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Creating a data dictionary

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10.3 · Chapter 10 · AS Level IT · Hodder Education

Creating a Data Dictionary

What a data dictionary is, what it must contain, and how to build one from a database — with a full worked example from the Jobs and Employee tables.

📋 Definition

🏷️ Components

📝 Worked Example

The Big Idea: A data dictionary describes the structure of a database — what tables exist, what fields are in each table, their data types, sizes, key fields, and any metadata like input masks or validation rules. It is created before the database as an alternative planning tool to an ERD.

10.3What is a Data Dictionary?

Definition

A data dictionary is a file (often a word-processed document) containing descriptions of and information about the structure of data in a database. It is often designed before a database is created as an alternative to an entity relationship diagram (ERD).

While an ERD shows the relationships between tables visually, a data dictionary provides a detailed written record of every field in every table — making it a practical reference for anyone building or maintaining the database.

10.3.1Components of a Data Dictionary

📋 Table Names

The name of each database table. Short, meaningful, no spaces (e.g. Jobs, Employee, Employer, Type).

🏷️ Field Names

The name of each field within the table. Short, meaningful, no spaces or underscores (depends on the task specification).

🔢 Data Types

The type of data each field holds — Alphanumeric/Text, Integer, Decimal, Currency, Date/Time, Boolean. Include sub-types where appropriate (e.g. Currency, 2 decimal places).

📏 Field Size

The maximum number of characters for alphanumeric fields only. Count the longest data item in the actual data and set the size to that number. Numeric fields do not need a size.

🔑 Key Field

Whether the field is a Primary Key (PK), Foreign Key (FK), or part of a Compound Key. Leave blank if the field is neither.

⚙️ Metadata

Any additional rules for the field: input masks, validation rules, default values — where applicable. Many fields will have no metadata.

⚠️ Field size only applies to alphanumeric (text) fields. Numeric, Currency, Date/Time, and Boolean fields do not have a character field size — leave this column blank or state "N/A" for those rows.

10.3.2How to Create a Data Dictionary

Create a table in your word processor for each database table

Use the table name as a heading above the table. Set up columns: Field name | Data type | Field size | Key field | Metadata.

List all field names in the left column

One row per field. Keep names short, meaningful, consistent in style, no spaces.

Determine the data type for each field

Remember: only use numeric if the data will be used in calculations. Use alphanumeric for text, IDs, phone numbers, postcodes. Include sub-type where relevant (e.g. Currency 2dp, Date/Time).

Set field sizes for alphanumeric fields only

Examine the actual data. Find the longest entry for each field. Count its characters. Set that as the field size. E.g. "Trainee Snowboard Instructor" = 28 characters → field size = 28.

Identify key fields

Add metadata where applicable

If the task specifies an input mask or validation rule, record it here. Most fields will have none — leave blank or dash.

Example

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📋 Summary & Review
✅ Concepts Checklist
🧠 What to Memorise
🎯 Exam Tips

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