SQL Server 2008 R2 : Defining Table Location, Defining Table Constraints

Defining Table Location

The mechanism for specifying the physical table location is the filegroup. Filegroups are aligned to physical data files. By default, each database has a primary filegroup and a data file that matches the name of the database. You can create additional filegroups and align them to other data files. When these filegroups are created, SQL Server enables you to create your database tables on a specific filegroup.

The placement of tables on separate filegroups has some distinct advantages, including performance benefits. You can achieve performance improvements by storing filegroups on different disks. You can also achieve some manageability improvements by using filegroups because you can back up and manipulate filegroups separately. This capability is particularly important for large tables.

You specify the location of a table by using the ON clause during table creation. Listing 1 shows an example of creating two filegroups in the BigPubs2008 database, followed by the creation of two new tables on those filegroups. Note that the filegroups must exist before the tables are created. 

Listing 1. An Example of Creating Tables on Specific Filegroups

--Add the filegroups
ALTER DATABASE BigPubs2008 ADD FILEGROUP FG1
ALTER DATABASE BigPubs2008 ADD FILEGROUP FG2
GO
--Add files to the filegroups
ALTER DATABASE BigPubs2008 ADD FILE
(   NAME = FG1_File,
    FILENAME = 'c:\BigPubs2008FG1.ndf',
    SIZE = 2MB) TO FILEGROUP FG1
go

ALTER DATABASE BigPubs2008 ADD FILE
(   NAME = FG2_File,
    FILENAME = 'c:\BigPubs2008FG2.ndf',
    SIZE = 2MB) TO FILEGROUP FG2
go

CREATE TABLE [dbo].[authors_NEW](
    [au_id] [dbo].[id] NOT NULL,
    [au_lname] [varchar](40) ,
    [au_fname] [varchar](20) ,
    [phone] [char](12),
    [address] [varchar](40)  NULL,
    [city] [varchar](20)  NULL,
    [state] [char](2)  NULL,
    [zip] [char](5)  NULL,
    [contract] [bit] NOT NULL,
) ON FG1
go

CREATE TABLE [dbo].[titles_NEW](
    [title_id] [dbo].[tid] NOT NULL,
    [title] [varchar](80)  NOT NULL,
    [type] [char](12)  NOT NULL,
    [pub_id] [char](4)  NULL,
    [price] [money] NULL,
    [advance] [money] NULL,
    [royalty] [int] NULL,
    [ytd_sales] [int] NULL,
    [notes] [varchar](400)  NULL,
    [pubdate] [datetime] NOT NULL,
) ON FG2

Defining Table Constraints

Constraints provide a means to enforce data integrity. In addition to NULL/NOT NULL, SQL Server provides five constraint types: PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK, and DEFAULT. These constraints help further define the type of data you can store in tables.

You can add constraints at the time of table creation, or you can add them after a table has been created, by using the ALTER TABLE statement.

Listing 2 shows a CREATE TABLE statement that has an example of each one of the five constraint types listed. The PRIMARY KEY constraint is created at the bottom of the script and is named PK_TitleHistory. The FOREIGN KEY constraint is created on the title_id column and is named FK_titles_titleHistory. The UNIQUE constraint is part of the primary key and can be identified with the UNIQUE keyword. The CHECK constraint is created on the price column; it checks to make sure the price is greater than zero. Finally, a DEFAULT constraint is created on the modify_user column; it sets the user to the value of system if no explicit value is specified.

Listing 2. Example of Creating Constraints with CREATE TABLE

CREATE TABLE dbo.TitleHistory(
   title_id dbo.tid
      CONSTRAINT FK_titles_titleHistory
      REFERENCES titles (title_id)NOT NULL ,
   change_date datetime NOT NULL,
   title varchar(80) NOT NULL,
   type char(12)  NOT NULL,
   price money NULL
      CONSTRAINT CK_TitleHistory_Price CHECK  (Price>0),
   modify_user nchar(10) NOT NULL
      CONSTRAINT DF_TitleHistory_modify_user  DEFAULT (N'system'),
 CONSTRAINT PK_TitleHistory UNIQUE CLUSTERED
(  title_id ASC,
   change_date ASC ) )

You can create the same constraints as in Listing 2 by using the ALTER TABLE statement. This means you can first create the table (without the constraints) and then add the constraints afterward. Listing 3 shows the creation of the same titleHistory table, with the constraints added later via the ALTER TABLE statement.

Listing 3. Example of Creating Constraints with ALTER TABLE

IF  EXISTS (SELECT * FROM dbo.sysobjects
WHERE id = OBJECT_ID(N'[dbo].[TitleHistory]')
AND OBJECTPROPERTY(id, N'IsUserTable') = 1)
DROP TABLE [dbo].[TitleHistory]

CREATE TABLE dbo.TitleHistory(
   title_id dbo.tid NOT NULL,
   change_date datetime NOT NULL,
   title varchar(80) NOT NULL,
   type char(12) NOT NULL,
   price money NULL,
   modify_user nchar(10) NOT NULL )
GO

--PRIMARY KEY/UNIQUE CONSTRAINT
ALTER TABLE dbo.TitleHistory
   ADD  CONSTRAINT PK_TitleHistory UNIQUE CLUSTERED
    (
      title_id ASC,
      change_date ASC
    )WITH (SORT_IN_TEMPDB = OFF, ONLINE = OFF)
go
--FOREIGN KEY CONSTRAINT
ALTER TABLE dbo.TitleHistory  WITH CHECK
   ADD  CONSTRAINT FK_titles_titleHistory FOREIGN KEY(   title_id)
    REFERENCES dbo.titles (title_id)
GO
--CHECK CONSTRAINT
ALTER TABLE dbo.TitleHistory  WITH CHECK
   ADD  CONSTRAINT CK_TitleHistory_Price CHECK  ((Price>(0)))
GO
--DEFAULT CONSTRAINT
ALTER TABLE dbo.TitleHistory
   ADD  CONSTRAINT DF_TitleHistory_modify_user
     DEFAULT (N'system') FOR modify_user