Fix NULL parameter type mapping for VARBINARY columns (#458)

Author: gargsaumya

mssql_python/cursor.py

@@ -392,7 +392,7 @@ def _map_sql_type(  # pylint: disable=too-many-arguments,too-many-positional-arg
         if param is None:
             logger.debug("_map_sql_type: NULL parameter - index=%d", i)
             return (
-                ddbc_sql_const.SQL_VARCHAR.value,
+                ddbc_sql_const.SQL_UNKNOWN_TYPE.value,
                 ddbc_sql_const.SQL_C_DEFAULT.value,
                 1,
                 0,
@@ -2185,6 +2185,16 @@ def executemany(  # pylint: disable=too-many-locals,too-many-branches,too-many-s
                     min_val=min_val,
                     max_val=max_val,
                 )
+                
+                # For executemany with all-NULL columns, SQL_UNKNOWN_TYPE doesn't work
+                # with array binding. Fall back to SQL_VARCHAR as a safe default.
+                if (
+                    sample_value is None
+                    and paraminfo.paramSQLType == ddbc_sql_const.SQL_UNKNOWN_TYPE.value
+                ):
+                    paraminfo.paramSQLType = ddbc_sql_const.SQL_VARCHAR.value
+                    paraminfo.columnSize = 1
+                
                 # Special handling for binary data in auto-detected types
                 if paraminfo.paramSQLType in (
                     ddbc_sql_const.SQL_BINARY.value,

mssql_python/pybind/ddbc_bindings.cpp

@@ -471,14 +471,19 @@ SQLRETURN BindParameters(SQLHANDLE hStmt, const py::list& params,
                         hStmt, static_cast<SQLUSMALLINT>(paramIndex + 1), &describedType,
                         &describedSize, &describedDigits, &nullable);
                     if (!SQL_SUCCEEDED(rc)) {
+                        // SQLDescribeParam can fail for generic SELECT statements where
+                        // no table column is referenced. Fall back to SQL_VARCHAR as a safe default.
                         LOG("BindParameters: SQLDescribeParam failed for "
-                            "param[%d] (NULL parameter) - SQLRETURN=%d",
+                            "param[%d] (NULL parameter) - SQLRETURN=%d, falling back to SQL_VARCHAR",
                             paramIndex, rc);
-                        return rc;
+                        sqlType = SQL_VARCHAR;
+                        columnSize = 1;
+                        decimalDigits = 0;
+                    } else {
+                        sqlType = describedType;
+                        columnSize = describedSize;
+                        decimalDigits = describedDigits;
                     }
-                    sqlType = describedType;
-                    columnSize = describedSize;
-                    decimalDigits = describedDigits;
                 }
                 dataPtr = nullptr;
                 strLenOrIndPtr = AllocateParamBuffer<SQLLEN>(paramBuffers);