CODEWITHSUNDEEP
relational-databaserelational-algebra
Mikkel bruun
Mikkel bruun

Reputation: 139

Selecting from relation based on other relation

Given a relation A(a,b,c), and a relation B(a,d,e), using projection to isolate 'a' in 'B' like so 'B_=projection_{a}(B)', is there a way to exclude all tupples in 'A', that does not have an 'a' in common with 'B'?

Note that i'm only using relational algebra, and not the extended version.

Upvotes: 0

Views: 69

Answers (1)

AntC
AntC

Reputation: 2806

is there a way to exclude all tupples in 'A', that does not have an 'a' in common with 'B'?

That's a double negative: "exclude ... not ...". Let's turn it into a positive:

"Show all tuples in A that do have an a in common with B."

That is, you want a subset of the tuples. It's important here that a is the only attribute name in common between the two relations. Then here's a start with Natural Join.

A ⋈ B

That will produce a result with all attributes {a, b, c, d, e}. Not yet what you want, so you're on the right track with a projection. I'll use Codd's original operator (π). There's two ways; these are equivalent:

A ⋈ (π{a}( B ))            // take just {a} from B
π{a, b, c}( A ⋈ B )        // take {a, b, c} from the result of Join

It's a commonly-needed operation, so there's also a shorthand amongst the "extended" set of operators, to avoid that projection, called (left) SemiJoin

A ⋉ B

Also called 'Matching'.

Upvotes: 2

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