Introduction to Relational Databases

Monday Jun 24th 2002 by Ian Gilfillan
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Many web developers are self-taught, learning HTML, then moving on to a programming language such as PHP. From there, they often learn to integrate this with a database. Too few though have a good theoretical knowledge of databases. Mention foreign keys, or referential integrity, and you're met with a blank stare. Small databases can be easily designed with little database theory knowledge. But large databases can easily get out of hand when badly designed, leading to poor performance, and resulting in the whole database needing to be rebuilt later. This article from Ian Gilfillan is a brief introduction to the topic of relational databases and will hopefully whet your appetite for further exploration.


Introduction

Many web developers are self-taught, learning HTML, then moving on to a programming language such as PHP. From there, they often learn to integrate this with a database. Too few though have a good theoretical knowledge of databases. Mention foreign keys, or referential integrity, and you're met with a blank stare. Small databases can be easily designed with little database theory knowledge. But large databases can easily get out of hand when badly designed, leading to poor performance, and resulting in the whole database needing to be rebuilt later. This article is a brief introduction to the topic of relational databases, and will hopefully whet your appetite for further exploration.


The Relational Database Model

A database can be understood as a collection of related files. How those files are related depends on the model used. Early models included the hierarchical model (where files are related in a parent/child manner, with each child file having at most one parent file), and the network model (where files are related as owners and members, similar to the network model except that each member file can have more than one owner).

The relational database model was a huge step forward, as it allowed files to be related by means of a common field. In order to relate any two files, they simply need to have a common field, which makes the model extremely flexible.

Poet
CodeFirst NameSurnameAge
1MonganeAfrika62
2StephenSerote58
3TatumkhuluWatson29

Poem
TitlePoet
Wakening Night1
Thrones of Darkness2
Once3

These two tables relate through the code field in the poet table, and the poet field in the poem table. We can see who wrote the poem 'Once' by following the relationship, and see that it was poet 3, or Tatumkhulu Watson.

In 1970, when E.F. Codd developed the model, it was thought to be hopelessly impractical, as the machines of the time could not cope with the overhead necessary to maintain the model. Of course, hardware since then has come on in huge strides, so that today even the most basic of PC's can run sophisticated relational database management systems. Together with this went the development of SQL. SQL is relatively easy to learn and allows people to quickly learn how to perform queries on a relational database. This simplicity is part of the reason that relational databases now form the majority of databases to be found.

Basic Terms

An understanding of relational databases requires an understanding of some of the basic terms.

  • Data are the values stored in the database. On its own, data means very little. "43156" is an example.
  • Information is data that is processed to have a meaning. For example, "43156" is the population of the town of Littlewood.
  • A database is a collection of tables.
  • Each table contains records, which are the horizontal rows in the table. These are also called tuples.
  • Each record contains fields, which are the vertical columns of the table. These are also called attributes. An example would be a product record.
  • Fields can be of many different types. There are many standard types, and each DBMS (database management system, such as Oracle or MySQL) can also have their own specific types, but generally they fall into at least three kinds - character, numeric and date. For example, a product description would be a character field, a product release date would be a date field, and a product quantity in stock would be a numeric field.
  • The domain refers to the possible values each field can contain (it's sometimes called a field specification). For example, a field entitled "marital_status" may be limited to the values "Married" and "Unmarried".
  • A field is said to contain a null value when it contains nothing at all. Fields can create complexities in calculations and have consequences for data accuracy. For this reason, many fields are specifically set not to contain NULL values.
  • A key is a logical way to access a record in a table. For example, in the product table, the product_id field could allow us to uniquely identify a record. A key that uniquely identifies a record is called a primary key.
  • An index is a physical mechanism that improves the performance of a database. Indexes are often confused with keys. However, strictly speaking they are part of the physical structure, while keys are part of the logical structure.
  • A view is a virtual table made up of a subset of the actual tables.
  • A one-to-one (1:1) relationship occurs where, for each instance of table A, only one instance of table B exists, and vice-versa. For example, each vehicle registration is associated with only one engine number, and vice-versa
  • A one-to-many (1:m) relationship is where, for each instance of table A, many instances of the table B exist, but for each instance of table B, only once instance of table A exists. For example, for each artist, there are many paintings. Since it is a one-to-many relationship, and not many-to-many, in this case each painting can only have been painted by one artist.
  • A many to many (m:n) relationship occurs where, for each instance of table A, there are many instances of table B, and for each instance of table B, there are many instances of the table A. For example, a poetry anthology can have many authors, and each author can appear in many poetry anthologies.
  • A mandatory relationship exists where, for each instance of table A, one or more instances of table B must exist. For example, for a poetry anthology to exist, there must exist at least one poem in the anthology. The reverse is not necessarily true though, as for a poem to exist, there is no need for it to appear in a poetry anthology.
  • An optional relationship is where, for each instance of table A, there may exist instances of table B. For example, a poet does not necessarily have to appear in a poetry anthology. The reverse isn't necessarily true though, for example for the anthology to be listed, it must have some poets.
  • Data integrity describes the accuracy, validity and consistency of data. An example of poor integrity would be where a poet's name is stored differently in two different places.
  • Database normalization is a technique that helps us to reduce the occurrence of data anomalies and poor data integrity.


Page 2: Table Keys



A key is the tool to unlock access to database tables. By knowing the key, we know how to locate specific records, and traverse the relationships between tables.

A candidate key is any field, or combination of fields, that uniquely identifies a record. The field/s of the candidate key must contain unique values (if the values were duplicated, they would be no longer identify unique records), and cannot contain a null value.

A primary key is the candidate key that has been chosen to identify unique records in a particular table.
Examine the following table:

Poet
CodeFirst NameSurnameAge
1MonganeAfrika62
2StephenSerote58
3TatumkhuluWatson29

At first it seems there are two candidate keys for this table. Both code and the combination of first_name and surname would suffice. It is always better to choose the candidate key with the least number of fields for the actual primary key, so we would choose code in this case. Also, if we thought about it some more, we'd soon realize that there is a possibility of the second combination not being unique. The combination of first_name and surname could be duplicated. So to consider this for a choice of primary key, we'd have to be sure that none of our poets could ever have the same name. This is the reason we assign code fields. Codes are assigned by our system, so we can ensure there are never any duplicate codes. After the primary key has been assigned, any remaining candidate keys are labeled alternate keys.

Foreign Keys

A relation between two tables is created by creating a common field to the two tables. The common field must be a primary key to the one table (the table that would be the one component of the one-to-many relationship). Consider a relation between a poet table and a poetry anthology table. The relation is of little use if instead of using the primary key from the poet table, code, to create the relationship with the anthology table, we use another field that is not unique, such as the poet's surname. We would never know for certain which poet we're referring to in the poetry anthology. The poet_code field is called the foreign key in the anthology table, which means it's the primary key(code) in the poet table.

Anthology
  • Anthology_code
  • Poet_code
Poet
  • Code
  • Firstname
  • Surname
  • Age

Foreign keys allow us to ensure what is called referential integrity. This means that if a foreign key contains a value, the value must refer to an existing record in the related table. For example, take a look at these two tables:

Poet
CodeFirstnameSurname
1MonganeAfrika
2StephenSerote
3TatumkhuluWatson

Poem
TitlePoet
Wakening Night1
Thrones of Darkness2
Once3

Referential integrity exists here, as all the poets listed in the poem table exist in the poet table. Stephen Serote now pulls out of the anthology, and we delete him from the poet table. In a situation where referential integrity is not enforced, he would not appear in the poet table, but the code he used to have would still appear in the poem table. So, when we look up the poet who wrote Thrones of Darkness (which is poet code 2) we are sent to a non-existent record.

Poet
CodeFirstnameSurname
1MonganeAfrika
3TatumkhuluWatson

Poem
TitlePoet
Wakening Night1
Thrones of Darkness2
Once3

The delete results in poor data integrity.

Foreign keys also allow what are called cascading deletes and updates. This means that we can delete Stephen Serote from the poet table, and all poems written by him, with one SQL statement. The delete "cascades" through the relevant tables, removing all relevant records. Foreign keys can contain null values if the relationship is optional, which indicates that no relationship exists. If the relationship is mandatory, the foreign key cannot contain nulls.


Page 3: Views



Views are virtual tables. They do not contain any data themselves - rather they're a structure to allow us to access data, or a subset of the data. A view can consist of a subset of one table, such as in this example:

Poet
  • Code
  • First name
  • Surname
  • Age
  • Address
  • Telephone

This is the complete list of fields from the poet table

Subscriber view
  • Code
  • First_name
  • Surname
  • Grade

This view could be used to allow others to see the poet's code, name and surname, but not allow them access to personal information. Or, a view could be a combination of a number of tables, such as in this example

: Poet
  • Code
  • First name
  • Surname
  • Age
  • Address
  • Telephone

Poem
  • Poem code
  • Poet code
  • Title

Subscriber view
  • First_name
  • Surname
  • Poem title

Views are often used for security purposes. Junior developers may need access to certain portions of a table, but they do not need access to all the data. What they don't need, even if it is from the same table, is hidden and safe from manipulation or viewing. Also, views allow SQL queries to be much simpler. For example, without views, a developer may have to use the following query:

	
SELECT first_name,surname,poem FROM poet,poem
WHERE poem.poet_code=poet.code AND poet.title='Once';

With the view, a developer could do the same with:

SELECT first_name,surname,poem from subscriber_view;

Much more simple for a junior developer who hasn't yet learnt how to do joins across multiple tables, and less hassle for a senior developer too!

This has been a brief introduction to relational databases. Hopefully it's put some of the terms you've come across in context, and whetted your appetite to explore further.

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