relational algebra, am I correct?

StackOverflow https://stackoverflow.com/questions/22137801

  •  19-10-2022
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Question

I doing some relational algebra and kinda hit with this question, I did it, but doubt I have done it correctly. So these are the 3 relations:

Musicians: ssn, name, annualIncome
Instruments: instrID, iname, key
Plays: ssn, instrID

The questions is find instrID played by a musician name John.

What I did was:

T1 = proj  [condition ssn] ( select [name = John] Musician)
T2 = Plays natural joins Instruments

Result = proj [instruID] (T1 natural joins T2)

It seems like I get every John that play the instruments instead of one specific john... how can I pick one john using relational algebra?

Was it helpful?

Solution

The underlying assumption is that there is only one musician called John: otherwise the question does not seem to make sense.

proj[instrID] (proj[ssn]( select[name = John] Musician) naturalJoin Plays)

should answer the question. You do not need table Instruments since you can get the instrID already from Plays.

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