Batch number auto generation query is not working properly

declare @Var as nvarchar(25)

declare @Var1 as nvarchar(25)

declare @Var2 as nvarchar(25)

declare @Var3 as nvarchar(25)

declare @Var4 as nvarchar(25)

select @Var4 =N''

select @Var4 = @Var4

if @var4 = '01'

begin

set @var4 ='D'

select @Var =max(cast(batchnum as NVARCHAR(25))) from oibt where batchnum like '%zd%'

select @Var2 = substring(@Var,3,3)

select @Var3 = substring(@Var,3,6)

end

else

begin

set @var4 ='I'

select @Var = max(cast(batchnum as NVARCHAR(25))) from oibt where batchnum like '%zi%'

select @Var2 = substring(@Var,3,3)

select @Var3 = substring(@Var,3,6)

end

select @Var1 = substring(@Var,5,8)

select @Var1 = @Var3 + 1

select @Var2= 'Z'+ @var4 + @Var1

select @var2

this query is not fetching the maximum value of batch number so kindly help me out in this as soon as possible

regards