MDAC 2.5 SDK - ODBC Programmer's Reference
Chapter 17: Programming Considerations
Behavioral changes are those changes for which the syntax of the interface remains the same, but the semantics have changed. For these changes, functionality used in ODBC 2.x behaves differently than the same functionality in ODBC 3.x.
Whether an application exhibits ODBC 2.x behavior or ODBC 3.x behavior is determined by the SQL_ATTR_ODBC_VERSION environment attribute. This 32-bit value is set to SQL_OV_ODBC2 to exhibit ODBC 2.x behavior, and SQL_OV_ODBC3 to exhibit ODBC 3.x behavior.
The SQL_ATTR_ODBC_VERSION environment attribute is set by a call to SQLSetEnvAttr. After an application calls SQLAllocHandle to allocate an environment handle, it must call SQLSetEnvAttr immediately to set the behavior it exhibits. (As a result, there is a new environment state to describe the environment handle in an allocated, but versionless, state.) For more information, see Appendix B, "ODBC State Transition Tables."
An application states what behavior it exhibits with the SQL_ATTR_ODBC_VERSION environment attribute, but the attribute has no effect on the application's connection with an ODBC 2.x or ODBC 3.x driver. An ODBC 3.x application can connect to either an ODBC 2.x or 3.x driver, no matter what the setting of the environment attribute.
ODBC 3.x applications should never call SQLAllocEnv. As a result, if the Driver Manager receives a call to SQLAllocEnv, it recognizes the application as an ODBC 2.x application.
The SQL_ATTR_ODBC_VERSION attribute affects three different aspects of an ODBC 3.x driver's behavior:
The setting of the SQL_ATTR_ODBC_VERSION environment attribute does not affect SQLSetParam or SQLBindParam. SQLColAttribute is also not affected by this bit. Although SQLColAttribute returns attributes that are affected by the version of ODBC (date type, precision, scale and length), the intended behavior is determined by the value of the FieldIdentifier argument. When FieldIdentifier is equal to SQL_DESC_TYPE, SQLColAttribute returns the ODBC 3.x codes for date, time, and timestamp; when FieldIdentifier is equal to SQL_COLUMN_TYPE, SQLColAttribute returns the ODBC 2.x codes for date, time, and timestamp.