SQL 文的一些操作

一：格式转化类
     1.cast Syntax is: cast(expression as date_type[(length)])
                  Example: cast('20070601' varchar(12))
     2.Convert  Syntax :Convert(date_type[(Length)],expression [,Style])
                 Example; Convert(varchar(10),Getdate(),112)
二：操作字符串类
     1.Substring(expression,start,length)
                 Example: Substring("zhangzeshuai",0,4)
                 Result: zhan
三：日期加减
     1.DateAdd(datepart,number,date)
                Example:DateAdd('yy',1,'20070101')
                Result:20080101
     2.Fecth this month's First day
           Declare   @DateTime  datetime
           Set   @DateTime='2006-02-21' 
           Select           DateAdd(dd,-1,Convert(Varchar(8),
                                      DateAdd(mm,1,@DateTime),120)+'01') 
四：查询条件的拼接
     1.SELECT * from table_name
        where (id = @id or @id is null)
       Explain: if the parameter @id is null,this row isn't transact(执行)