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TABLE 18-2 SQL Standard Data Type Codes
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Code 4 5 2 3 6 7 8 1 12 14 15 9 10 1 2 4 3 5 1 2 3 4 5 6 7 8 9 10 11 12 13
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Programming with SQL
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For example, if you set the data type for a particular item to indicate an integer, the standard says that the corresponding information in the descriptor that tells the length of the same item will be reset to some implementation-dependent value. Normally this doesn t impact your programming; however, you can t assume that just because you set some value within the descriptor previously, it still retains the same value. It s best to fill the descriptor hierarchically, starting with higher-level information (for example, the number of items and their data types) and then proceeding to lower-level information (data type lengths, subtypes, whether NULL values are allowed, and so on). The flow of the program in Figure 18-8 can now continue unmodified. The PREPARE statement compiles the dynamic UPDATE statement, and its form does not change for standard SQL. The program then enters the for loop, prompting the user for parameters. Here again, the concepts are the same, but the details of manipulating the SQLDA structure and the SQL descriptor differ. If the user indicates that a NULL value is to be assigned (by typing an asterisk in response to the prompt), the program in Figure 18-8 sets the parameter indicator buffer appropriately with the statement:
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*(parmvar -> sqlind) = -1;
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and if the value is not NULL, the program again sets the indicator buffer with the statement:
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*(parmvar -> sqlind) = 0;
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For the SQL descriptor, these statements would again be converted to a pair of SET DESCRIPTOR statements:
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SET DESCRIPTOR parmdesc VALUE(:j + l) INDICATOR = -1; SET DESCRIPTOR parmdesc VALUE(:j + 1) INDICATOR = 0;
Note again the use of the loop control variable to specify which item in the descriptor is being set, and the direct passing of data (in this case, constants) rather than the use of pointers to buffers in the SQLDA structure. Finally, the program in Figure 18-8 passes the actual parameter value typed by the user to the DBMS, via the SQLDA. The statements at callout 8 accomplish this for data of different types, by first converting the typed characters into binary representations of the data and placing the binary data into the data buffers pointed to by the SQLDA. Again, the conversion to standard SQL involves replacing these pointers and direct SQLDA manipulation with a SET DESCRIPTOR statement. For example, these statements pass the data and its length for a variable-length character string:
length = strlen(inbuf); SET DESCRIPTOR parmdesc VALUE(:j + 1) DATA = :inbuf; SET DESCRIPTOR parmdesc VALUE(:j + 1) LENGTH = :length;
For data items that do not require a length specification, passing the data is even easier, since only the DATA form of the SET DESCRIPTOR statement is required. It s also useful to note that the SQL standard specifies implicit data type conversions between host variables (such as inbuf) and SQL data types. Following the SQL standard, it would be necessary for the program in Figure 18-8 to perform all of the data type conversion in the sscanf() functions.
18:
Dynamic SQL*
Instead, the data could be passed to the DBMS as character data, for automatic conversion and error detection. With the SQLDA finally set up as required, the program in Figure 18-8 executes the dynamic UPDATE statement with the passed parameters at callout 9, using an EXECUTE statement that specifies a SQLDA. The conversion of this statement to a SQL descriptor is straightforward; it becomes
EXECUTE updatestmt USING SQL DESCRIPTOR parmdesc;
The keywords in the EXECUTE statement change slightly, and the name of the descriptor is specified instead of the name of the SQLDA. Finally, the program in Figure 18-8 should be modified like this to tell the DBMS to deallocate the SQL descriptor. The statement that does this is
DEALLOCATE DESCRIPTOR parmdesc;
In a simple program like this one, the DEALLOCATE is not necessary, but in a more complex real-world program with multiple descriptors, it s a very good idea to deallocate the descriptors when the program no longer requires them.
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