Database Management Systems (DBMS)

Computer Science AS · Databases. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A DBMS is the software that creates and manages a database. All programs and users reach the data through it and never open the files directly. Each item is stored once, so the redundancy and inconsistency of a file-based approach are reduced.

The structure of the data is held by the DBMS and not inside each program. A program can then stay the same when the structure changes. This is called program-data independence.

The DBMS keeps a data dictionary, which is data about the data. It also protects the database with access rights and backups, and it gives tools to developers and users.

Rules to remember

Data dictionary: metadata such as table names, field names, data types, field lengths, keys, validation rules and relationships. It holds no actual records.
Logical schema: a description of the design of the database (entities, attributes and relationships) that does not depend on how the data is physically stored.
Data modelling: planning the structure of the data, for example with an E-R diagram.
Data integrity: the DBMS applies validation rules, data types and referential integrity.
Data security: user accounts and passwords, access rights for individuals and groups, and regular backups so data can be restored after loss.
Developer interface: lets a developer create tables, forms, reports and queries.
Query processor: takes a query (written in SQL), checks it, finds an efficient way to run it and returns the result.

Common mistake

The data dictionary is often described as holding the data. It only holds the description of the data (metadata), such as field names and data types.

Worked example

A school database is used by teachers and by office staff. Teachers must change marks but must not see fee payments. Office staff must see fee payments but only view marks. Which DBMS feature provides this, and how?

The need is to limit what each kind of user can do, so this is a data security feature, not a backup or integrity feature.
The DBMS gives access rights to groups of users. Each person logs in with a user account that belongs to a group.
The teacher group gets read and write access to marks and no access to fees. The office group gets read-only access to marks and read access to fees.

Answer: Access rights set for groups of users, enforced by the DBMS through user accounts.

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