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Database in Depth by C.J. Date

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Chapter Seven. Database Design Theory

The goal of data independence has the direct implication that logical and physical database design are different disciplines: logical design is concerned with what the database looks like to the user, and physical design is concerned with how the logical design maps to physical storage. The primary focus of this chapter is on logical design, and I'll use the unqualified term design to mean logical design specifically, until further notice.

One point I want to stress right away is this. Recall that "the" relvar constraint for relvar R can be regarded as the system's approximation to the predicate for R; recall too that the predicate for R is the intended interpretation, or meaning, for R. It follows that constraints and predicates are highly relevant to the business of logical design! Indeed, logical design is, in essence, precisely a process of pinning down the predicates as carefully as possible and then mapping those predicates to relvars and constraints. Of course, those predicates are necessarily somewhat informal (they're what some people like to call "business rules"); by contrast, the relvar and constraint definitions are necessarily formal.

Incidentally, the foregoing state of affairs explains why I'm not much of a fan of entity/relationship (E/R) modeling and similar pictorial methodologies. The problem with E/R diagrams and similar pictures is that they're completely incapable of representing all but a few rather specialized constraints. ...

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