MDAC 2.5 SDK - ODBC Programmer's Reference
Appendix D: Data Types
The SQL_C_TCHAR type identifier does not actually identify a data type; it is a macro that exists within the header file for Unicode conversion. It is replaced by SQL_C_CHAR or SQL_C_WCHAR depending on the setting of the UNICODE #define. It is useful for an application transferring character data that is compiled as both an ANSI and a Unicode application.