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CHAPTER 11 MULTIPLE ASSIGNMENT
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several portions of several rows at the same time within table EMP.6 It resembles Dijkstra s concurrent assignment in that: a. All of the expressions on the right sides of assignments in the SET clause are evaluated before any of those assignments are performed. b. No two of those assignments are allowed to specify the same target column. (Actually, we re simplifying matters just slightly here. See Appendix A for further discussion of this point.) Incidentally, please note that we said it might be claimed that the SQL UPDATE statement constitutes an example of multiple assignment. We should make it clear, however, that we wouldn t make any such claim ourselves. In fact, we ve argued at length, in The Third Manifesto and elsewhere, that UPDATE in SQL and in a relational language like Tutorial D, come to that is best understood as shorthand for assigning a single value (namely, a relation value) to a single variable (namely, a relation variable); in other words, it s really another case of nonscalar single assignment. Analogous remarks apply to INSERT and DELETE, of course. Be that as it may, we can summarize the discussions of this section by observing that there are at least four constructs in the literature already that might lay some claim to being called multiple assignment : 1. An assignment that assigns the same source value (more precisely, the result of evaluating the same source expression) to several targets 2. Nonscalar assignment 3. Dijkstra s concurrent assignment 4. SQL s UPDATE statement However, the kind of multiple assignment we believe we need, and the kind we ll be discussing in the rest of this chapter, is different from all of these! Dijkstra s notion of concurrent assignment is, perhaps, closest to what we have in mind. Unlike Dijkstra, however, we do not require that the target variables all be distinct; in some cases, in fact, we explicitly want to be able to specify the same target variable more than once, for reasons that should become clear as we proceed.
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6. In this chapter we use the terms table, row, and column in SQL contexts (mainly in Appendix A), the more formal terms relation, tuple, and attribute elsewhere.
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CHAPTER 11 MULTIPLE ASSIGNMENT
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A Multiple Assignment Example
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At this point, it would be nice if we could say exactly what our multiple assignment operator is. As you might expect, however, the fact that we want to be able to specify the same target variable more than once causes complications, and we aren t yet in a position to explain just what those complications are. Until further notice, therefore, we focus not so much on what the operator actually is, but rather on why we need it and why systems should support it and we hope our examples will be sufficient, for now, to give some idea as to how it s supposed to work. As a basis for our first example, consider the usual suppliers-and-parts database, with definition as follows (Tutorial D syntax): VAR S RELATION /* suppliers */ { S# S#, SNAME CHAR, STATUS INTEGER, CITY CHAR } KEY { S# } ; VAR P RELATION { P# P#, PNAME CHAR, COLOR COLOR, WEIGHT WEIGHT, CITY CHAR } KEY { P# } ; /* parts */
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VAR SP RELATION /* shipments */ { S# S#, P# P#, QTY QTY } KEY { S#, P# } FOREIGN KEY { S# } REFERENCES S FOREIGN KEY { P# } REFERENCES P ; S, P, and SP here are relation variables or relvars (see The Third Manifesto for further explanation). Sample values for those relvars are shown in Figure 11-1.
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