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Reference : Designing ODBC Applications for Performance Optimization : Managing Connections and Updates : Using SQLSpecialColumns

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Using SQLSpecialColumns
Use SQLSpecialColumns to determine the optimal set of columns to use in the Where clause for updating data. Often, pseudo-columns provide the fastest access to the data, and these columns can only be determined by using SQLSpecialColumns.
Some applications cannot be designed to take advantage of positioned updates and deletes. These applications typically update data by forming a Where clause consisting of some subset of the column values returned in the result set. Some applications may formulate the Where clause by using all searchable result columns or by calling SQLStatistics to find columns that are part of a unique index. These methods typically work, but can result in fairly complex queries.
Consider the following example:
rc = SQLExecDirect (hstmt, "SELECT first_name, last_name, ssn, address, city, state, zip FROM emp", SQL_NTS);
// fetchdata
rc = SQLExecDirect (hstmt, "UPDATE EMP SET ADDRESS = ? WHERE first_name = ? AND last_name = ? AND
   ssn = ? AND address = ? AND city = ? AND state = ? AND zip = ?", SQL_NTS);
// fairly complex query
Applications should call SQLSpecialColumns/SQL_BEST_ROWID to retrieve the optimal set of columns (possibly a pseudo-column) that identifies a given record. Many databases support special columns that are not explicitly defined by the user in the table definition but are "hidden" columns of every table (for example, ROWID and TID). These pseudo-columns provide the fastest access to data because they typically point to the exact location of the record. Because pseudo-columns are not part of the explicit table definition, they are not returned from SQLColumns. To determine if pseudo-columns exist, call SQLSpecialColumns.
Consider the previous example again:
rc = SQLSpecialColumns (hstmt, ..... ’emp’, ...);
rc = SQLExecDirect (hstmt, "SELECT first_name, last_name, ssn, address, city, state, zip, ROWID
   FROM emp", SQL_NTS);
// fetch data and probably "hide" ROWID from the user
rc = SQLExecDirect (hstmt, "UPDATE emp SET address = ? WHERE ROWID = ?",SQL_NTS);
// fastest access to the data!
If your data source does not contain special pseudo-columns, the result set of SQLSpecialColumns consists of columns of the optimal unique index on the specified table (if a unique index exists). Therefore, your application does not need to call SQLStatistics to find the smallest unique index.