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The Relational Database Model

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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 Code First Name Surname Age

1 Mongane Afrika 62

2 Stephen Serote 58

3 Tatumkhulu Watson 29

Poem Title Poet

Wakening Night 1

Thrones of Darkness 2

Once 3

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.

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