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The result of the natural join is the set of all combinations of tuples in R and S that are equal on their common attribute names.
The result of the semijoin is only the set of all tuples in R for which there is a tuple in S that is equal on their common attribute names.
The point is that natural join is a set of all combinations and semijoin is only the tuples from the first relation not a combination between the two.
For example:
R1 R2
A B B C
1 2 2 3
1 3 3 4
1 4 5 3
R1 (natural join) R2 =
A B C
1 2 3
1 3 4
whereas R1(semijoin) R2 =
A B
1 2
1 3
So in a way semijoin selects and returns a table of only the tuples from R1 that have an equal attribute with R2
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∙ 13y ago
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