise, show an example of the content of R, S and T and the result of each of the RA expressions. |
In question 5 - can we provide the empty relation as an answer to section א? | |
No, there exists a non-empty content for the relations that satisfies the question's demands. |
In question 5 section ב - how is a semijoin defined between two relations that don't have any common attributes (their schema is completely disjoint)? | |
If R and S don't have any common attributes (for example: R(A,B), S(C,D) ) and S is NOT empty then - R semijoin S = R Otherwise, if S is empty - R semijoin S = empty |
In question 4 - you mentioned that you have changed the diagram and added a company name, but the only section in this question (א), doesn't seem to have anything to do with it - are we missing something here? | |
There was an additional section but we've decided to cancel it. So - the company name indeed doesn't have anything to do with section א' |
In question 3 section ב - can we switch the places of the "source" and the "destination", and then in the first transfer write address A as source and address B as destination, and in the second transfer switch their places? | |
No, each of the arrows that connect the "transfer" relation has a role - one of them is the source and the other one is the destination. So, the values must remain the same places in both of the transfers. |
In question 3 section ג - can a relationship have keys? | |
Yes, in this case the relationship's table will include the following keys: the relationship's keys, and the keys of each of the members in the relationship. |
In an IS-A relationship - can the superclass participate in another relationship R? | |
Yes. In this case the meaning is that each of the subclasses can participate in the other relationship R. |
In question 4 - if we join the table "transfer" with itself, how should we handle the duplicate column names? | |
"address" is the key of apartment, so if two apartments (in this question, "source" and "address") are involved in a "join", we simply denote them as "source.address" and "destination.address" |