不同表上列的唯一约束

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

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 2

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 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