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Encoder QR-Code in Software PART V

PART V
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In many cases, a SQL statement must be repeatedly executed with changes only in some of the values that it specifies. For example, an INSERT statement to add an order to the sample database is identical for every order except for the specific information about the customer number, product and manufacturer, and quantity ordered. As described in 18, for dynamic embedded SQL, such statements can be processed efficiently by specifying the variable parts of the statement as input parameters. The statement text passed to the SQLPrepare() call has a parameter marker a question mark ( ) in its text at each position where a parameter value is to be inserted. When the statement is later executed, values must be supplied for each of its input parameters.
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Part V:
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/* Directly execute a SQL statement */ SQLSMALLINT SQLExecDirect ( SQLINTEGER SQLCHAR SQLSMALLINT stmtHdl, textlen) /* IN: /* IN: /* IN: statement handle */ SQL statement text */ statement text length */ *stmttext,
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/* Prepare a SQL statement */ SQLSMALLINT SQLPrepare ( SQLINTEGER SQLCHAR SQLSMALLINT stmtHdl, textlen) /* IN: /* IN: /* IN: statement handle */ SQL statement text */ statement text length */ *stmttext,
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/* Execute a previously prepared SQL statement */ SQLSMALLINT SQLExecute ( SQLINTEGER stmtHdl) /* IN: statement handle */
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/* Bind a SQL statement parameter to a program data area */ SQLSMALLINT SQLBindParam ( SQLINTEGER SQLSMALLINT SQLSMALLINT SQLSMALLINT SQLSMALLINT SQLSMALLINT void *value, *lenind) SQLINTEGER stmtHdl, parmnr, valtype, parmtype, colsize, /* IN: /* IN: /* IN: /* IN: /* IN: /* IN: /* IN: statement handle */ parameter number (1,2,3...) */ data type of value supplied */ data type of parameter */ column size */ number of decimal digits */ pointer to parameter value buf */ pointer to length/indicator buf */
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decdigits, /* IN:
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/* Get parameter-tag for next required dynamic parameter */ SQLSMALLINT SQLParamData ( SQLINTEGER void stmtHdl, /* IN: stmt handle w/dynamic parms */ *prmtag) /* OUT: returned parameter-tag value */
/* Obtain detailed info about an item described by a CLI descriptor */ SQLSMALLINT SQLPutData ( SQLINTEGER void SQLSMALLINT stmtHdl, /* IN: /* IN: stmt handle w/dynamic parms */ buffer with data for parameter */ parameter length or NULL ind */ *prmdata,
prmlenind) /* IN:
FIGURE 19-20
CLI statement-processing routines
19:
SQL APIs
The most straightforward way to supply input parameter values is with the SQLBindParam() call. Each call to SQLBindParam() establishes a linkage between one of the parameter markers in the SQL statement (identified by number) and a variable in the application program (identified by its memory address). In addition, an association is optionally established with a second application program variable (an integer) that provides the length of variable-length input parameters. If the parameter is a NULL-terminated string like those used in C programs, a special negative code value, defined in the header file as the symbolic constant SQL_NTS, can be passed, indicating that the string length can be obtained from the data itself by the CLI routines. Similarly, a negative code is used to indicate a NULL value for an input parameter. If three input parameter markers are in the statement, three calls will be made to SQLBindParam(), one for each input parameter. Once the association between application program variables (more accurately, program storage locations) and the statement parameters is established, the statement can be executed with a call to SQLExecute(). To change the parameter values for subsequent statements, it is only necessary to place new values in the application program buffer areas before the next call to SQLExecute(). Alternatively, the parameters can be rebound to different data areas within the application program by subsequent calls to SQLBindParam(). Figure 19-21 shows a program that includes a SQL statement with two input parameters. The program repeatedly prompts the user for a customer number and a new credit limit for the customer. The values provided by the user become the input parameters to an UPDATE statement for the CUSTOMERS table. The SQLParamData() and SQLPutData() functions provide an alternative method of passing parameter data at runtime, called deferred parameter passing. The selection of this technique for a particular statement parameter is indicated in the corresponding call to SQLBindParam(). Instead of actually supplying a program data location to which the parameter is bound, the SQLBindParam() call indicates that deferred parameter passing will be used and provides a value that will later be used to identify the particular parameter being processed in this way. After statement execution is requested (by a SQLExecute() or SQLExecDirect() call), the program calls SQLParamData() to determine whether deferred parameter data is required by the statement. If so, the CLI returns a status code (SQL_NEED_DATA) along with an indicator of which parameter needs a value. The program then calls SQLPutData() to actually provide the value for the parameter. Typically, the program then calls SQLParamData() again to determine if another parameter requires dynamic data. The cycle repeats until all required dynamic data has been supplied, and SQL statement execution then continues normally. This alternative parameter-passing method is considerably more complex than the straightforward process of binding parameters to application program locations. It has two advantages. The first is that the actual passing of data values (and the allocation of storage to contain those values) can be delayed until the last possible moment when the data is actually needed. The second advantage is that the technique can be used to pass very long parameter values piece by piece. For selected long data types, the CLI allows repeated calls to SQLPutData() for the same parameter, with each call passing the next part of the data. For example, the text of a document that is supplied as a parameter for the VALUES clause of an INSERT statement might be passed in 1000-character pieces through repeated SQLPutData() calls until all of the document has been passed. This avoids the need to allocate a single very large memory buffer within the application program to hold the entire parameter value.
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