这个自增字段，怎么写？
CREATE TABLE [ProductCartons] (
[ProductID] [int] NOT NULL ,
[Carton#] [int] not null,
[CLength] [float] NULL DEFAULT (0),
[CWidth] [float] NULL DEFAULT (0),
[CHeight] [float] NULL DEFAULT (0))
GO

--ID 为1的第一个Carton，[Carton#]自增为1
insert ProductCartons([ProductID],[CLength],[CWidth],[CHeight])
values(1,18,15,19)

--ID 为1的第二个Carton，[Carton#]自增为2
insert ProductCartons([ProductID],[CLength],[CWidth],[CHeight])
values(1,36,30,38)

怎么把[Carton#]设为自增字段

如果插入新的ProductID，[Carton#]为1

如果插入的ProductID已存在，[Carton#]=该ProductID的最大[Carton#]+1

高手帮忙，谢谢！！！




[解决办法]

CREATE TABLE [ProductCartons] (
[ID] int identity(1, 1),
[ProductID] [int] NOT NULL ,
[Carton#] [int] null,
[CLength] [float] NULL DEFAULT (0),
[CWidth] [float] NULL DEFAULT (0),
[CHeight] [float] NULL DEFAULT (0))
GO

create trigger tr on ProductCartons
for insert
as
declare @Carton int
select @Carton=max(A.[Carton#] )
from ProductCartons as A, inserted as B
where A.[ProductID]=B.[ProductID]

if @Carton is null
set @Carton=1
else
set @Carton=@Carton+1

update A set A.[Carton#]=@Carton
from ProductCartons as A, inserted as B
where A.ID=B.ID
go

insert ProductCartons([ProductID],[CLength],[CWidth],[CHeight])
values(1,18,15,19)

insert ProductCartons([ProductID],[CLength],[CWidth],[CHeight])
values(1,36,30,38)

insert ProductCartons([ProductID],[CLength],[CWidth],[CHeight])
values(2,36,30,38)

select * from ProductCartons

--result
ID ProductID Carton# CLength CWidth CHeight
----------- ----------- ----------- ----------------------------------------------------- ----------------------------------------------------- -----------------------------------------------------
1 1 1 18.0 15.0 19.0
2 1 2 36.0 30.0 38.0
3 2 1 36.0 30.0 38.0

(3 row(s) affected)