create table tb(docid int,doctitle nvarchar(10))
insert into tb select 1,'hello'
insert into tb select 2,'nihao'
insert into tb select 3,'你好'
insert into tb select 120,'你好aaaa'

declare @str varchar(2000)='1,3,3,120,2,1'
set @str='select a.* from tb a,(select '+REPLACE(@str,',',' as id union all select ')
    +') b where a.docid=b.id'
print @str
exec (@str)

/*
docid       doctitle
----------- ----------
1           hello
3           你好
3           你好
120         你好aaaa
2           nihao
1           hello

------解决方案--------------------

SQL code
　　use tempdb
　　go
　　--测试数据
　　declare @s varchar(1000)
　　set @s='ak47,mp5,1,23'
　　/*要求输出结果
　　S
　　----
　　ak47
　　mp5
　　1
　　23
　　*/
　　--3种方法对比:
　　--1.[朴实]动态Exec方法:
　　declare @s1 varchar(1000)
　　set @s1=right(replace(','+@s,',',''' as S union select '''),len(replace(','+@s,',',''' as S union select '''))-12)+''''
　　exec(@s1)
　　--2.[变通]表交叉方法:
　　select replace(reverse((left(s,charindex(',',s)))),',','') as S from(
　　select r,reverse(left(@s,r))+',' as s
　　from(
　　select (select count(*) from sysobjects where name<=t.name ) as r
　　from sysobjects t
　　)a where r<=len(@s)
　　and left(@s+',',r+1) like '%,'
　　)t order by r
　　--3.[高级]XML方法:
　　DECLARE @idoc int;
　　DECLARE @doc xml;
　　set @doc=cast('<Root><item><S>'+replace(@s,',','</S></item><item><S>')+'</S></item></Root>' as xml)
　　EXEC sp_xml_preparedocument @Idoc OUTPUT, @doc
　　SELECT * FROM OPENXML (@Idoc, '/Root/item',2)
　　WITH (
　　[S] varchar(10)
　　)