今天百度知道那里，看到个问题
说 需要一个因日期变更的归零自增字段
在今天0409他会以0409001开始无限增加，
到0410这天会以0410001开始无限增加，以此类推。。
关键是解决001这三位数怎样随日期变更而重置？？

下面是测试表，只描述几个关键的字段：
也就是 
p_id ：自增主键
p_year : 记录的日期
p_num : 准备作为那个流水号的字段.

create table Pdms_polling(
 p_id int primary key IDENTITY(1,1) ,
 p_year datetime ,
 p_num varchar(30) 
);
go


-- 这个是我一开始写的触发器
-- 算法是使用 ROW_NUMBER() 对 日期进行分组，按自增主键进行排序
-- 最后是形成一个按日期递增 序号的列表
-- 然后使用 right( cast(power(10,3) as varchar) + 序号, 3) 来将其格式化为 000 的格式

CREATE TRIGGER TriAutoNum
 ON Pdms_polling
FOR INSERT
AS 
  DECLARE
    @newID  INT,
    @newNum    varchar(30),
    @oldNum    varchar(30);
BEGIN
  -- 定义游标.
  DECLARE c_test_main CURSOR FAST_FORWARD FOR
    SELECT 
      p_id, 
      p_num,
      Convert(varchar(10), p_year, 112) +
      right(
        cast(power(10,3) as varchar)
        + ROW_NUMBER() OVER(PARTITION BY Convert(varchar(10), p_year, 112) ORDER BY p_id)
        , 3)
   FROM 
     Pdms_polling
   WHERE
     Convert(varchar(10), p_year, 112) 
       IN ( SELECT DISTINCT Convert(varchar(10), p_year, 112) FROM inserted );
     
  -- 打开游标.
  OPEN c_test_main;

  -- 填充数据.
  FETCH NEXT FROM c_test_main INTO @newID, @oldNum, @newNum;

  -- 假如检索到了数据，才处理.
  WHILE @@fetch_status = 0
  BEGIN
    
    IF @oldNum IS NULL BEGIN
      UPDATE
        Pdms_polling
      SET
        p_num = @newNum
      WHERE
        p_id = @newID;
    END;
    -- 填充下一条数据.
    FETCH NEXT FROM c_test_main INTO @newID, @oldNum, @newNum;
  END;
  
  -- 关闭游标
  CLOSE c_test_main;
  -- 释放游标.
  DEALLOCATE c_test_main;
END;
go




-- 下面是测试插入一条记录
1> INSERT INTO Pdms_polling (p_year) VALUES( GETDATE() );
2> go

(1 行受影响)
1>
2> SELECT * FROM Pdms_polling;
3> go
p_id        p_year                  p_num
----------- ----------------------- ------------------------------
          1 2011-04-09 22:53:32.640 20110409001

(1 行受影响)


1> -- 下面是测试插入多条记录
2> INSERT INTO Pdms_polling (p_year)
3>   SELECT GETDATE()
4>   UNION ALL SELECT GETDATE()
5>   UNION ALL SELECT GETDATE();
6> go

(1 行受影响)
1>
2> SELECT * FROM Pdms_polling;
3> go
p_id        p_year                  p_num
----------- ----------------------- ------------------------------
          1 2011-04-09 22:53:32.640 20110409001
          2 2011-04-09 22:53:32.840 20110409002
          3 2011-04-09 22:53:32.840 20110409003
          4 2011-04-09 22:53:32.840 20110409004

(4 行受影响)
1>
2> INSERT INTO Pdms_polling (p_year)
3>   SELECT '2011-04-08 10:00:00'
4>   UNION ALL SELECT '2011-04-09 10:00:00'
5>   UNION ALL SELECT '2011-04-10 10:00:00';
6> go

(1 行受影响)


1> -- 下面是测试插入多条记录 不同天的。
2> SELECT * FROM Pdms_polling;
3> go
p_id        p_year                  p_num
----------- ----------------------- ------------------------------
          1 2011-04-09 22:53:32.640 20110409001
          2 2011-04-09 22:53:32.840 20110409002
          3 2011-04-09 22:53:32.840 20110409003
          4 2011-04-09 22:53:32.840 20110409004
          5 2011-04-08 10:00:00.000 20110408001
          6 2011-04-09 10:00:00.000 20110409005
          7 2011-04-10 10:00:00.000 20110410001

(7 行受影响)




写好以后，又回去看看那个问题。
发现别人用 CTE 来处理，效果也不错。

回头我再来修改修改我的触发器，也使用 CTE 来处理。
这样就可以不用游标了。代码也简短了不少。


ALTER TRIGGER TriAutoNum
 ON Pdms_polling
FOR INSERT
AS 
BEGIN
  
  WITH
    cte
  AS
  (
    SELECT 
      p_id, 
      p_num,
      Convert(varchar(10), p_year, 112) +
      right(
        cast(power(10,3) as varchar)
        + ROW_NUMBER() OVER(PARTITION BY Convert(varchar(10), p_year, 112) ORDER BY p_id)
        , 3)  AS new_p_num
    FROM 
      Pdms_polling
    WHERE
      Convert(varchar(10), p_year, 112) 
        IN ( SELECT DISTINCT Convert(varchar(10), p_year, 112) FROM inserted )
  )
  UPDATE
    Pdms_polling
  SET
    Pdms_polling.p_num = cte.new_p_num
  FROM 
    Pdms_polling 
      JOIN cte 
      ON (Pdms_polling.p_id = cte.p_id)
  WHERE
    Pdms_polling.p_num IS NULL;

END;
go


-- 下面是测试插入一条记录
1> INSERT INTO