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CHAPTER 11 MULTIPLE ASSIGNMENT
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this implicit support any further in this appendix; however, we do have more to say regarding the UPDATE statement in particular (that s the subject of the final subsection).
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The SET Statement: Single Assignment
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As we saw in the body of the chapter, the single-assignment version of the SQL SET statement takes the form SET target = source ; The target can be a variable or a pseudovariable.16 We consider the variable case first. Here are some examples. First a self-explanatory scalar example: DECLARE I INTEGER ; SET I = 0 ; Note: All DECLARE statements shown in this appendix are SQL statements specifically (i.e., they have the effect of defining some SQL variable). Like SET, DECLARE was introduced into the SQL standard as part of the Persistent Stored Modules feature, SQL/PSM. Next, an array example: DECLARE H INTEGER ARRAY [10] ; DECLARE K INTEGER ARRAY [10] ; SET H = K ; Note carefully that the SET statement here is indeed a single assignment (see the discussion of this issue in the section A Little History in the body of the chapter). It s also a nonscalar assignment. Incidentally, though it s not relevant to our main purpose in this chapter, we remark that arrays in SQL are varying-length ; thus, the two [10] s in the declarations of H and K each define an upper bound, and the variables can each contain any number N of elements (N 0), up to but not greater than that upper bound. And if, for example, the variable H currently contains exactly three elements, then those elements are precisely H[1], H[2], and H[3]. Third and last, a row example: DECLARE NAME ROW ( FIRST VARCHAR(25), LAST VARCHAR(25), MI CHAR(1) ) ; SET NAME = ROW ( 'Truman', 'Harry', 'S' ) ; Again the SET statement here is a single, nonscalar assignment (in fact it s SQL s counterpart to Tutorial D s <tuple assign>, which was mentioned in the body of this chapter, though not discussed in detail). The expression on the right side is a row value constructor. The keyword ROW is optional in such constructors.
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16. It can also be a parameter (within the body of some procedure). And in certain kinds of triggered procedures it can be specified as a column reference, in which case SET can be used to achieve a database updating effect after all (albeit only indirectly).
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CHAPTER 11 MULTIPLE ASSIGNMENT
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Now we turn to pseudovariables. SQL supports three kinds: elements of arrays, fields within rows, and something analogous (somewhat) to our THE_ pseudovariables.17 We consider each in turn. Here first is an example involving an array element: DECLARE IA INTEGER ARRAY [5] ; SET IA = ARRAY [ 0, 0, 0, 0, 0 ] ; SET IA[4] = 8 ; The first SET statement here assigns an array of five elements, all of them zero, to the array variable IA (the expression on the right side is an array value constructor). The second SET statement then replaces the value of the fourth element of that array variable by the value eight. In fact, that second SET statement is effectively shorthand for this one: SET IA = ARRAY [ IA[1], IA[2], IA[3], 8, IA[5] ] ; Note: We could replace the first SET statement in the example by an appropriate DEFAULT clause on the variable declaration, as here: DECLARE IA INTEGER ARRAY [5] DEFAULT ARRAY [ 0, 0, 0, 0, 0 ] ; The keyword DEFAULT is a little misleading, however, inasmuch as what SQL s DEFAULT clause really does is assign an initial value to the variable in question (just as the INIT specification does in Tutorial D). Now we turn to an example involving assignment to a field within a row variable: DECLARE NAME ROW ( FIRST VARCHAR(25), LAST VARCHAR(25), MI CHAR(1) ) ; SET NAME.LAST = 'Potter' ; Expanded form of the SET statement: SET NAME = ROW ( NAME.FIRST, 'Potter', NAME.MI ) ; A note on terminology: The term field referring to a component of a row type, row value, or row variable was first introduced into SQL when explicit row type support was added, in SQL:1999. Of course, tables contain rows too, and the components of those rows were historically, and rather awkwardly, called columns. Now, however, they too are referred to as fields. Now we consider SQL s analog of our THE_ pseudovariables. Here first is an example: CREATE TYPE POINT AS ( X REAL, Y REAL ) ; DECLARE P POINT ; SET P.Y = 5.0 ; This SET statement expands to: SET P = P.Y ( 5.0 ) ; This expansion requires a certain amount of explanation.
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17. Note, incidentally, that it does not support anything analogous to PL/I s SUBSTR pseudovariables.
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