declare @nd varchar(10) set @nd=2012
declare @yf varchar(10) set @yf=7
select (cast(@nd + '-'+@yf + '-01' as datetime)) as rq 

/*
rq
-----------------------
2012-07-01 00:00:00.000
*/

declare @nd1 varchar(10) set @nd1='2001~2021'
declare @yf1 varchar(10) set @yf1='1~12'

select 
convert(varchar(10),cast(left(@nd1,charindex('~',@nd1)-1)+
'-'+left(@yf1,charindex('~',@yf1)-1)+'-01' as datetime),120)+'~'+
convert(varchar(10),cast(right(@nd1,len(@nd1)-charindex('~',@nd1))+
'-'+right(@yf1,len(@yf1)-charindex('~',@yf1))+'-01' as datetime),120) as rq
/*
rq
---------------------
2001-01-01~2021-12-01
*/

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


Select CONVERT(varchar(10), (cast(nd + '-'+yf + '-01' as datetime)), 120)as rq from tb

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

select cast(DATENAME (YEAR, getdate())+'-'+DATENAME (MONTH, getdate())+'-01' as datetime)

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

--理解有误，不好意思
--try
declare @nd varchar(20),@yf varchar(6)
set @nd='2001~2021'
set @yf='1~12'
select 
substring(@nd,1,charindex('~',@nd)-1)+'-1-1~'+convert(varchar,convert(int,substring(@nd,charindex('~',@nd)+1,4))-1)+'-12-01'