Question

In an Alexander Kuznetsov article, he presents the follow code snippet:

CREATE TABLE dbo.Vehicles(
    ID INT NOT NULL, 
    [Type] VARCHAR(5) NOT NULL,
    CONSTRAINT Vehicles_PK PRIMARY KEY(ID),
    CONSTRAINT Vehicles_UNQ_ID_Type UNIQUE(ID, [Type]),
    CONSTRAINT Vehicles_CHK_ValidTypes CHECK([Type] IN ('Car', 'Truck'))
);

This snippet raises a few questions for me.

1. Why is it necessary to include both ID and Type in the unique constraint? If just ID is unique, then the combination of the two columns will always be unique as well.

2. Also, I know how to set a primary key and specify if it unique in SSMS. But how would I specify a primary key on one column, and make a unique constraint on a combination of columns? Does this create two indexes?

This came up because I’m trying to implement similar code, which does not create a composite primary key, and I get the following error. So I’m trying to understand this code better.

The columns in table ‘MyTable’ do not match an existing primary key or UNIQUE constraint.

---

EDIT

I was able to get this working by simply creating a composite primary key in MyTable. The actual table definition is shown below. Again, this works. But it is not the same as the code quoted above. And I’m not sure if it would be better if I did it the other way.

CREATE TABLE [dbo].[MessageThread](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [MessageThreadType] [int] NOT NULL,
    CONSTRAINT [PK_MessageThread_1] PRIMARY KEY CLUSTERED 
    (
        [Id] ASC,
        [MessageThreadType] 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].[MessageThread]  WITH CHECK ADD  CONSTRAINT [CK_MessageThread_ValidType] CHECK  (([MessageThreadType]=(2) OR [MessageThreadType]=(1)))
GO

ALTER TABLE [dbo].[MessageThread] CHECK CONSTRAINT [CK_MessageThread_ValidType]
GO

Answer 1

1 : I am not sure of the specific purpose of the given schema. But note that a unique constraint can be applied for multiple reasons, most commonly: (a) to enforce uniqueness and (b) to provide the optimizer with more information to base decisions.

2 : A unique constraint does not create two indexes. It creates a single index with one of the columns as the leading key column. It enforces uniqueness on both. So a unique constraint on a,b could have:

a    b    
---- ----
1    1
1    2
2    1
2    2

Notice that neither of the columns enforce uniqueness individually. I am not a big fan of using the table designer in SSMS (it has tons of bugs and doesn’t support all functionality) but here is how to do it:

a) right-click the grid and choose Indexes/Keys...

b) choose multiple columns using the […] button in the Columns grid

c) change Type to Unique Key

d) change the Name if desired

Here’s an example of a table that already has a primary key. I could add one or more unique indexes if I wanted to: