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Long Data and SQLSetPos and SQLBulkOperations

  • Άρθρο

As is the case with parameters in SQL statements, long data can be sent when updating rows with SQLBulkOperations or SQLSetPos or when inserting rows with SQLBulkOperations. The data is sent in parts, with multiple calls to SQLPutData. Columns for which data is sent at execution time are known as data-at-execution columns.

Note

An application actually can send any type of data at execution time with SQLPutData, although only character and binary data can be sent in parts. However, if the data is small enough to fit in a single buffer, there is generally no reason to use SQLPutData. It is much easier to bind the buffer and let the driver retrieve the data from the buffer.

Because long data columns typically are not bound, the application must bind the column before calling SQLBulkOperations or SQLSetPos and unbind it after calling SQLBulkOperations or SQLSetPos. The column must be bound because SQLBulkOperations or SQLSetPos operates only on bound columns and must be unbound so that SQLGetData can be used to retrieve data from the column.

To send data at execution time, the application does the following:

  1. Places a 32-bit value in the rowset buffer instead of a data value. This value will be returned to the application later, so the application should set it to a meaningful value, such as the number of the column or the handle of a file containing data.

  2. Sets the value in the length/indicator buffer to the result of the SQL_LEN_DATA_AT_EXEC(length) macro. This value indicates to the driver that the data for the parameter will be sent with SQLPutData. The length value is used when sending long data to a data source that needs to know how many bytes of long data will be sent so that it can preallocate space. To determine whether a data source requires this value, the application calls SQLGetInfo with the SQL_NEED_LONG_DATA_LEN option. All drivers must support this macro; if the data source does not require the byte length, the driver can ignore it.

  3. Calls SQLBulkOperations or SQLSetPos. The driver discovers that a length/indicator buffer contains the result of the SQL_LEN_DATA_AT_EXEC(length) macro and returns SQL_NEED_DATA as the return value of the function.

  4. Calls SQLParamData in response to the SQL_NEED_DATA return value. If long data needs to be sent, SQLParamData returns SQL_NEED_DATA. In the buffer pointed to by the ValuePtrPtr argument, the driver returns the unique value that the application placed in the rowset buffer. If there is more than one data-at-execution column, the application uses this value to determine which column to send data for; the driver is not required to request data for data-at-execution columns in any particular order.

  5. Calls SQLPutData to send the column data to the driver. If the column data does not fit in a single buffer, as is often the case with long data, the application calls SQLPutData repeatedly to send the data in parts; it is up to the driver and data source to reassemble the data. If the application passes null-terminated string data, the driver or data source must remove the null-termination character as part of the reassembly process.

  6. Calls SQLParamData again to indicate that it has sent all of the data for the column. If there are any data-at-execution columns for which data has not been sent, the driver returns SQL_NEED_DATA and the unique value for the next data-at-execution column; the application returns to step 5. If data has been sent for all data-at-execution columns, the data for the row is sent to the data source. SQLParamData then returns SQL_SUCCESS or SQL_SUCCESS_WITH_INFO and can return any SQLSTATE that SQLBulkOperations or SQLSetPos can return.

After SQLBulkOperations or SQLSetPos returns SQL_NEED_DATA and before data has been completely sent for the last data-at-execution column, the statement is in a Need Data state. In this state, the application can call only SQLPutData, SQLParamData, SQLCancel, SQLGetDiagField, or SQLGetDiagRec; all other functions return SQLSTATE HY010 (Function sequence error). Calling SQLCancel cancels execution of the statement and returns it to its previous state. For more information, see Appendix B: ODBC State Transition Tables.