using system.collections.generic;
using system;
using system.text;
///
/// 构造分页后的sql语句
///
public static class paginghelper
{
    ///
    /// 获取分页sql语句，排序字段需要构成唯一记录
    ///
    /// 记录总数
    /// 每页记录数
    /// 当前页数
    /// sql查询语句
    /// 排序字段，多个则用“,”隔开
    /// 分页sql语句
    public static string createpagingsql(int _recordcount, int _pagesize, int _pageindex, string _safesql, string _orderfield)
    {
        //重新组合排序字段，防止有错误
        string[] arrstrorders = _orderfield.split(new char[] { ',' }, stringsplitoptions.removeemptyentries);
        stringbuilder sboriginalorder = new stringbuilder(); //原排序字段
        stringbuilder sbreverseo教程rder = new stringbuilder(); //与原排序字段相反，用于分页
        for (int i = 0; i < arrstrorders.length; i++)
        {
            arrstrorders[i] = arrstrorders[i].trim();  //去除前后空格
            if (i != 0)
            {
                sboriginalorder.append(", ");
                sbreverseorder.append(", ");
            }
            sboriginalorder.append(arrstrorders[i]);
            int index = arrstrorders[i].indexof(" "); //判断是否有升降标识
            if (index > 0)
            {
                //替换升降标识，分页所需
                bool flag = arrstrorders[i].indexof(" desc", stringcomparison.ordinalignorecase) != -1;
                sbreverseorder.appendformat("{0} {1}", arrstrorders[i].remove(index), flag ? "asc" : "desc");
            } www.
            else
            {
                sbreverseorder.appendformat("{0} desc", arrstrorders[i]);
            }
        }
        //计算总页数
        _pagesize = _pagesize == 0 ? _recordcount : _pagesize;
        int pagecount = (_recordcount + _pagesize - 1) / _pagesize;
        //检查当前页数
        if (_pageindex < 1)
        {
            _pageindex = 1;
        }
        else if (_pageindex > pagecount)
        {
            _pageindex = pagecount;
        }
        stringbuilder sbsql = new stringbuilder();
        //第一页时，直接使用top n，而不进行分页查询
        if (_pageindex == 1)
        {
            sbsql.appendformat(" select top {0} * ", _pagesize);
            sbsql.appendformat(" from ({0}) as t ", _safesql);
            sbsql.appendformat(" order by {0} ", sboriginalorder.tostring());
        }
        //最后一页时，减少一个top n
        else if (_pageindex == pagecount)
        {
            sbsql.append(" select * from ");
            sbsql.append(" ( ");
            sbsql.appendformat(" select top {0} * ", _recordcount - _pagesize * (_pageindex - 1));
            sbsql.appendformat(" from ({0}) as t ", _safesql);
            sbsql.appendformat(" order by {0} ", sbreverseorder.tostring());
            sbsql.append(" ) as t ");
            sbsql.appendformat(" order by {0} ", sboriginalorder.tostring());
        }
        //前半页数时的分页
        else if (_pageindex < (pagecount / 2 + pagecount % 2))
        {
            sbsql.append(" select * from ");
            sbsql.append(" ( ");
            sbsql.appendformat(" select top {0} * from ", _pagesize);
            sbsql.append(" ( ");
            sbsql.appendformat(" select top {0} * ", _pagesize * _pageindex);
            sbsql.appendformat(" from ({0}) as t ", _safesql);
            sbsql.appendformat(" order by {0} ", sboriginalorder.tostring());
            sbsql.append(" ) as t ");
            sbsql.appendformat(" order by {0} ", sbreverseorder.tostring());
            sbsql.append(" ) as t ");
            sbsql.appendformat(" order by {0} ", sboriginalorder.tostring());
        }
        //后半页数时的分页
        else
        {
            sbsql.appendformat(" select top {0} * from ", _pagesize);
            sbsql.append(" ( ");
            sbsql.appendformat(" select top {0} * ", ((_recordcount % _pagesize) + _pagesize * (pagecount - _pageindex)));
            sbsql.appendformat(" from ({0}) as t ", _safesql);
            sbsql.appendformat(" order by {0} ", sbreverseorder.tostring());
            sbsql.append(" ) as t ");
            sbsql.appendformat(" order by {0} ", sboriginalorder.tostring());
        }
        return sbsql.tostring();
    }
    ///
    /// 获取记录总数sql语句
    ///
    /// 限定记录数
    /// sql查询语句
    /// 记录总数sql语句
    public static string createtopnsql(int _n, string _safesql)
    {
        return string.format(" select top {0} * from ({1}) as t ", _n, _safesql);
    }
    ///
    /// 获取记录总数sql语句
    ///
    /// sql查询语句
    /// 记录总数sql语句
    public static string createcountingsql(string _safesql)
    {
        return string.format(" select count(1) as recordcount from ({0}) as t ", _safesql);
    }
}