By David M. Rothwell
This ebook is a realistic textual content designed to permit the reader to exploit the database INGRES, with the minimal quantity of attempt. It presents the fundamental origin for turning into both knowledgeable person of the process or getting to know database layout. Combining a realistic method with a theoretical realizing, this article permits the reader to turn into educated in INGRES and to appreciate what gains are getting used and why.
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Additional info for An Introduction to Relational Database Theory
The value specified in an INIT clause is commonly called the initial value of the variable in question. It remains that value until it is replaced by subsequent invocation of an update operator such as assignment. A variable is a container or holder for a value. To answer the question posed in the section heading, we can now see that a variable: x x x has a name; has a declared type, namely the type of the values it can legally hold; always “has” a value (of its declared type). The name and declared type remain the same throughout the existence of the variable, but its value can change from time to time, which is of course why it is called a variable.
Consider again the invocation SID('S1'), a literal of type SID. Recall that SID is an operator that, when invoked with a suitable character string, returns a value of type SID; also that every value of type SID can be denoted by some invocation of the operator SID. I have explained that we call such an operator a selector (for values of the type in question). The parameters of a selector correspond to components of what we call a possible representation (possrep for short). I will explain why we use the word “possible” here in just a moment.
In general, a type is used for constraining the values that are permitted to be used for some purpose. In particular, for constraining: x x x x the values that can be assigned to a variable the values that can be substituted for a parameter the values that an operator can yield when invoked the values that can appear for a given attribute of a relation In each of the above cases, the type used for the purpose in question is the declared type of the variable, parameter, operator, or attribute, respectively.
An Introduction to Relational Database Theory by David M. Rothwell