public IOqlExpressionVisitor VisitSubQuery(Expression expression)
        {
            IOqlExpressionVisitor visitor = new OqlExpressionVisitor(m_syntax_context)
                                            .ExecuteVisit(expression);

            m_query_builder.AppendBeginExpression();
            m_query_builder.AppendToken(visitor);
            m_query_builder.AppendEndExpression();
            return(visitor);
        }