refactor error

error.go

@@ -26,6 +26,8 @@ var (
 	ErrNotImplemented = errors.New("Not implemented")
 	// ErrConditionType condition type unsupported
 	ErrConditionType = errors.New("Unsupported condition type")
+	// ErrUnSupportedSQLType parameter of SQL is not supported
+	ErrUnSupportedSQLType = errors.New("unsupported sql type")
 )
 
 // ErrFieldIsNotExist columns does not exist

statement.go

@@ -133,7 +133,7 @@ func (statement *Statement) SQL(query interface{}, args ...interface{}) *Stateme
 		statement.RawSQL = query.(string)
 		statement.RawParams = args
 	default:
-		statement.lastError = errors.New("unsupported sql type")
+		statement.lastError = ErrUnSupportedSQLType
 	}
 
 	return statement