MDAC 2.5 SDK - ODBC Programmer's Reference
Chapter 4: ODBC Fundamentals
ODBC defines the C data types that are used by application variables and their corresponding type identifiers. These are used by the buffers that are bound to result set columns and statement parameters. For example, suppose an application wants to retrieve data from a result set column in character format. It declares a variable with the SQLCHAR * data type and binds this variable to the result set column with a type identifier of SQL_C_CHAR. For a complete list of C data types and type identifiers, see Appendix D, "Data Types."
ODBC also defines a default mapping from each SQL data type to a C data type. For example, a 2-byte integer in the data source is mapped to a 2-byte integer in the application. To use the default mapping, an application specifies the SQL_C_DEFAULT type identifier. However, use of this identifier is discouraged for interoperability reasons.
All integer C data types defined in ODBC 1.x were signed. Unsigned C data types and their corresponding type identifiers were added in ODBC 2.0. Because of this, applications and drivers need to be particularly careful when dealing with 1.x versions.