--create table Tid
--(
--task_id  varchar(20),
--members  varchar(30)
--)
--
--create table Tname 
--(
--[user_id] varchar(20),
--username varchar(50)
--)
--
--insert  Tid
--select 1,'34,35,36'
--union select 2,'33,36'
--insert Tname 
--select 33,N'劉二'
--union select 34,N'張三'
--union select 35,N'李四'
--union select 36,N'王五'

declare @Id int, @Members nvarchar(20)
select @Members ='34,35,36'

--select username from Tname where user_id in (@Members)

EXEC ('select username from Tname where user_id in ('+@Members+')')

/*
張三
李四
王五*/

------解决方案--------------------
你这样select username from Tname where user_id in (@Members)传值进去相当于
select username from Tname where user_id in ('34,35,36')当成一个字符串了，当然没有结果了
可以
select username from Tname where CHARINDEX(',' + CAST(user_id AS VARCHAR(10)) + ',',',' + @Members + ',') > 0

------解决方案--------------------
SQL code

declare @Id int, @Members nvarchar(20)
select @Members ='34,35,36'
 exec('select username from Tname where user_id in ('+''''+@Members+''''+')')
 select username from Tname where user_id in (34,35,36)

------解决方案--------------------
要么动态拼接，要么用CHARINDEX。