Question

The semijoin of relations R and S \[R \ltimes S\] is the set of tuples t in R such that there is at least one tuple in S that agrees with t in all attributes that R and S have in common. Give three different expressions of relational algebra that are equivalent to \[R \ltimes S\]

Answer 1

Here's one:\[\pi_{a_1,a_2,a_3,...,a_n}(R \bowtie S)\]where a_1,...,a_n are attributes of R

Answer 2

Of course you can replace the natural join by Codd's primitives. That's another possible way.

Answer 3

Just as an aside, I learned SQL before I learned relational algebra so seeing terms like "semijoin" are still weird to me. heh.