How to find missing ID Number Gridview control?

1. Fetch the First row id and last row id.
2. create Enumerable range

IEnumerable<int> nos= Enumerable.Range(firstrowid, lastrowid);

3. use linq to get missing nos.

var missingid = from a in nos
              where !(from b in gvRows.AsEnumerable()
                        select b["id"])
                        .Contains(a)
              select a;

This question often comes up, and sadly, the most common (and most portable) answer is to create a temporary table to hold the IDs that should be there, and do a left join. The syntax is pretty similar between MySQL and SQL Server. The only real difference is the temporary tables syntax.

declare @id int
declare @maxid int

set @id = 1
select @maxid = max(id) from tbl

create table #IDSeq
(
    id int
)

while @id < @maxid --whatever you max is
begin
    insert into #IDSeq values(@id)

    set @id = @id + 1
end

select
    s.id
from
    #idseq s
    left join tbl t on
        s.id = t.id
 where t.id is null

 drop table #IDSeq