While studying database related content, I came across the following:

When table R is vertically partitioned into R1, R2, R3..., Rn, it is said that it can be expressed as R = R1⋈R2⋈R3...⋈Rn. (⋈ is the join symbol)

However, if the join symbol is used without any special conditions, it becomes a Cartesian product, so doesn't that result in much more tuples than in the original table R? Can you please explain why the existing table is represented as a join of a partitioned table?

CodePudding user response：

That operator means natural join. See https://en.m.wikipedia.org/wiki/Relational_algebra:

Natural join (⋈) is a binary operator that is written as (R ⋈ S) where R and S are relations.[2] 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.