仿orm自动生成分页SQL分享

先看看目前这4种数据库的分页写法:

代码如下:

-- Oracle
SELECT * FROM (
    SELECT ROWNUM RN,  PageTab.* FROM 
                (
                SELECT * FROM User_Tables order by id desc
                ) PageTab  where ROWNUM <= 3010
            ) Where RN>= 3001

-- SQLite   
select * from User_Tables order by id desc limit 3001,10

-- SQL2000
SELECT TOP 100 PERCENT  * FROM (
    SELECT TOP 10 * FROM (
        SELECT TOP 3010 * from User_Tables  order by id desc ) PageTab order by id ASC 
) PageTab2 order by id desc

-- SQL2005+   
Select PageTab.* from ( 
    Select top 3010 ROW_NUMBER() over (order by id desc) RN , * from User_Tables 
) PageTab Where RN >= 3001

其中针对 Oracle和Sql2005+的分页写法做个说明。

Oracle使用ROWNUM要比Row_Number()要快。sql示例中均是查询 [3001,3010] 区间的数据，在Sql语句中，尽可能在子查询中减少查询的结果集行数，然后针对排序过后的行号，在外层查询中做条件筛选。 如Oracle写法中 子查询有ROWNUM <= 3010 ,Sql2005 中有 top 3010 * 。

当然今天要讨论的问题，不是分页语句的性能问题，如果你知道更好更快的写法，欢迎交流。

上面的分页写法，基于的查询sql语句是:

代码如下:

select * from User_Tables order by id desc

首先要从Sql语句中分析出行为，我把该Sql拆成了n部分，然后完成了以上拼接功能。按照模子往里面套数据，难度不大。

逆序分页

我们来描述另外一种场景，刚刚演示的sql是查询 满足条件下行数在[3001,3010]之间的数据，如果说总行数仅仅只有3500行，那么结果则是需要查询出3010行数据，并取出最后10条，而前面3000条数据，是没用的。

所以借鉴以前的经验，姑且叫它 逆序分页 。在知道总行数的前提下，我们可以进行分析，是否需要逆序分页，因为逆序分页得到分页Sql语句，也是需要时间的，并非所有的情况都有必要这么做。之前有假设，数据仅仅有3500行，我们期望取出 按照id 倒叙排序后的[3001,3010]数据，换种方式理解，若按照id升序，我们期望取出的数据则是[491,500] 这个区间，然后将这个数据，再按照id倒叙排序，也就是我们需要的数据了。

理论知识差不多就说完了，需要了解更多的话，百度一下，你就知道。下面是代码，有点长，展开当心:

代码如下:

public enum DBType
    {
        SqlServer2000,
        SqlServer,
        Oracle,
        SQLite
    }

public class Page
    {
        /// <summary>
        /// 数据库类别
        /// </summary>
        public DBType dbType = DBType.Oracle;
        /// <summary>
        /// 逆序分页行数，总行数大于MaxRow，则会生成逆序分页SQL
        /// </summary>
        public int MaxRow = 1000;//临时测试，把值弄小点

/// <summary>
        /// 匹配SQL语句中Select字段
        /// </summary>
        private Regex rxColumns = new Regex(@"\A\s*SELECT\s+((?:\((?>\((?<depth>)|\)(?<-depth>)|.?)*(?(depth)(?!))\)|.)*?)(?<!,\s+)\bFROM\b", RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.Compiled);
        /// <summary>
        /// 匹配SQL语句中Order By字段
        /// </summary>
        private Regex rxOrderBy = new Regex(@"\b(?<ordersql>ORDER\s+BY\s+(?:\((?>\((?<depth>)|\)(?<-depth>)|.?)*(?(depth)(?!))\)|[\w\(\)\.])+)(?:\s+(?<order>ASC|DESC))?(?:\s*,\s*(?:\((?>\((?<depth>)|\)(?<-depth>)|.?)*(?(depth)(?!))\)|[\w\(\)\.])+(?:\s+(?:ASC|DESC))?)*", RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.Compiled);
        /// <summary>
        /// 匹配SQL语句中Distinct
        /// </summary>
        private Regex rxDistinct = new Regex(@"\ADISTINCT\s", RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.Compiled);
        private string[] SplitSqlForPaging(string sql)
        {
            /*存储分析过的SQL信息 依次为:
             * 0.countsql
             * 1.pageSql(保留位置此处不做分析)
             * 2.移除了select的sql
             * 3.order by 字段 desc
             * 4.order by 字段
             * 5.desc
             */
            var sqlInfo = new string[6];
            // Extract the columns from "SELECT <whatever> FROM"
            var m = rxColumns.Match(sql);
            if (!m.Success)
                return null;

// Save column list and replace with COUNT(*)
            Group g = m.Groups[1];
            sqlInfo[2] = sql.Substring(g.Index);

if (rxDistinct.IsMatch(sqlInfo[2]))
                sqlInfo[0] = sql.Substring(0, g.Index) + "COUNT(" + m.Groups[1].ToString().Trim() + ") " + sql.Substring(g.Index + g.Length);
            else
                sqlInfo[0] = sql.Substring(0, g.Index) + "COUNT(*) " + sql.Substring(g.Index + g.Length);

// Look for an "ORDER BY <whatever>" clause
            m = rxOrderBy.Match(sqlInfo[0]);
            if (!m.Success)
            {
                sqlInfo[3] = null;
            }
            else
            {
                g = m.Groups[0];
                sqlInfo[3] = g.ToString();
                //统计的SQL 移除order
                sqlInfo[0] = sqlInfo[0].Substring(0, g.Index) + sqlInfo[0].Substring(g.Index + g.Length);
                //存储排序信息
                sqlInfo[4] = m.Groups["ordersql"].Value;//order by xxx
                sqlInfo[5] = m.Groups["order"].Value;//desc

//select部分 移除order
                sqlInfo[2] = sqlInfo[2].Replace(sqlInfo[3], string.Empty);
            }

return sqlInfo;
        }

/// <summary>
        /// 生成逆序分页Sql语句
        /// </summary>
        /// <param name="sql"></param>
        /// <param name="sqls"></param>
        /// <param name="start"></param>
        /// <param name="limit"></param>
        /// <param name="total"></param>
        public void CreatePageSqlReverse(string sql,ref string[] sqls, int start, int limit, int total = 0)
        {
            //如果总行数不多或分页的条数位于前半部分，没必要逆序分页
            if (total < 100 || start <= total / 2)
            {
                return;
            }

//sql正则分析过后的数组有5个值，若未分析，此处分析
            if (sqls == null || sqls.Length == 6)
            {
                sqls = SplitSqlForPaging(sql);
                if (sqls == null)
                {
                    //无法解析的SQL语句
                    throw new Exception("can't parse sql to pagesql ,the sql is " + sql);
                }
            }

//如果未定义排序规则，则无需做逆序分页计算
            if (string.IsNullOrEmpty(sqls[5]))
            {
                return;
            }

//逆序分页检查
            string sqlOrder = sqls[3];
            int end = start + limit;

//获取逆序排序的sql
            string sqlOrderChange = string.Compare(sqls[5], "desc", true) == 0 ?
                string.Format("{0} ASC ", sqls[4]) :
                string.Format("{0} DESC ", sqls[4]);

/*理论
             * total:10000 start:9980 limit:10
             * 则 end:9990 分页条件为 RN >= 9980+1 and RN <= 9990
             * 逆序调整后
             * start = total - start = 20
             * end = total - end = 10
             * 交换start和end，分页条件为 RN >= 10+1 and RN<= 20
             */
            //重新计算start和end
            start = total - start;
            end = total - end;
            //交换start end
            start = start + end;
            end = start - end;
            start = start - end;

//定义分页SQL
            var pageSql = new StringBuilder();

if (dbType == DBType.SqlServer2000)
            {
                pageSql.AppendFormat("SELECT TOP @PageLimit * FROM ( SELECT TOP @PageEnd {0} {1} ) ", sqls[2], sqlOrderChange);
            }
            else if (dbType == DBType.SqlServer)
            {
                //组织分页SQL语句
                pageSql.AppendFormat("SELECT PageTab.* FROM ( SELECT TOP @PageEnd ROW_NUMBER() over ({0}) RN , {1}  ) PageTab ",
                    sqlOrderChange,
                    sqls[2]);

//如果查询不是第一页，则需要判断起始行号
                if (start > 1)
                {
                    pageSql.Append("Where RN >= :PageStart ");
                }
            }
            else if (dbType == DBType.Oracle)
            {
                pageSql.AppendFormat("SELECT ROWNUM RN,  PageTab.* FROM  ( Select {0} {1} ) PageTab  where ROWNUM <= :PageEnd ", sqls[2], sqlOrderChange);

//如果查询不是第一页，则需要判断起始行号
                if (start > 1)
                {
                    pageSql.Insert(0, "SELECT * FROM ( ");
                    pageSql.Append(" ) ");
                    pageSql.Append(" WHERE RN>= :PageStart ");
                }
            }
            else if (dbType == DBType.SQLite)
            {
                pageSql.AppendFormat("SELECT * FROM ( SELECT {0} {1} limit  @PageStart,@PageLimit ) PageTab ", sqls[2], sqlOrderChange);
            }

//恢复排序
            pageSql.Append(sqlOrder);

//存储生成的分页SQL语句 
            sqls[1] = pageSql.ToString();

//临时测试
            sqls[1] = sqls[1].Replace("@", "").Replace(":", "").Replace("PageStart", ++start + "").Replace("PageEnd", end + "").Replace("PageLimit", limit + "");

Console.WriteLine("【count】{0}", sqls[0]);
            Console.WriteLine("【page】{0}", sqls[1]);
            Console.WriteLine();
        }

/// <summary>
        /// 生成常规Sql语句
        /// </summary>
        /// <param name="sql"></param>
        /// <param name="sqls"></param>
        /// <param name="start"></param>
        /// <param name="limit"></param>
        /// <param name="createCount"></param>
        public void CreatePageSql(string sql, out string[] sqls, int start, int limit, bool createCount = false)
        {
            //需要输出的sql数组
            sqls = null;

//生成count的SQL语句 SqlServer生成分页，必须通过正则拆分
            if (createCount || dbType == DBType.SqlServer || dbType == DBType.SqlServer2000)
            {
                sqls = SplitSqlForPaging(sql);
                if (sqls == null)
                {
                    //无法解析的SQL语句
                    throw new Exception("can't parse sql to pagesql ,the sql is " + sql);
                }
            }
            else
            {
                sqls = new string[2];
            }

//组织分页SQL语句
            var pageSql = new StringBuilder();

var end = start + limit;
            if (dbType == DBType.SqlServer2000)
            {
                pageSql.AppendFormat("SELECT TOP @PageEnd {0} {1}", sqls[2], sqls[3]);

if (start > 1)
                {
                    var orderChange = string.IsNullOrEmpty(sqls[5]) ? null :
                        string.Compare(sqls[5], "desc", true) == 0 ?
                        string.Format("{0} ASC ", sqls[4]) :
                        string.Format("{0} DESC ", sqls[4]);
                    pageSql.Insert(0, "SELECT TOP 100 PERCENT  * FROM (SELECT TOP @PageLimit * FROM ( ");
                    pageSql.AppendFormat(" ) PageTab {0} ) PageTab2 {1}", orderChange, sqls[3]);
                }
            }
            else if (dbType == DBType.SqlServer)
            {
                pageSql.AppendFormat(" Select top @PageEnd ROW_NUMBER() over ({0}) RN , {1}",
                    string.IsNullOrEmpty(sqls[3]) ? "ORDER BY (SELECT NULL)" : sqls[3],
                    sqls[2]);

//如果查询不是第一页，则需要判断起始行号
                if (start > 1)
                {
                    pageSql.Insert(0, "Select PageTab.* from ( ");
                    pageSql.Append(" ) PageTab Where RN >= @PageStart");
                }
            }
            else if (dbType == DBType.Oracle)
            {
                pageSql.Append("select ROWNUM RN,  PageTab.* from ");
                pageSql.AppendFormat(" ( {0} ) PageTab ", sql);
                pageSql.Append(" where ROWNUM <= :PageEnd ");

//如果查询不是第一页，则需要判断起始行号
                if (start > 1)
                {
                    pageSql.Insert(0, "select * from ( ");
                    pageSql.Append(" ) Where RN>= :PageStart ");
                }
            }
            else if (dbType == DBType.SQLite)
            {
                pageSql.AppendFormat("{0} limit @PageStart,@PageLimit", sql, start, limit);
            }

//存储生成的分页SQL语句 
            sqls[1] = pageSql.ToString();

//临时测试
            sqls[1] = sqls[1].Replace("@", "").Replace(":", "").Replace("PageStart", ++start + "").Replace("PageEnd", end + "").Replace("PageLimit", limit + "");

Console.WriteLine("【count】{0}", sqls[0]);
            Console.WriteLine("【page】{0}", sqls[1]);
            Console.WriteLine();
        }
    }

1.交换2个整数用了这样的算法。交换a和b，a=a+b;b=a-b;b=a-b;这是原来找工作的时候被考到的，如果在不使用第三方变量的情况下交换2个整数。

2.Sql2000下由于是使用top进行分页，除非条件一条数据都查不到，否则在分页start和limit参数超过了总行数时，也会查询出数据。

3.拆分Sql语句，参考了PetaPoco的部分源代码。

4.我的应用场景则是在dbhelp类，某个方法传递sql，start，limit参数即可对sql查询出来的结果进行分页。其中start：查询结果的起始行号(不包括它)，limit：需要取出的行数。如 start:0,limit:15 则是取出前15条数据。