By David M. Rothwell
This ebook is a realistic textual content designed to allow the reader to take advantage of the database INGRES, with the minimal volume of attempt. It offers the fundamental origin for turning into both a professional person of the procedure or gaining knowledge of database layout. Combining a realistic process with a theoretical knowing, this article permits the reader to turn into trained in INGRES and to appreciate what gains are getting used and why.
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Additional resources for An Introduction to Relational Database Theory
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. 5 a value of type SID. The variable SN is an example of a scalar variable in Tutorial D.
This literal is an invocation of the operator whose name happens to be the same as that of the type for student identifiers, SID. This operator has a single parameter whose declared type, CHAR, is that of the representation (character strings) chosen for student identifiers. In this invocation the CHAR literal 'S1' denotes the argument corresponding to that parameter. The result of the invocation is not a character string but a value of type SID. We call the operator SID a selector, because it can be used to “select” any value of type SID.
8: x x THE_C (SN) denotes the value of the C component of the current value of SN under the possrep (SID) declared for type SID. LEFT ( THE_C (SN), 1 ) therefore denotes the leftmost character of that sting, which will of course be the letter S. || is the concatenation operator for strings. LEFT ( THE_C (SN), 1 ) || '5' concatenates the letter S with the digit 5 to produce the string 'S5'. 7 this string is then operated on by the SID selector to yield the corresponding value of type SID, which is assigned to SN.