sql - 如何创建由其他列分段的自动增量列

Question

我需要创建一个包含递增 ID 的表，但我希望 ID 能够根据其他列自动分段。这是我想要的:

CREATE TABLE dbo.MyTable (
    myKey INT IDENTITY PRIMARY KEY,
    category INT,
    incrementalId INT
);

INSERT INTO dbo.MyTable (category) VALUES (100);
INSERT INTO dbo.MyTable (category) VALUES (200);
INSERT INTO dbo.MyTable (category) VALUES (100);
INSERT INTO dbo.MyTable (category) VALUES (100);
INSERT INTO dbo.MyTable (category) VALUES (100);
INSERT INTO dbo.MyTable (category) VALUES (200);

SELECT *
FROM dbo.MyTable;

我希望它显示如下内容:

myKey       category    incrementalId
----------- ----------- -------------
1           100         1
2           200         1
3           100         2
4           100         3
5           100         4
6           200         2

意思是我希望每个类别的 incrementalId 自动递增，并且对于插入的任何新类别从 1 重新开始。我希望在表中的任何插入操作上自行完成此操作(我不想在插入此表时记住这样做)。

我认为这可能是通过窗口函数和触发器来完成的，但我只是不知道如何做。

编辑:

我希望保留数据以避免在发生数据删除时移动 incrementalId。此外，理想情况下，在删除行时不会重新给出相同的 ID(与序列或 IDENTITY 的工作方式相同)

有什么想法吗？

Answer 1

CREATE TABLE dbo.MyTable (
  myKey INT IDENTITY PRIMARY KEY,
  category INT,
  incrementalId INT
);
GO

create table dbo.nextCategoryID (
  category int,
  nextidvalue int,
  constraint PK_nextCategoryID primary key clustered( category, nextidvalue )
);
GO

create trigger numberByCategory on dbo.MyTable
after insert as 

-- Automatically add any net new category
insert into dbo.nextCategoryID ( category, nextidvalue )
    select distinct category, 1 as nextidvalue
    from inserted
    where not exists ( select * from dbo.nextCategoryID s
        where s.category = inserted.category );


-- Number the new rows in each incoming category
with numberedrows as (
    select 
        i.myKey, 
        i.category, 
        n.nextidvalue - 1 + row_number() over ( partition by i.category order by i.category ) as incrementalId
    from inserted i
    join dbo.nextCategoryID n on i.category = n.category
)
update m
    set incrementalId = n.incrementalId
from dbo.MyTable m
join inserted i on m.myKey = i.myKey
join numberedrows n on n.myKey = i.myKey;


update dbo.nextCategoryID
    set nextidvalue = 1 + ( select max( m.incrementalId ) 
        from inserted i 
        join dbo.MyTable m on i.myKey = m.myKey
        where i.category = nextCategoryID.category 
    )
where exists ( select * 
    from inserted i
    where i.category = nextCategoryID.category
);

GO

-- Test data 

INSERT INTO dbo.MyTable (category) VALUES (100);
INSERT INTO dbo.MyTable (category) VALUES (200);
INSERT INTO dbo.MyTable (category) VALUES (100);
INSERT INTO dbo.MyTable (category) VALUES (100);
INSERT INTO dbo.MyTable (category) VALUES (100);
INSERT INTO dbo.MyTable (category) VALUES (200);

insert into dbo.MyTable (category) 
values 
    ( 200 ),
    ( 200 ),
    ( 100 ),
    ( 300 ),
    ( 400 ),
    ( 400 )


SELECT *
FROM dbo.MyTable;