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CHAPTER 11 MULTIPLE ASSIGNMENT
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Second expansion: BEGIN DECLARE TEMP ROW ( F1 SET TEMP = ROW ( 7.0, SET P = P.X ( TEMP.F1 SET P = P.Y ( TEMP.F2 END ; ..., F2 ... ) ; 5.0 ) ; ) ; ) ;
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The first assignment to P here sets the X coordinate to seven, leaving the Y coordinate unchanged; the second then sets the Y coordinate to five, leaving the X coordinate unchanged. The final result is thus that P contains the point with coordinates (7.0,5.0), as desired. Note carefully that, in contrast with the analogous example in the body of the chapter, we do not have to collect together the two constituent assignments in order to achieve the desired result. Why not Well, in order to answer this question, it helps to examine a more complex example: CREATE TYPE ELLIPSE AS ( A INTEGER, B INTEGER, CTR POINT ) ; DECLARE E ELLIPSE ; SET ( E.A, E.B ) = ROW ( 7, 3 ) ; For brevity, let s combine the two expansions this time. The overall result looks like this: BEGIN DECLARE TEMP ROW ( F1 SET TEMP = ROW ( 7, 3 SET E = E.A ( TEMP.F1 SET E = E.B ( TEMP.F2 END ; ..., F2 ... ) ; ) ; ) ; ) ;
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Now suppose the previous value of E had a = 10 and b = 8. Then the first assignment to E yields an ellipse with a = 7 and b = 8 in other words, an ellipse with a < b. However, this state of affairs does not cause a run-time error (and the second assignment to E thus goes on to yield an ellipse with a = 7 and b = 3, as required). Why is there no run-time error Because SQL is unaware of the type constraint to the effect that a must be greater than or equal to b (observe that the definition of type ELLIPSE included no such constraint); in fact, SQL doesn t support the concept of type constraints at all (we can t even state them). In our opinion, this lack of type constraint support on SQL s part constitutes a very serious flaw. While it does make it easier to support multiple assignment, as we ve just seen,21 it also means there are many real world types that can t be properly specified in SQL! For example, suppose we want to define a type called LENGTH (with the obvious meaning). Then we might specify, say, integers as the associated representation, and of course that specification does impose an a priori constraint on the values that go to make up the type. Considered as type constraints, however, such a priori constraints are extremely weak. In the case at hand, for example, there s no
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21. As we ll see in the next subsection, however, it turns out (in another context) that SQL still has to do the same kind of thing we do anyway i.e., it still has to collect various constituent assignments together so it can hardly be argued that the fact that it doesn t have to do so in the case of multiple assignment is a significant advantage.
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CHAPTER 11 MULTIPLE ASSIGNMENT
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way to specify that lengths mustn t be negative. As we ve shown elsewhere (in The Third Manifesto in particular), therefore, SQL necessarily permits negative lengths, noncircular circles, nonsquare squares, and all kinds of similar nonsenses. Note: Possible reasons why type constraint support is omitted from SQL are discussed in the book mentioned several times already, An Introduction to Database Systems, 8th edition, by C. J. Date (Addison-Wesley, 2004). We close this subsection by noting that, owing to its lack of proper support for multiple table assignment, SQL can t directly handle either (a) the example discussed in the section A Multiple Assignment Example, or (b) the example discussed at the end of the section Assigning to a Variable More than Once, in the body of the chapter. Instead, it has to use several separate assignment statements in both cases, and bundle those statements up into a transaction. It also has to make sure the pertinent constraint checking is deferred.22
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