CODEWITHSUNDEEP
relational-algebra
Raj Kona
Raj Kona

Reputation: 9

Relational algebra statement to eliminate duplicates

Given the following table schema

customer (name: string, credit: integer)
allowance (no: string, type: string)
asker(cname: string, lno: string)

asker.cname and asker.lno are foreign keys referencing customer, respectively allowance , whose keys are name, respectively no (number)

I am trying to write the relational algebra for the query to find pairs of names of customers who share the same allowance. Avoid listing a customer with himself (Tim, Tim) and avoid listing the same over and over (ex. (Tim, Jane) and (Jane, Tim) should be one)

I have tried is: 

 ρ(Cust1, π no (allownace)
 ρ(Cust2, π no (allownace)
 π name, name((Cust1 ∩ Cust2)(customer))

I believe this is incorrect. Specially, I am having trouble where I need to find customers with the same allowance and also avoiding the customer himself and repeating.

Upvotes: 0

Views: 3353

Answers (1)

philipxy
philipxy

Reputation: 15118

Eliminate (Tim, Tim) via σ (name1 <> name2). (The relation(ship) is non-reflexive.)

Eliminate just one of (Tim, Jane) & (Jane, Tim) but not (Tim, Tim) via σ (name1 op name2) where op is one of <= or >=. (The relation(ship) is anti-symmetric.)

Eliminate both cases by σ (name1 op name2) where op is one of < or >. (The relationship is non-reflexive and anti-symmetric.)

(Re querying & algebra see this answer.)

Upvotes: 1

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