Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?

Lauri Pietarinen wrote:
>
>what is your take on Garcia-Molina, Ullman and Jennifer Widom
>regarding their stand on duplicates?
>
>(see http://www.dbdebunk.com/cjddtdt.htm and cjddtdt2)
>
>Are they just realists accepting that SQL is the de facto
>standard, so one might as well take it as a basis?

There are two separate questions here:
1. Do we want duplicates in the data model, i.e., in the original relations

   and the results of queries?
2. Do we want duplicates in intermediate results?

I'm not completely sure what their answer to 1. is but I suspect it is something like "probably not". But how your algebra looks depends on how you answer question 2, because query optimization is the main raison d'etre of the algebra, and there it is a completely different story. It can for example be more efficient to postpone duplicate elimination. If you don't have a bag algebra you cannot express this in your algebra.

Note that in the writings you mention Date only addresses the first question, where what you actually asked concerned mostly the second question.

• Jan Hidders