Consider two tables in a relational database
1|A 2|B 1|X 1|Y 2|Z
whose join on the first column is (if we drop the column used for the join)
A|X A|Y B|Z
How is called the converse operation yielding the original tables from the result of the join?
In the general case, we may want to distinguish between the various flavours of joins and to tell the operation which columns belong to the left table and which to the right table. I do not want to go into these details here – unless this is of critical importance to give a name to this operation!