Databases Reference
In-Depth Information
as indicated in Fig. 3.2 (which shows, of course, values for those relvars corresponding to the sample value shown
for relvar S in Fig. 3.1).
SNC CT
┌─────┬───────┬────────┐ ┌────────┬────────┐
│ SNO │ SNAME │ CITY │ │ CITY │ STATUS │
├═════┼───────┼────────┤ ├════════┼────────┤
│ S1 │ Smith │ London │ │ Athens │ 30 │
│ S2 │ Jones │ Paris │ │ London │ 20 │
│ S3 │ Blake │ Paris │ │ Paris │ 30 │
│ S4 │ Clark │ London │ └────────┴────────┘
│ S5 │ Adams │ Athens │
└─────┴───────┴────────┘
Fig. 3.2: Relvars SNC and CT—sample values
Points arising from this example:
First, the decomposition certainly eliminates the redundancy—the fact that a given city has a given status
now appears exactly once.
Second, the decomposition process is basically a process of taking projections —the relations shown in
Fig. 3.2 are each projections of the relation shown in Fig. 3.1. 2 In fact, we can write a couple of equations:
SNC = S { SNO , SNAME , CITY }
CT = S { CITY , STATUS }
(Recall from Chapter 2 that the Tutorial D syntax for projection takes the form r { A1 ,..., An }, where r is some
relational expression and A1 , ..., An are attribute names.) 3
Third, the decomposition process is nonloss (also called lossless )—no information is lost in the process,
because the relation shown in Fig. 3.1 can be reconstructed by taking the join of the relations shown in
Fig. 3.2:
S = JOIN { SNC , CT }
( Tutorial D syntax again.) Thus, we can say the relation in Fig. 3.1 and the pair of relations in Fig. 3.2 are
information equivalent —or, to state the matter more precisely, for any query that can be performed against
the relation of Fig. 3.1, there's a corresponding query that can be performed against the relations of Fig. 3.2
2 Other kinds of decomposition are possible, but I'll assume until further notice that “decomposition,” unqualified, means decomposition via
projection specifically.
3 Tutorial D also supports syntax of the form R {ALL BUT B1 ,..., Bm }, which denotes the projection of r on all of its attributes except B1 , ..., Bm .
For example, the projection corresponding to SNC in the example could alternatively be expressed thus: S {ALL BUT STATUS}.
Search WWH ::

Custom Search