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Databases

The data dictionary

  • 3 things to know
  • A2 Unit 1

Overview

Data dictionary, field length and default value.

What you need to know

Answer each one before you reveal it.

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Key term

What is a data dictionary?

Key term

Why is a data dictionary useful when a new developer joins a project?

Key term

Why does a data dictionary record validation rules?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is a data dictionary?

    Model answer

    A data dictionary is a store of information about the data held in a database rather than the data itself, recording what each table and column is, what it may contain and how it relates to the rest. It is often called metadata.

  2. 2

    Name the components a data dictionary records for each attribute.

    Model answer

    For each attribute a data dictionary records the field name, a description of its meaning, its data type, its field length or size, its format, any validation rules, its default value, and whether it may be left empty.

  3. 3

    What does a data dictionary record about tables?

    Model answer

    For each table a data dictionary records the table name and purpose, the attributes it contains, its primary key, its foreign keys and the relationships they implement, and any indexes defined on it.

  4. 4

    Why does a data dictionary record validation rules?

    Model answer

    Recording the validation rules in one place means every program that writes to the column applies the same rule, and a developer or auditor can see what values the column is supposed to admit without reading the application code.

  5. 5

    How does a data dictionary help maintenance?

    Model answer

    A data dictionary tells a maintainer what a column means and what else depends on it, so the effect of a proposed change can be assessed before it is made and a column is not altered in ignorance of the reports that read it.

  6. 6

    What is an active data dictionary?

    Model answer

    An active data dictionary is maintained by the database management system itself and is consulted whenever data is accessed, so it cannot fall out of step with the database. A passive dictionary is kept separately and must be updated by hand.

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