barcode generator in c# code project DATA REDUNDANCY AND DATABASE DESIGN in Font

Encoder QR in Font DATA REDUNDANCY AND DATABASE DESIGN

CHAPTER 12 DATA REDUNDANCY AND DATABASE DESIGN
Print QR Code ISO/IEC18004 In None
Using Barcode creator for Font Control to generate, create QR-Code image in Font applications.
www.OnBarcode.com
Encode PDF 417 In None
Using Barcode creation for Font Control to generate, create PDF-417 2d barcode image in Font applications.
www.OnBarcode.com
6-7
Encode QR In None
Using Barcode creator for Font Control to generate, create QR image in Font applications.
www.OnBarcode.com
Code 39 Generation In None
Using Barcode encoder for Font Control to generate, create USS Code 39 image in Font applications.
www.OnBarcode.com
6 6PLWK 6PLWK -RQHV -RQHV
Encoding EAN 128 In None
Using Barcode maker for Font Control to generate, create UCC-128 image in Font applications.
www.OnBarcode.com
Barcode Creator In None
Using Barcode generation for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
0DWK 3K\VLFV 0DWK 3K\VLFV
Generating UPC-A In None
Using Barcode generator for Font Control to generate, create UPC Symbol image in Font applications.
www.OnBarcode.com
Creating USD - 8 In None
Using Barcode generator for Font Control to generate, create Code11 image in Font applications.
www.OnBarcode.com
7 3URI 3URI 3URI 3URI :KLWH *UHHQ :KLWH %URZQ
Make QR In VS .NET
Using Barcode generator for Visual Studio .NET Control to generate, create QR Code JIS X 0510 image in .NET applications.
www.OnBarcode.com
Painting QR Code In Java
Using Barcode generation for Java Control to generate, create QR Code image in Java applications.
www.OnBarcode.com
CHAPTER 12 DATA REDUNDANCY AND DATABASE DESIGN
Recognizing Barcode In Java
Using Barcode Control SDK for BIRT reports Control to generate, create, read, scan barcode image in BIRT reports applications.
www.OnBarcode.com
GS1-128 Creator In Objective-C
Using Barcode drawer for iPad Control to generate, create EAN / UCC - 14 image in iPad applications.
www.OnBarcode.com
Normalization Is Not Enough
UCC - 12 Generator In VS .NET
Using Barcode maker for .NET framework Control to generate, create Universal Product Code version A image in .NET applications.
www.OnBarcode.com
Generating QR-Code In None
Using Barcode encoder for Word Control to generate, create QR Code 2d barcode image in Office Word applications.
www.OnBarcode.com
The message in the title of this section should already be clear from the SJT example. However, I want to make two further observations as part of the same overall point. The first is this: Contrary to popular misconception, normalization is certainly not enough to eliminate redundancy entirely (even if we ignore the preserve dependencies principle) by definition, the best it can do is eliminate redundancies that can be eliminated by means of nonloss decomposition. Here s a counterexample, again taken from An Introduction to Database Systems. We re given a relvar CTXD, with attributes COURSE, TEACHER, TEXT, and DAYS, with predicate: Teacher TEACHER spends DAYS days with text TEXT on course COURSE. Figure 12-3 shows a sample value for this relvar.
Generating Code128 In .NET Framework
Using Barcode maker for Reporting Service Control to generate, create Code-128 image in Reporting Service applications.
www.OnBarcode.com
Code 128 Generation In None
Using Barcode printer for Word Control to generate, create Code128 image in Word applications.
www.OnBarcode.com
&7;'
Making GTIN - 128 In Java
Using Barcode maker for Eclipse BIRT Control to generate, create GS1 128 image in BIRT reports applications.
www.OnBarcode.com
Decode QR Code 2d Barcode In None
Using Barcode reader for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
&2856( 3K\VLFV 3K\VLFV 3K\VLFV 3K\VLFV 0DWK 0DWK 0DWK
Recognizing Data Matrix ECC200 In Visual Basic .NET
Using Barcode recognizer for .NET framework Control to read, scan read, scan image in Visual Studio .NET applications.
www.OnBarcode.com
Recognizing Barcode In None
Using Barcode decoder for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
7($&+(5 3URI 3URI 3URI 3URI 3URI 3URI 3URI *UHHQ *UHHQ %URZQ %URZQ *UHHQ *UHHQ *UHHQ
7(;7 %DVLF 0HFKDQLFV 3ULQFLSOHV RI 2SWLFV %DVLF 0HFKDQLFV 3ULQFLSOHV RI 2SWLFV %DVLF 0HFKDQLFV 9HFWRU $QDO\VLV 7ULJRQRPHWU\
'$<6       
5. Actually it s in sixth normal form (6NF) as well. See Example 11 in the section Other Kinds of Redundancy, later in this chapter.
CHAPTER 12 DATA REDUNDANCY AND DATABASE DESIGN
Now I turn to my second point. Consider the following decomposition of relvar S: SNT { S#, SNAME, STATUS } KEY { S# } STC { S#, STATUS, CITY KEY { S# } }
Sample values are shown in Figure 12-4. As the figure shows, this decomposition is hardly very sensible (in particular, the fact that any given supplier has a given status appears twice), and yet it satisfies all of the normalization principles both projections are in 5NF, the decomposition is nonloss (in the strong sense of satisfying the fourth principle as well as the first), and dependencies are preserved.
6 6 6 6 6 6
61$0( 6PLWK -RQHV %ODNH &ODUN $GDPV
67$786     
67&
6 6 6 6 6 6
67$786     
&,7< /RQGRQ 3DULV 3DULV /RQGRQ $WKHQV
Orthogonality (I)
Intuitively, the problem with the design illustrated in Figure 12-4 is clear: It has the property that (to use an obvious shorthand notation for tuples) the tuple <s#,n,st> appears in SNT if and only if the tuple <s#,st,c> appears in STC; equivalently, the tuple <s#,st> appears in the projection of SNT on S# and STATUS if and only if that very same tuple <s#,st> appears in the projection of STC on S# and STATUS meaning the projections in question are equal. In other words, the design implies some redundancy. So the principle we re looking for is obviously going to say something along the lines of: Don t do that! Here s a first attempt: Let A and B be distinct projections arising from nonloss decomposition of some relvar R. Then there should not exist nonloss decompositions of A and B into projections A1, A2, ..., Am and B1, B2, ..., Bn, respectively, such that some Ai (i = 1, 2, ..., m) is identically equal to some Bj (j = 1, 2, ..., n).
CHAPTER 12 DATA REDUNDANCY AND DATABASE DESIGN
Or equivalently (my reason for the following restatement should become clear in the section Orthogonality (II) later in this chapter): Let A and B be distinct projections arising from nonloss decomposition of some relvar R. Then there should not exist nonloss decompositions of A and B into projections A1, A2, ..., Am and B1, B2, ..., Bn, respectively, such that a given tuple appears in some Ai (i = 1, 2, ..., m) if and only if it appears in some Bj (j = 1, 2, ..., n). Now, we could think of this principle, if we liked, as a fifth principle of normalization ; in fact, however, it s really just a special case or a logical consequence, rather of The Principle of Orthogonal Design, which I ll be discussing in detail in the section Orthogonality (II). Be that as it may, it should be obvious right away that there s no need to limit our attention to the situation in which A and B are obtained via nonloss decomposition from the same relvar R it applies to pairs of relvars in general. More generally, therefore, we have: Let A and B be distinct relvars. Then there should not exist nonloss decompositions of A and B into projections A1, A2, ..., Am and B1, B2, ..., Bn, respectively, such that a given tuple appears in some Ai (i = 1, 2, ..., m) if and only if it appears in some Bj (j = 1, 2, ..., n). As an aside, it s interesting to note that Codd himself attempted to deal with the kind of redundancy under discussion here in his very first two papers. In the 1969 paper, he said this: A set of relations is strongly redundant if it contains at least one relation [that] is derivable from the rest of the [relations in the set]. And he tightened up this definition somewhat in the 1970 paper: A set of relations is strongly redundant if it contains at least one relation that possesses a projection [that] is derivable from other projections of relations in the set. I should explain that when Codd says a relation r is derivable from a set S of relations, he means r is equal to the result of applying some sequence of relational operations join, projection, and so forth to relations from S. I do have a few comments on his definitions, however: First, the term relation should be replaced by the term relvar throughout (of course, this latter term wasn t introduced until many years later, and Codd never used it at all). Second, I think we can ignore the qualifier strongly; Codd was distinguishing between strong redundancy and something he called weak redundancy, but I don t think that distinction is relevant for present purposes (I mean, I don t think we need to consider weak redundancy here at all). The 1969 definition is subsumed by the 1970 definition, of course, because as we know every relvar R is identically equal to a certain projection of R namely, the identity projection.
Copyright © OnBarcode.com . All rights reserved.