What are the pros and cons of the following two design methods ?
(1) Using foreign keys to form a composite primary key of a child
tables -- as in Example.
POOR MAN'S ERD PROVIDED FOR SUMMARY OVERVIEW (Code provided below)
*** Example 1 COMPOSITE FOREIGN KEY ***
PK = Primary Key
FK = Foreign Key
Relationships:
Language to Brochure = one-to-many
Brochure to Heading = many-to-many
Heading to Paragraph = one-to-many
tbLanguage tbBrochure
---------------------- ----------------------
- LanguageId (PK) - -----\ - BrochureId (PK) -
- LangName - \----> - LanguageId (PK)(FK)-
---------------------- / - Title -
/ / ---------------------
/ /
/ /
tbBrochureHeadingMap / / tbHeading
---------------------- <-----/ / ----------------------
- BrochureId (PK)(FK)- <------/ -- - HeadingId (PK) -
- LanguageId (PK)(FK)- / - HeadingText -
- HeadingId (PK)(FK)- <--------/ ----------------------
---------------------- |
/
tbParagraph /
---------------------- /
- BrochureId (PK)(FK)- <----------/
- LanguageId (PK)(FK)-
- HeadingId (PK)(FK)-
- SequenceNo (PK) -
- ParagraphText -
----------------------
(2) Using a new key to form a single primary key of a table, and
placing parent tables as only foreign keys -- as in Example 2.
*** Example 2 SINGLE PRIMARY KEY ***
Relationships:
Language to Brochure = one-to-many
Brochure to Heading = many-to-many
Heading to Paragraph = one-to-many
tbLanguage tbBrochure
---------------------- ----------------------
- LanguageId (PK) - -----\ - BrochureId (PK) -
- LangName - \----> - LanguageId (FK) -
---------------------- - Title -
----------------------
|
|
tbBrochureHeadingMap | tbHeading
---------------------------- / ----------------------
- BrochureHeadingMapId (PK)- / - HeadingId (PK) -
- BrochureId (FK) - <---/ / - HeadingText -
- HeadingId (FK) - <--------/ ----------------------
---------------------------- |
|
tbParagraph /
---------------------- /
- ParagraphId(PK) - /
- HeadingId (FK) - <--------------/
- SequenceNo -
- ParagraphText -
----------------------
It has been argued that Example 1: COMPOSITE FOREIGN KEY has the
following pros, over Example 2:
1) Fewer indexes are needed. Five (5) Indexes in Example 1 instead of
Nine (9) in Example 2.
2) Queries can be created with fewer joins.
For example: (one join in Example 1)
SELECT b.Title,
p.ParagraphText
FROM tbBrochure b
INNER JOIN tbParagraph p
ON (
b.BrochureId = p.BrochureId and
b.LanguageId = p.LanguageId
)
Instead Of: (two joins in Example 2)
SELECT b.Title,
p.ParagraphText
FROM tbBrochure b
INNER JOIN tbBrochureHeadingMap bhm
ON bhm.BrochureId = b.BrochureId
INNER JOIN tbParagraph p
ON p.HeadingId = bhm.HeadingId
Can anyone see any advantages of using the Example 2 over using
Example 1 method ?
-- *** Example 1 COMPOSITE FOREIGN KEY Code (SQL Server 2000) ***
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbParagraph]') and OBJECTPROPERTY(id, N'IsUserTable') =
1)
drop table [tbParagraph]
GO
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbBrochureHeadingMap]') and OBJECTPROPERTY(id,
N'IsUserTable') = 1)
drop table [tbBrochureHeadingMap]
GO
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbBrochure]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [tbBrochure]
GO
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbLanguage]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [tbLanguage]
GO
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbHeading]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [tbHeading]
GO
CREATE TABLE tbLanguage
(
LanguageId int identity(1,1) not null,
LangName varchar(255) not null,
PRIMARY KEY CLUSTERED (LanguageId)
)
go
CREATE TABLE tbBrochure
(
BrochureId int identity(1,1) not null,
LanguageId int not null,
Title varchar(255) not null
PRIMARY KEY CLUSTERED(BrochureId,LanguageId),
FOREIGN KEY (LanguageId)
REFERENCES tbLanguage(LanguageId)
)
go
CREATE TABLE tbHeading
(
HeadingId int identity(1,1) not null,
HeadingText varchar(1000) not null,
PRIMARY KEY CLUSTERED (HeadingId)
)
go
CREATE TABLE tbBrochureHeadingMap
(
BrochureId int not null,
LanguageId int not null,
HeadingId int not null,
PRIMARY KEY CLUSTERED (BrochureId,LanguageId,HeadingId),
FOREIGN KEY (BrochureId,LanguageId)
REFERENCES tbBrochure (BrochureId,LanguageId),
FOREIGN KEY (HeadingId)
REFERENCES tbHeading (HeadingId)
)
go
CREATE TABLE tbParagraph
(
BrochureId int not null,
LanguageId int not null,
HeadingId int not null,
SequenceNo int not null,
ParagraphText varchar(4000) not null,
PRIMARY KEY CLUSTERED (BrochureId,LanguageId,HeadingId,SequenceNo),
FOREIGN KEY (BrochureId,LanguageId)
REFERENCES tbBrochure (BrochureId,LanguageId),
FOREIGN KEY (HeadingId)
REFERENCES tbHeading (HeadingId)
)
go
-- *** Example 2 SINGLE PRIMARY KEY Code (SQL Server 2000) ***
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbParagraph]') and OBJECTPROPERTY(id, N'IsUserTable') =
1)
drop table [tbParagraph]
GO
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbBrochureHeadingMap]') and OBJECTPROPERTY(id,
N'IsUserTable') = 1)
drop table [tbBrochureHeadingMap]
GO
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbBrochure]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [tbBrochure]
GO
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbLanguage]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [tbLanguage]
GO
if exists (select * from dbo.sysobjects where id =
object_id(N'[tbHeading]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [tbHeading]
GO
CREATE TABLE tbLanguage
(
LanguageId int identity(1,1) not null,
LangName varchar(255) not null,
PRIMARY KEY CLUSTERED (LanguageId)
)
go
CREATE TABLE tbBrochure
(
BrochureId int identity(1,1) not null,
LanguageId int not null,
Title varchar(255) not null
PRIMARY KEY CLUSTERED(BrochureId),
FOREIGN KEY (LanguageId)
REFERENCES tbLanguage(LanguageId)
)
go
CREATE NONCLUSTERED INDEX ix_tbBrochure_LanguageId ON tbBrochure
(LanguageId)
go
CREATE TABLE tbHeading
(
HeadingId int identity(1,1) not null,
HeadingText varchar(1000) not null,
PRIMARY KEY CLUSTERED (HeadingId)
)
go
CREATE TABLE tbBrochureHeadingMap
(
BrochureHeadingMapId int identity(1,1) not null,
BrochureId int not null,
HeadingId int not null,
PRIMARY KEY CLUSTERED (BrochureHeadingMapId),
FOREIGN KEY (BrochureId)
REFERENCES tbBrochure (BrochureId),
FOREIGN KEY (HeadingId)
REFERENCES tbHeading (HeadingId)
)
go
CREATE NONCLUSTERED INDEX ix_tbBrochureHeadingMap_BrochureId ON
tbBrochureHeadingMap (BrochureId)
go
CREATE NONCLUSTERED INDEX ix_tbBrochureHeadingMap_HeadingId ON
tbBrochureHeadingMap (HeadingId)
go
CREATE TABLE tbParagraph
(
ParagraphId int identity(1,1) not null,
HeadingId int not null,
SequenceNo int not null,
ParagraphText varchar(4000) not null,
PRIMARY KEY CLUSTERED (ParagraphId),
FOREIGN KEY (HeadingId)
REFERENCES tbHeading (HeadingId)
)
go
CREATE NONCLUSTERED INDEX ix_tbParagraph_BrochureId ON
tbBrochureHeadingMap (HeadingId)
go
