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We have defined a new primitive operator that we call multiple assignment.13 The fundamental purpose of that operator is, in effect, to allow the checking of certain constraints to be deferred for a little while, without the possibility that the user will ever see an inconsistent state of the database. However, the operator does have a number of subsidiary uses as well, which the chapter has also briefly examined.
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Appendix A: Multiple Assignment in SQL
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The SQL standard includes several features that can fairly be regarded as multiple assignment support. Some of those features are new with the most recent version of the standard (SQL:2003); others go all the way back to the very first version (SQL:1986); the rest were introduced at various points along the way. This appendix briefly surveys the features in question. One point that s worth making right away is this: As just indicated, SQL s multiple assignment features were incorporated into the language piecemeal, and they don t seem to have been perceived as different aspects of the same general problem. As a consequence, they do suffer from a certain lack of orthogonality and parsimony in their design, as will be seen. The reader (or, more to the point, the user) is warned.
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13. By the way, the word multiple, which in practice is very often abused, is indeed the mot juste here. In general, a good way to tell whether the word is being used appropriately is to see whether it makes sense to replace it by, say, triple or quadruple. This simple test shows immediately that remarks such as There are multiple reasons to vote for Arnold are not well expressed ( triple reasons ). By contrast, triple assignment is a perfectly reasonable construction, and so is multiple assignment.
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CHAPTER 11 MULTIPLE ASSIGNMENT
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The appendix is divided into four subsections. In the first, we present an overview of SQL s support for assignment in general, in order to lay some necessary groundwork. In the next two, we discuss the principal SQL assignment statement i.e., the SET statement in detail; the first covers single assignment and the second multiple. Finally, the fourth subsection describes certain relevant aspects of the regular SQL UPDATE statement.
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Overview
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SQL has always supported the well-known INSERT, DELETE, and UPDATE statements, of course (more recently it has added support for a MERGE statement, which is shorthand for a certain combination of INSERT and UPDATE). These statements can all be characterized as table-level assignments, though obviously they don t use conventional assignment syntax. Furthermore, they re all single assignments (i.e., they all assign a single source value to a single target variable).14 Second, SQL has also always supported the SELECT INTO and FETCH statements, which in fact are multiple assignments. For example, the following statement against the suppliersand-parts database SELECT INTO FROM WHERE S.STATUS, S.CITY XST, XSC S S# = S#('S1') ;
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effectively assigns values to the two variables XST and XSC at the same time. Note: Whether SELECT INTO and FETCH are scalar or nonscalar assignments (or even a mixture) depends on the types of the source expressions and the types of the target variables. The one thing they re not, though, is table-level assignments, because SQL does not permit tables to contain columns that contain further tables in turn. Third, SQL also supports a variety of miscellaneous statements (e.g., GET DESCRIPTOR, GET DIAGNOSTICS) that can also be regarded as assignments of a kind. For simplicity we ignore such statements for the remainder of this appendix. Fourth, and most important, SQL now includes an explicit assignment statement called SET (introduced with the Persistent Stored Modules feature, SQL/PSM). SET as originally defined supported single assignment only, but SQL:2003 extends it to support multiple assignment as well. It also supports both scalar and (some) nonscalar assignments, but nonscalar here unfortunately does not include tables; in fact, it includes arrays, rows, and nothing else.15 It follows that SET cannot normally be used to update the database. Fifth and last, SQL s regular UPDATE statement might be regarded as a kind of multiple assignment (see the discussion of this issue near the end of the section Assigning to Several Variables at Once in the body of the chapter). It also provides implicit support for multiple table assignment through (a) its ability to perform certain compensating actions (e.g., cascade delete) and (b) its ability to update certain views (join views in particular). We do not discuss
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14. How best to characterize the positioned or CURRENT forms of DELETE and UPDATE we leave as an exercise for the reader. 15. Some might argue that it also includes structured types. Certainly SET can be used to assign structured values to structured variables (as we ll see), but whether such values and variables are scalar or nonscalar is a matter of some debate. The issue is explored in detail in An Introduction to Database Systems, 8th edition, by C. J. Date (Addison-Wesley, 2004).
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