/// <summary>
/// 必要时使用字段的全称（表名加字段名）
/// </summary>
/// <param name="orderByClause"></param>
public void AddOrderByClause(OrderByClause orderByClause)
{
_orderByClauses.Add(orderByClause);
}
/// <summary>
/// 添加排序字段
/// </summary>
/// <param name="feildName"></param>
/// <param name="isDesc"></param>
public void AddOrderByClause(string feildName, bool isDesc)
{
OrderByClause order = new OrderByClause(feildName, isDesc);
_orderByClause.Add(order);
}