Data Dictionaries

• Can be logical data dictionary which lays down what data is required in a general sense (e.g. "What data should be in a sales contract?")
• Can also be a physical data dictionary design tool e.g. in the sales contract record there is a sale_date (type date), customer_ID (text, 10 characters) etc."
• See here ... http://www.modernanalyst.com/Careers/InterviewQuestions/tabid/128/articleType/ArticleView/articleId/1392/categoryId/45/What-is-a-logical-data-dictionary-and-what-are-the-benefits-of-maintaining-one.aspx for info

(PHYSICAL) DATA DICTIONARY

A data dictionary (as its name implies), defines the data used in a system. It is a complete and authoritative definition of all the data in the system. A data dictionary may cover a whole organisation, a part of the organisation or a database.

In its simplest form, the data dictionary is only a collection of data element definitions, according to descriptions below. More advanced data dictionary contains database schema with reference keys, still more advanced data dictionary contains entity-relationship model of the data elements or objects.

A data dictionary may contain such things as:

* Data element name (e.g. a field's name, like "Surname")
* Short description
* Data type (characters, numeric, etc.), size
* Null value allowed: Null or non-existing data value may or may not be allowed for an element.
* Default value: Data element may have a default value. Default value may be a variable, like current date and time of the day
* Allowed values and validation
* Whether the field is a key field.
* Rules used in calculations producing the values are usually written here.

2. Database table definitions may include

* Table name
* Table owner or database name
* List of data element (column) names and details
* Key order for all the elements, which are possible keys
* Possible information on indexes
* Possible information on table organisation
* Duplicate rows allowed or not allowed
* Possible detailed data element list with complete data element definitions
* Possible data on the current contents of the table
* Security classification of the table

Example: