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Variable part one occurrence per item (parameter or query results column):
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them, placing data items into them, extracting data from them via a new set of dynamic SQL statements specially designed for that purpose. Figure 18-19 summarizes these SQL2 descriptor management statements. To understand how the SQL2 descriptor management statements work, it s instructive to reexamine the dynamic SQL update program in Figure 18-8. This program illustrates the use of a DB2-style SQLDA in an EXECUTE statement. The flow of the program remains identical if a SQL2 descriptor is used instead, but the specifics change quite a lot. Before using the descriptor, the program must create it, using the statement:
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ALLOCATE DESCRIPTOR parmdesc WITH MAX :parmcnt;
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This statement replaces the allocation of storage for the parmda data structure at callout 1 in Figure 18-8. The descriptor (named parmdesc) will perform the same functions as the parmda. Note that the program in Figure 18-8 had to calculate how much storage would be required for the parmda structure before allocating it. With the
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Figure 18-19.
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18:
Dynamic SQL *
SQL2 descriptor, that calculation is eliminated, and the host program simply tells the DBMS how many items the variable part of the descriptor must be able to hold. The next step in the program is to set up the descriptor so that it describes the parameters to be passed their data types, lengths, and so on. The loop at callout 2 of the program remains intact, but again, the details of how the descriptor is initialized differ from those for the SQLDA. At callout 3 and callout 4, the data type and length for the parameter are specified with a form of the SET DESCRIPTOR statement, with this code excerpt:
typecode = columns[i].typecode; length = columns[i].buflen; SET DESCRIPTOR parmdesc VALUE (:i + l) TYPE = :typecode SET DESCRIPTOR parmdesc VALUE (:i + l) LENGTH = :length;
The differences from Figure 18-8 are instructive. Because the descriptor is maintained by the DBMS, the data type and length must be passed to the DBMS, through the SET DESCRIPTOR statement, using host variables. In this particular example, the simple variables typecode and length are used. Additionally, the data type codes in Figure 18-8 were specific to DB2. The fact that each DBMS vendor used different codes to represent different SQL data types was a major source of portability problems in dynamic SQL. The SQL2 standard specifies integer data type codes for all of the data types specified in the standard, eliminating this issue. The SQL2 data type codes are summarized in Table 18-2. So, in addition to the other changes, the data type codes in the columns structure of Figure 18-8 would need to be modified to use these SQL2 standard data type codes.
Data Type
Data Type Codes (TYPE) INTEGER SMALLINT NUMERIC DECIMAL FLOAT Table 18-2.
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4 5 2 3 6
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Data Type
REAL DOUBLE PRECISION CHARACTER CHARACTER VARYING BIT BIT VARYING DATE/TIME/TIMESTAMP INTERVAL Date/Time Subcodes (Interval_Code) DATE TIME TIME WITH TIME ZONE TIMESTAMP TIMESTAMP WITH TIME ZONE YEAR MONTH DAY HOUR MINUTE SECOND YEAR MONTH DAY HOUR DAY MINUTE DAY SECOND Table 18-2.
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7 8 1 12 14 15 9 10 1 2 4 3 5 1 2 3 4 5 6 7 8 9 10
Date/Time Subcodes (Interval_Precision)
SQL2 Data Type Codes (continued)
18:
Dynamic SQL *
The statements at callouts 5 and 6 in Figure 18-8 were used to bind the SQLDA structure to the program buffers used to contain the parameter data and the corresponding indicator variable. Effectively, they put pointers to these program buffers into the SQLDA for the use of the DBMS. With SQL2 descriptors, this type of binding is not possible. Instead, the data value and indicator value are specifically passed as host variables, later in the program. Thus, the statements at callouts 5 and 6 would be eliminated in the conversion to SQL2. The statement at callout 7 in Figure 18-8 sets the SQLDA to indicate how many parameter values are actually being passed to the DBMS. The SQL2 descriptor must similarly be set to indicate the number of passed parameters. This is done with a form of the SET DESCRIPTOR statement:
SET DESCRIPTOR parmdesc COUNT = :parmcnt;
Strictly speaking, this SET DESCRIPTOR statement should probably be placed earlier in the program and should be executed before those for the individual items. The SQL2 standard specifies a complete set of rules that describe how setting values in some parts of the descriptor causes values in other parts of the descriptor to be reset. For the most part, these rules simply specify the natural hierarchy of information. 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, but it does mean that you can t assume that just because you set some value within the descriptor previously, that 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 SQL2. 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 SQL2 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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and if the value is not NULL, the program again sets the indicator buffer with the statement:
*(parmvar->sqlind) = 0;
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