select action_id,data from [dbo].[list] where start_id>@start_id
order by start_id

update [dbo].[clients_list] set start_id=
(select isnull(max(start_id),0) from [dbo].[list])  -- можно ли избежать этого подзапроса?
where client_id=@client_id;

declare @t table (action_id int,data varchar(255),start_id int)
declare @maxstartid int

insert into @t (action_id,data, start_id)
select action_id,data, start_id from [dbo].[list] where start_id>@start_id

select action_id,data
FROM @t
order by start_id

set @maxstartid =  (select top (1) start_id from @t order by start_id desc);

update [dbo].[clients_list] set start_id=@maxstartid 
where client_id=@client_id;