UNIQUE Constraint For Columns On Different Tables - sql

Question

I have three tables a,b,and c each with an int IDENTITY PK field that relates to the child table.
CREATE TABLE [dbo].[a]([aID] [int] IDENTITY(1,1) NOT NULL,[aCode] [varchar](20) NOT NULL,
CONSTRAINT [PK_a] PRIMARY KEY CLUSTERED([aID] ASC)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]
GO
CREATE TABLE [dbo].[b]([bID] [int] IDENTITY(1,1) NOT NULL,[aID] [int] NOT NULL, [bCode] [varchar](20) NOT NULL,
CONSTRAINT [PK_b] PRIMARY KEY CLUSTERED([bID] ASC)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]
GO
CREATE TABLE [dbo].[c]([cID] [int] IDENTITY(1,1) NOT NULL,[bID] [int] NOT NULL,[cCode] [varchar](20) NOT NULL,
CONSTRAINT [PK_c] PRIMARY KEY CLUSTERED([cID] ASC)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]
GO
ALTER TABLE [dbo].[b] WITH CHECK ADD CONSTRAINT [FK_b_a] FOREIGN KEY([aID])
REFERENCES [dbo].[a] ([aID])
GO
ALTER TABLE [dbo].[b] CHECK CONSTRAINT [FK_b_a]
GO
ALTER TABLE [dbo].[c] WITH CHECK ADD CONSTRAINT [FK_c_b] FOREIGN KEY([bID])
REFERENCES [dbo].[b] ([bID])
GO
ALTER TABLE [dbo].[c] CHECK CONSTRAINT [FK_c_b]
GO
How do I create a CONSTRAINT that enforces a unique condition for a.aID, c.cCode?

Answer 1

If you are looking for a unique constraint over a combination of columns a.aID, c.cCode one way is to alter table c and add [aid] column to it and have a composite unique key.
ALTER TABLE [dbo].[c](
[cID] [int] IDENTITY(1,1) NOT NULL,
[bID] [int] NOT NULL,[cCode] [varchar](20) NOT NULL,
[cCode] [varchar](20) NOT NULL,
[aID] INT FOREIGN KEY REFERENCES [a]([aID]) NOT NULL,
CONSTRAINT uq_cCode_aid UNIQUE NONCLUSTERED (cCode,aID),
CONSTRAINT [PK_c] PRIMARY KEY CLUSTERED([cID] ASC)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
ON
[PRIMARY]) ON [PRIMARY]
GO

Answer 2

You can add a unique constraint as follows:
ALTER TABLE [C] ADD CONSTRAINT [uc_C_cCode] UNIQUE NONCLUSTERED [cCode];
You cannot however enforce a constraint across tables, which is what it sounds like you are trying to do.
As for table A.aID, its a primary key, it will already have a unique constraint.

Answer 3

I found a way to get what I wanted (I think) using an indexed view. Please comment on if this seems appropriate. My tests so far conclude that it does work.
I created a view with schemabinding on joining c to a (going through b).
I created an clustered index to enforce unique on aID and cCode.
This allows me to not need to include aID within the c table but still enforces the unique constraint.
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE VIEW [dbo].[vw.aIDcCode] WITH SCHEMABINDING
AS
SELECT dbo.a.aID, dbo.c.cCode
FROM dbo.a
INNER JOIN
dbo.b ON dbo.a.aID = dbo.b.aID
INNER JOIN
dbo.c ON dbo.b.bID = dbo.c.bID
GO
SET ARITHABORT ON
GO
SET CONCAT_NULL_YIELDS_NULL ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_NULLS ON
GO
SET ANSI_PADDING ON
GO
SET ANSI_WARNINGS ON
GO
SET NUMERIC_ROUNDABORT OFF
GO
CREATE UNIQUE CLUSTERED INDEX [IDX_Unique_aIDcCode] ON [dbo].[vw.aIDcCode]
([aID] ASC, [cCode] ASC)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
ON [PRIMARY]
GO