barcode generator in c# code project MULTIPLE ASSIGNMENT in Font

Generate QR in Font MULTIPLE ASSIGNMENT

CHAPTER 11 MULTIPLE ASSIGNMENT
QR Code Encoder In None
Using Barcode creator for Font Control to generate, create QR Code JIS X 0510 image in Font applications.
www.OnBarcode.com
Code 128C Generator In None
Using Barcode generator for Font Control to generate, create Code 128 image in Font applications.
www.OnBarcode.com
This latter design requires (among many other things) a database constraint actually a foreign key constraint to say that every supplier number appearing in relvar SS must also appear in relvar ST ( every supplier must have exactly one status ). With the design of Figure 11-1, by contrast, the desired effect is achieved by the primary key constraint on relvar S (a relvar constraint). Thus, if we agree that relvar constraints must be checked immediately, it follows that database constraints must be checked immediately as well. Third, the ability to perform semantic optimization requires the database to be consistent at all times (more precisely, at statement boundaries), not just at transaction boundaries. Note: Semantic optimization is a technique for using integrity constraints to simplify queries in order to improve performance. Clearly, if the constraints aren t satisfied, then the simplifications won t be valid. See the book mentioned at the beginning of this section (An Introduction to Database Systems) for further discussion. Fourth, we don t want every application and the code of every user-defined operator to have to cater for the possibility that the database might be inconsistent. There is a severe loss of orthogonality if a procedure that assumes consistency becomes unsafe to use while constraint checking is deferred. Having presented all of these arguments, we now have to admit that of course the conventional wisdom is that database constraint checks, at least, surely have to be deferred. By way of example, consider again the constraint from the section A Multiple Assignment Example to the effect that supplier S1 and part P1 must never be in different cities, implying among other things that if supplier S1 moves to Paris, then part P1 must move to Paris as well. The conventional solution to this problem is (a) to define the constraint to be deferred (meaning it s checked only at end-of-transaction) and then (b) to wrap the two updates up into a single transaction, like this: BEGIN TRANSACTION ; UPDATE S WHERE S# = S# ('S1') ( CITY := 'Paris' ) ; UPDATE P WHERE P# = P# ('P1') ( CITY := 'Paris' ) ; COMMIT ; In this conventional solution, the constraint is checked at COMMIT, and the database is inconsistent between the two UPDATEs.8 Note in particular that if the transaction were to ask the question Are supplier S1 and part P1 in different cities between the two UPDATEs, it would get the answer yes. And this fact is a major reason why we want to support multiple assignment! With multiple assignment, as we know, we can perform both UPDATEs as a single operation, thus: UPDATE S WHERE S# = S# ('S1') ( CITY := 'Paris' ) , UPDATE P WHERE P# = P# ('P1') ( CITY := 'Paris' ) ; Now no integrity checking is done until both UPDATEs have been done (i.e., until we reach the semicolon ), and there s no way for the transaction to see an inconsistent state of the database between the two UPDATEs, because the notion of between the two UPDATEs now has no meaning.
Barcode Maker In None
Using Barcode creation for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
Create Code-39 In None
Using Barcode drawer for Font Control to generate, create Code 39 Extended image in Font applications.
www.OnBarcode.com
8. The conventional solution therefore can t be expressed in Tutorial D even though we ve pretended for the sake of the example that it can precisely because Tutorial D doesn t support deferred constraints.
Print UPCA In None
Using Barcode creator for Font Control to generate, create UCC - 12 image in Font applications.
www.OnBarcode.com
EAN13 Drawer In None
Using Barcode generation for Font Control to generate, create EAN 13 image in Font applications.
www.OnBarcode.com
CHAPTER 11 MULTIPLE ASSIGNMENT
Barcode Creator In None
Using Barcode encoder for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
Print USD8 In None
Using Barcode maker for Font Control to generate, create USD8 image in Font applications.
www.OnBarcode.com
Assigning to a Variable More Than Once
Quick Response Code Drawer In Objective-C
Using Barcode printer for iPhone Control to generate, create QR Code ISO/IEC18004 image in iPhone applications.
www.OnBarcode.com
Decode QR Code JIS X 0510 In None
Using Barcode reader for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
Recall that assignment to a THE_ pseudovariable (like assignment to any pseudovariable, in fact) is really shorthand for a longer assignment that doesn t involve pseudovariables at all. For example, the assignment THE_Y ( P ) := 5.0 ; is shorthand for the following expanded form: P := POINT ( THE_X ( P ), 5.0 ) ; Suppose now that the initial value of the variable P is the origin, which we will denote by (0.0,0.0) for brevity, and consider the following multiple assignment: THE_X ( P ) := 7.0 , THE_Y ( P ) := 5.0 ; Expanding the two constituent assignments gives: P := POINT ( 7.0, THE_Y ( P ) ) , P := POINT ( THE_X ( P ), 5.0 ) ; What are the semantics of this expanded version Well, suppose we apply the usual rule namely, we evaluate all of the right sides first (i.e., before doing any assignments), and then we go on to perform the actual assignments. Then the net result is that P is assigned the point value (0.0,5.0) not the point value (7.0,5.0) as required; in other words, the assignment to THE_X has had no lasting effect. (To be more precise, it does have an effect in the first of the two constituent assignments, but the second of those assignments then causes the result of the first to be overwritten.) Clearly, what we need to do is collect together the two constituent assignments somehow, such that the result of the first is combined with that of the second instead of being overwritten by it. We ll show how this can be done in the next section. First, however, we want to say a little more regarding the foregoing example specifically. Now, you ve probably realized for yourself that the example in question involves, precisely, a multiple assignment in which the same variable, P, appears as a target more than once. What s more, this state of affairs is exactly what we want; we want to assign to the same variable more than once though, admittedly, we want to assign to different portions of that variable in different constituent assignments (speaking a trifle loosely). Thus, while we might agree with Dijkstra that a multiple assignment of the form X := 1 , X := 2 ; doesn t make much sense, we don t agree that assigning to the same variable more than once is necessarily an error. In particular, therefore, we don t want to impose a syntax rule to that effect. Let s look at a couple more examples. First, here s one involving arrays: VAR IA ARRAY INTEGER [5] INIT ARRAY INTEGER [ 1, 2, 3, 4, 5 ] ; IA[4] := 0 , IA[2] := 8 ;
Creating EAN-13 Supplement 5 In Java
Using Barcode drawer for Android Control to generate, create GS1 - 13 image in Android applications.
www.OnBarcode.com
Barcode Maker In None
Using Barcode creator for Online Control to generate, create Barcode image in Online applications.
www.OnBarcode.com
Barcode Generator In Java
Using Barcode creator for Java Control to generate, create Barcode image in Java applications.
www.OnBarcode.com
Barcode Reader In C#
Using Barcode Control SDK for Visual Studio .NET Control to generate, create, read, scan barcode image in VS .NET applications.
www.OnBarcode.com
Generating Barcode In Java
Using Barcode generation for Android Control to generate, create Barcode image in Android applications.
www.OnBarcode.com
EAN-13 Supplement 5 Drawer In Java
Using Barcode creator for Java Control to generate, create European Article Number 13 image in Java applications.
www.OnBarcode.com
Make Quick Response Code In None
Using Barcode printer for Office Excel Control to generate, create QR Code JIS X 0510 image in Excel applications.
www.OnBarcode.com
Barcode Decoder In Java
Using Barcode Control SDK for Eclipse BIRT Control to generate, create, read, scan barcode image in BIRT reports applications.
www.OnBarcode.com
Scanning GS1-128 In VB.NET
Using Barcode scanner for Visual Studio .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
Draw Code 3 Of 9 In Java
Using Barcode generation for Java Control to generate, create Code-39 image in Java applications.
www.OnBarcode.com
Copyright © OnBarcode.com . All rights reserved.