Feeds

Multivalued datatypes considered harmful

How dangerous can a data type be?

New hybrid storage solutions

Increasingly developers are required to write applications that interact with database engines – typically Oracle, SQL Server, DB2, MySQL or Access. In many ways the database engine is pretty much immaterial; no matter what the flavour it’s still simply a matter of tables, columns, rows and a variety of data types; text, memo, BLOB, numeric, whatever. However if you work with Access, a completely new data type is on the horizon for 2007 – multi-valued. Unfortunately, this isn’t just-another-data-type; this is a whole different ball game and a dangerous one – more like rollerball than baseball.

As the name suggests, a multi-valued field is one in which you can place more than one value. So, imagine that you design a table to store information about, say, customers.

CUSTOMER

CustID FName LName Hobbies
1 Fred Smith Fishing, Rollerball, Hockey
2 Sally Jones Sailing
3 Brian Wilson Gliding, Sailing, Singing, Hockey

The hobbies column is a multi-valued field. In some ways this is very neat because we have created a many-to-many join (many different customers can have many different hobbies) using a single table. The alternative, and traditional way, is to use three tables.

CUSTOMER

CustID FName LName
1 Fred Smith
2 Sally Jones
3 Brian Wilson

HOBBY

HobbyID Hobbies
1 Fishing,
2 Rollerball
3 Hockey
4 Sailing
5 Gliding
6 Singing

CUSTOMER/HOBBY

CustID HobbyID
1 1
1 2
1 3
2 4
3 5
3 4
3 6
3 3

Clearly, the solution using three tables is more complex and less intuitive; so what is wrong with the multi-valued data type solution? Well, in his initial set of rules defining relational databases, Ted Codd (the originator of the relational model) forbad their use.

Rule 2, the guaranteed access rule.

Each and every datum (atomic value) in a relational data base is guaranteed to be logically accessible by resorting to a combination of table name, primary key value and column name.

If we use the table name (Customer), Primary key value (1) and column name (Hobbies) we don’t get a single atomic value (such as ‘Fishing’); we get multiple pieces of data (Fishing, Rollerball, Hockey).

Now this is a killer argument if you are a database freak like me (“If Ted Codd forbad it, I want no further truck with your multi-valued data types.”) but I quite understand that, if you are an application developer, the finer points of relational database theory often sound like just so much academic nonsense. If a new feature makes life easier, who cares if it happens to break some arbitrary rule written over 20 years ago?

Fair enough. So let’s look at an intensely practical reason why multi-valued fields are so bad. We query databases using SQL. The design of SQL is based entirely on the assumption that each column contains atomic values. If we run a normal SQL query against our single table solution:

SELECT FName FROM CUSTOMER
WHERE Hobby = “Rollerball”

It will return zero rows; despite the fact that one of our customers plays rollerball, because there is no row with a field just containing “rollerball”.

If you want a challenge, try to construct the SQL necessary to find the names of the customers who both glide and sail. It is, of course, possible, but the solution is more complex than extracting the same information from the three table structure.

And if you are not convinced that we are plunging into deep water here, imagine that I store foreign key values (in the example shown, into a PRODUCT table) in a multi valued field:

ORDER

OrderID ODate Products
1 1/1/07 1,5,5,5,5,5,67,434,434,5654
2 1/1/07 45,67,454,454,454,65556
3 2/1/07 2,454,5677

Reducing the cost and complexity of web vulnerability management

More from The Register

next story
New 'Cosmos' browser surfs the net by TXT alone
No data plan? No WiFi? No worries ... except sluggish download speed
'Windows 9' LEAK: Microsoft's playing catchup with Linux
Multiple desktops and live tiles in restored Start button star in new vids
iOS 8 release: WebGL now runs everywhere. Hurrah for 3D graphics!
HTML 5's pretty neat ... when your browser supports it
'People have forgotten just how late the first iPhone arrived ...'
Plus: 'Google's IDEALISM is an injudicious justification for inappropriate biz practices'
Mathematica hits the Web
Wolfram embraces the cloud, promies private cloud cut of its number-cruncher
Mozilla shutters Labs, tells nobody it's been dead for five months
Staffer's blog reveals all as projects languish on GitHub
SUSE Linux owner Attachmate gobbled by Micro Focus for $2.3bn
Merger will lead to mainframe and COBOL powerhouse
iOS 8 Healthkit gets a bug SO Apple KILLS it. That's real healthcare!
Not fit for purpose on day of launch, says Cupertino
Not appy with your Chromebook? Well now it can run Android apps
Google offers beta of tricky OS-inside-OS tech
prev story

Whitepapers

Secure remote control for conventional and virtual desktops
Balancing user privacy and privileged access, in accordance with compliance frameworks and legislation. Evaluating any potential remote control choice.
WIN a very cool portable ZX Spectrum
Win a one-off portable Spectrum built by legendary hardware hacker Ben Heck
Intelligent flash storage arrays
Tegile Intelligent Storage Arrays with IntelliFlash helps IT boost storage utilization and effciency while delivering unmatched storage savings and performance.
High Performance for All
While HPC is not new, it has traditionally been seen as a specialist area – is it now geared up to meet more mainstream requirements?
Beginner's guide to SSL certificates
De-mystify the technology involved and give you the information you need to make the best decision when considering your online security options.