Unique constraint on two fields, and their opposite

Question

I have a data structure, where I have to store pairs of elements. Each pair has exactly 2 values in it, so we are employing a table, with the fields(leftvalue, rightvalue....). These pairs should be unique, and they are considered the same, if the keys are changed.

Example: (Fruit, Apple) is the same as (Apple, Fruit).

If it is possible in an efficient way, I would put a database constraint on the fields, but not at any cost - performance is more important.

We are using MSSQL server 2008 currently, but an update is possible.

Is there an efficient way of achieving this?

Answer 1

Two solutions, both really about changing the problem into an easier one. I'd usually prefer the T1 solution if forcing a change on consumers is acceptable:

create table dbo.T1 (
    Lft int not null,
    Rgt int not null,
    constraint CK_T1 CHECK (Lft < Rgt),
    constraint UQ_T1 UNIQUE (Lft,Rgt)
)
go
create table dbo.T2 (
    Lft int not null,
    Rgt int not null
)
go
create view dbo.T2_DRI
with schemabinding
as
    select
        CASE WHEN Lft<Rgt THEN Lft ELSE Rgt END as Lft,
        CASE WHEN Lft<Rgt THEN Rgt ELSE Lft END as Rgt
    from dbo.T2
go
create unique clustered index IX_T2_DRI on dbo.T2_DRI(Lft,Rgt)
go

In both cases, neither T1 nor T2 can contain duplicate values in the Lft,Rgt pairs.

Answer 2

If you always store the values in order but store the direction in another column,

CREATE TABLE [Pairs]
(
    [A] NVarChar(MAX) NOT NULL,
    [B] NVarChar(MAX) NOT NULL,
    [DirectionAB] Bit NOT NULL,
    CONSTRAINT [PK_Pairs] PRIMARY KEY ([A],[B]) 
)

You can acheive exaclty what you want with one clustered index, and optimize your lookups too.

So when I insert the pair 'Apple', 'Fruit' I'd do,

INSERT [Pairs] VALUES ('Apple', 'Friut', 1);

Nice and easy. Then I insert 'Fruit', 'Apple',

INSERT [Pairs] VALUES ('Apple', 'Fruit', 0); -- 0 becuase order is reversed.

The insert fails because this is a primary key violation. To further illustrate, the pair 'Coconuts', 'Bananas' would be stored as

INSERT [Pairs] VALUES ('Bananas', 'Coconuts', 0);

---

For additional lookup performance, I'd add the index

CREATE NONCLUSTERED INDEX [IX_Pairs_Reverse] ON [Pairs] ([B], [A]);

---

If you can't control inserts to the table, it may be necessary to ensure that [A] and [B] are inserted correctly.

CONSTRAINT [CK_Pairs_ALessThanB] CHECK ([A] < [B])

But this may be an unnecessary performance hit, depending on how controlled your inserts are.

Answer 3

One way would be to create a computed column that combines the two values and put a unique constraint upon it:

create table #test (
    a varchar(10) not null, 
    b varchar(10) not null, 
    both as case when a > b then a + ':' + b else b + ':' + a end persisted unique nonclustered
    )

so

insert #test
select 'apple', 'fruit'
insert #test
select 'fruit', 'apple'

Gives

(1 row(s) affected)
Msg 2627, Level 14, State 1, Line 3
Violation of UNIQUE KEY constraint 'UQ__#test_____55252CB631EC6D26'. Cannot insert duplicate key in object 'dbo.#test'.
The statement has been terminated.

Answer 4

Unique constraint on two/more fields is possible but on their opposite no...

SQL Server 2005 Unique constraint on two columns

Unique constraint on multiple columns

How do I apply unique constraint on two columns SQL Server?

http://www.w3schools.com/sql/sql_unique.asp