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);
}