Wprowadzenie do baz danych

It is worth to realise that databases are synonymous with storing huge amounts of data. Modern databases collect for instance the photographs of the universe, information about flights and plane passengers all over the world, the descriptions of medicines and their applications. It would be difficult to store such information traditionally, in a notebook or even in a file. Even if you had enough sheets of paper, finding needed information would be simply impossible. For comparison, a single modern personal computer can contain information written on over 20 million sheets of paper.

A database is a set of organized data associated with a specific issue.

Basically a database can be treated as a tool for:

• storing,

• browsing,

• searching,

• sorting information.

The basic terms related to databases are: a table, a recordrecordrecord, a fieldfieldfield and a primary keyprimary keyprimary key.

Data are stored in tables. A database can contain multiple tables. The tabletabletable must have its unique name in the database. The table is made up of fields, which correspond to columns in a spreadsheet, and data are collected in rows, which are called records. The records, contrary to the spreadsheet, are not assigned the row numbers and can be entered into the table in any order. All data values are based on simple data types. A primary keyprimary keyprimary key is a unique field throughout a table that clearly identifies a given record.

The types of databases

Simple databases:

• A file databasefile databasefile database is a database based on a single file, usually a text file; it can store data for particular applications or devices.

• A flat‑file databaseflat‑file databaseflat‑file database is a database in which every table is a single document and is not related to the other table.

• A hierarchical databasehierarchical databasehierarchical database is a database in which each recordrecordrecord, except from the root record, is linked to only one parent record. Data are organized into a tree‑like structure, similar to the structure of directories and files.

Complex databases:

• A relational databaserelational databaserelational database is a database in which multiple tables are linked with one another with the so called relations.

• An object databaseobject databaseobject database is a database which stores a set of objects along with the properties assigned to them.

• A temporal databasetemporal databasetemporal database is a variation of a relational database in which every record has got a timestamp, which defines the time in which a given value is true. Temporal databases enable storing the entire history of all data.

• XML is a data structure based on the markup language. Many databases can import data from the XML file or export data to such a file. Some programming languages enable operating on data directly in XML files.

The classification of databases by their reach:

• local databases,

• databases operating based on the client- server model,

• centralised databases – databases stored on a single server,

• distributed databases – databases stored on multiple servers,

• data warehouses – databases that collect huge amounts of data, which may come from multiple systems. Such databases do not focus on a single recordrecordrecord but analyse data from numerous records.

Database design

Creating databases should be preceded by their design. In order to design a good database, you must pay attention to the following issues:

• functionality according to assumptions,

• removing data redundancy (duplicating data),

• full data integrity (all data are consistent),

• the scalability (easy expansion) of a database,

• the security of collected data.

In the case of large databases the design process is crucial and unfortunately it is difficult to implement due to the specialised knowledge needed.

Table design

The data types in a single column should be identical and defined already at the stage of the table design. Various programmes may accept slightly different data types. However all of them enable storing text, numbers, dates and time, currencies, multimedia elements, links, etc. The names of fields, similar to tables, should be unique.

When you create the names of fields in a database tabletabletable, avoid names including spaces and the Polish diacritics.

Currently databases are applied almost in every fieldfieldfield of our life.

Exercises

Glossary

Keywords

databasedatabasedatabase

primary keyprimary keyprimary key

relationrelationrelation

tabletabletable