What are the consequences of eliminating duplicates in Projection Operator?And yes, why there is no duplicate elimination in real systems, unless specified?
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5Relational algebra deals with sets. Real systems deal with multisets. – Martin Smith Feb 14 '15 at 18:47
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I don't know which "real systems" you're thinking of, but real professional _information_ systems certainly avoid duplication of information. – Erwin Smout Feb 23 '15 at 14:04
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If something is true, then saying it twice won't make it any truer.
The required duplicate elimination was left out of the early implementations of "relational" systems because the engineers were afraid of the performance implications. And even when keywords like DISTINCT were newly introduced / newly supported in an existing system, the algorithms used for it were typically so naive that "performance implications" appeared to be very real and/or inevitable. Things have remained like that ever since.
Erwin Smout
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As Martin suggested, Relational Algebra deals with sets, projection, selection, union, intersection are all SET operations. And sets don't have duplicates. However real world systems take into considerations tuples or multisets, which can have duplicates.
liju
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