New PDF release: An Introduction to Relational Database Theory

By David M. Rothwell

This ebook is a realistic textual content designed to let the reader to take advantage of the database INGRES, with the minimal volume of attempt. It presents the fundamental origin for turning into both a professional person of the method or getting to know database layout. Combining a realistic method with a theoretical figuring out, this article permits the reader to turn into knowledgeable in INGRES and to appreciate what good points are getting used and why.

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Sample text

The box at the top of the upper pane, labelled “Location:”, identifies the directory containing the database you are working with. You can switch to another directory by clicking on the little button to the right of the box, labelled with three dots (…). 3. Type the following into the lower pane: output 2+2 ; Execute what you have typed, either by clicking on Evaluate (F5) shown at the bottom of the window or by pressing F5. Now delete the semicolon and try executing what remains. ) You will see how Rel handles errors.

2 is a picture of part of the function represented by the read-only operator +. 2 represents the predicate a + b = c. The relation attributes a and b can be considered as the parameters of the operator +. Each tuple maps a pair of values substituted for a and b to the result of their addition, which is substituted for c. The relation is a function because each unique pair maps to exactly one c valueno two tuples with the same a value also have the same b value, so, given an a and a b, so to speak, we know the (only) resulting c.

4. This exercise is merely to alert you to a certain awkwardness in Rel that has no real importance but might cause you to waste a lot of time if you are not warned about it. 0. Look closely at what happens. It doesn't work! Rel, like some other languages, treats INTEGER and RATIONAL as distinct types. If you want to do arithmetic on rational numbers, both operands must be rational numbers. Literals denoting rational numbers are distinguished from those denoting integers by the presence of a decimal point, and Rel follows the convention in the English-speaking community of using a full stop for this purpose (as opposed to the comma that is used throughout most of Europe, for example).

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