如果记录不存在，则插入并更新，否则竞争条件

if exists (select itemcode from item where itemcode=1120)
update item 
set itemname = \'laptop\'
where itemcode = 1120

else 
insert into item (itemcode,itemname)
values (1120,\'laptop\')

transaction

begin transaction

begin try

    if exists (select itemcode from item where itemcode=1120)
    BEGIN
    update item 
    set itemname = \'laptop\'
    where itemcode = 1120
    END

    else 
    BEGIN
    insert into item (itemcode,itemname)
    values (1120,\'laptop\')
    END

    commit transaction

end try

begin catch
  raiserror(\'Message here\', 16, 1)
  rollback transaction
end catch