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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!

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    Isn't it just normalization? – Ordous May 22 '15 at 16:59
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    the opposite of a join is a projection – Steven A. Lowe May 22 '15 at 19:31
  • @StevenA.Lowe: Projection is selecting only specific columns from a relation, but the result is still a single relation. None of the standard relational operations yields more than a single relation as result, so what the OP asks for is something different. – JacquesB May 22 '15 at 19:44
  • @JacquesB Actually I think StephenA.Lowe is right, if we agree to consider projections with a base, since the join is a fibered product. – Michael Le Barbier Grünewald May 22 '15 at 20:16
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If I were given a file with the information you want o break back apaort, ti woudl be part of an ETL operations (exptract, transform, load) hetransaformation step. And yes, you mighte well be normalizing the data depanding on the dat table structure (certainly your example would be). Theload part would come into play when you actually move teh dat to the differnt tables and could involve intermediate steps such as loading to eh parent table, getting the autogenreated ids and tehn loading to the child tables using those ids. It is not necessary that this transformation and load process strictly speaking have completly normalized tables at the end. Normalization happens when you design table structures (Or normally should). Transformation happens when you manipulate the data so that is can fit into those strucures.

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