auto increment on composite primary key

Question

I have a table called 'Workspaces' where the columns 'AreaID' and 'SurfaceID' work as a composite primary key. The AreaID references to another table called 'Areas' which only has AreaID as the primary key. What I want to do now is to make the surfaceID recound from 1 on every new AreaID. Right now I'm using the following code for the tables 'Areas' and 'Workspaces':

--Table 'Areas'
CREATE TABLE Areas (
AreaID INT IDENTITY(1,1) PRIMARY KEY,
Areaname VARCHAR(60) UNIQUE NOT NULL
)

--Table 'Workspaces'
CREATE TABLE Workspaces (
AreaID INT
CONSTRAINT ck_a_areaid REFERENCES Areas(AreaID)
ON DELETE CASCADE
ON UPDATE NO ACTION,
SurfaceID INT IDENTITY(1,1)
CONSTRAINT ck_surfaceid CHECK (surfaceid > 0 AND surfaceid < 1001),
Description VARCHAR(300) NOT NULL,
CONSTRAINT ck_workspaces PRIMARY KEY (AreaID, SurfaceID)
)

When I use the code above I get a result like this when creating new workspaces in different areas:

AreaID    SurfaceID
1         1
1         2
1         3
2         4
2         5
3         6
Etc...

But I want the SurfaceID to recount from 1 on every new areaID, so my desired result would be like this:

AreaID    SurfaceID
1         1
1         2
1         3
2         1
2         2
3         1
Etc...

Does anyone know how this can be fixed?

Answer 1

I am agree with Mr. Linoff's answer but if you want to store it phisicaly, you can do it within an insert trigger:

Update Your_Table
set SurfaceID =  ( select max(isnull(SurfaceID,0))+1 as max 
                  from Workspaces t
                  where t.AreaID = INSERTED.AreaID )

---

EDIT:*(as an example wanted for how to implement it)

In the question I have seen two table that's why I have wrote the code as above, but following is a sample for what I meant:

Sample table:

CREATE TABLE testTbl 
(
    AreaID INT,
    SurfaceID INT, --we want this to be auto increment per specific AreaID 
    Dsc VARCHAR(60)NOT NULL
)

Trigger:

CREATE TRIGGER TRG
ON testTbl
INSTEAD OF INSERT

AS

DECLARE @sid INT
DECLARE @iid INT
DECLARE @dsc VARCHAR(60)

SELECT @iid=AreaID FROM INSERTED
SELECT @dsc=DSC FROM INSERTED

--check if inserted AreaID exists in table -for setting SurfaceID
IF NOT EXISTS (SELECT * FROM testTbl WHERE AreaID=@iid)
SET @sid=1
ELSE
SET @sid=(  SELECT MAX(T.SurfaceID)+1 
            FROM testTbl T
            WHERE T.AreaID=@Iid
          )

INSERT INTO testTbl (AreaID,SurfaceID,Dsc)
            VALUES  (@iid,@sid,@dsc)

Insert:

INSERT INTO testTbl(AreaID,Dsc) VALUES (1,'V1');
INSERT INTO testTbl(AreaID,Dsc) VALUES (1,'V2');
INSERT INTO testTbl(AreaID,Dsc) VALUES (1,'V3');
INSERT INTO testTbl(AreaID,Dsc) VALUES (2,'V4');
INSERT INTO testTbl(AreaID,Dsc) VALUES (2,'V5');
INSERT INTO testTbl(AreaID,Dsc) VALUES (2,'V6');
INSERT INTO testTbl(AreaID,Dsc) VALUES (2,'V7');
INSERT INTO testTbl(AreaID,Dsc) VALUES (3,'V8');
INSERT INTO testTbl(AreaID,Dsc) VALUES (4,'V9');
INSERT INTO testTbl(AreaID,Dsc) VALUES (4,'V10');
INSERT INTO testTbl(AreaID,Dsc) VALUES (4,'V11');
INSERT INTO testTbl(AreaID,Dsc) VALUES (4,'V12');

Check the values:

SELECT * FROM testTbl

Output:

AreaID  SurfaceID   Dsc
   1       1        V1
   1       2        V2
   1       3        V3
   2       1        V4
   2       2        V5
   2       3        V6
   2       4        V7
   3       1        V8
   4       1        V9
   4       2        V10
   4       3        V11
   4       4        V12

IMPORTANT NOTICE: this trigger does not handle multi row insertion once and it is needed to insert single record once like the example. for handling multi record insertion it needs to change the body of and use row_number

Answer 2

You cannot easily do what you want. You can do it using triggers, but that is a rather ugly solution. You can get close to what you want by using a single identity primary key and then calculating the number you want on output:

CREATE TABLE Workspaces (
    WorkspacesId int not null identity(1, 1) primary key,
    AreaID INT,
    Description VARCHAR(300) NOT NULL,
    CONSTRAINT ck_a_areaid REFERENCES Areas(AreaID) ON DELETE CASCADE ON UPDATE NO ACTION,
);

Then when you query (or in a view):

select w.*, row_number() over (partition by areaId
                               order by WorkspaceId) as SurfaceId
from Workspaces

Note: This does not check the maximum value of surfaceId. If you really need to implement this logic, then you will need to use triggers.