在插入中将A字段设置为与ID（IDENTITY）相同

create table TestTable (id int identity, col1 int)
go
create trigger TestTrigger on TestTable after insert
    as begin
    update  TestTable 
    set     col1 = id
    where   col1 is null
            and id in (select id from inserted)
    end
go

insert TestTable default values
insert TestTable (col1) values (666)
insert TestTable default values
select * from TestTable

create procedure dbo.InsertTestRow(
    @col1 int)
as
insert  TestTable (col1) values (@col1)

if @col1 is null
    begin
    update  TestTable
    set     col1 = id
    where   id = SCOPE_IDENTITY()
    end

create table T (ID int identity, ID2 int)

insert into T values (default)
insert into T values (null)
insert into T values (78)

select
  ID, 
  coalesce(ID2, ID) as ID2
from T

ID  ID2
--  ---
1   1
2   2
3   78

ALTER TABLE MyTable
ADD CONSTRAINT MyTableCodeDefault
DEFAULT IDENT_CURRENT(\'MyTable\') FOR Code

INSERT INTO MyTable DEFAULT VALUES
INSERT INTO MyTable ({columns NOT including \'Code\'}) 
  VALUES ({value list matching insert columns})
INSERT INTO MyTable (Code) VALUES (666)
INSERT INTO MyTable (Code) SELECT 8 UNION SELECT 13 UNION SELECT 21

INSERT INTO MyTable ({columns NOT including \'Code\'})
SELECT {value list matching insert columns}
UNION 
SELECT {value list matching insert columns}
UNION
SELECT {value list matching insert columns}

if object_id(\'TempTable\') is not null drop table TempTable
create table TempTable (Id int identity(1,1), Code as Id)

insert into TempTable
default values

insert into TempTable
default values

insert into TempTable
default values

select * from TempTable

default values

if object_id(\'TempTable\') is not null drop table TempTable
create table TempTable (Id int identity(1,1), Code as Id, SomethingElse int)

insert into TempTable (SomethingElse)
select 10 union all
select 11 union all
select 12

select * from TempTable

SET IDENTITY_INSERT [tableName] ON