Metadata/Forms of Metadata

Structural metadata

The database schema used by Mediawiki, the software that powers Wikibooks. This schema is an example of structural metadata, because it describes how this project's data are structured.

Certain communities, such as database administrators, are more concerned with another type of metadata: structural metadata. This type of metadata documents how data is structured and stored.

For example, a store might keep a database that tracks how much money customers spend on groceries. In this case, we would know that the customer's name would always be a string of characters, and that the amount of money they spend would always be a positive number. Knowing this, we would be able to set up a database that stores the data more efficiently. While creating the database, they might issue a command like:

CREATE TABLE purchases(
 purchase_id UNSIGNED INTEGER,
 customer_name VARCHAR(50),
 amount_spent UNSIGNED INTEGER,
 purchase_date TIMESTAMP,
 PRIMARY KEY (customer_id)
);

This establishes a schema -- a set of constraints -- for our database table. Here are some of the constraints:

• The purchase_date must be in SQL's timestamp. The database will throw an error if somebody tries to enter Last Tuesday for a transaction's purchase_date, but will accept 2020-02-14 09:22:15.
• Customers cannot spend a negative amount of money during a purchase, because the amount_spent column only allows values with an UNSIGNED INTEGER type, which must be positive.
• Customers with very long names may find their names shortened in the database, since the VARCHAR(50) type only allows 50 characters of data in the customer_name column.

After the database has seen some use, we might have the following in our table:

purchase_id (UNSIGNED INTEGER)	customer_name (VARCHAR(50))	amount_spent (UNSIGNED INTEGER)	purchase_date (TIMESTAMP)
1	Ashok Kumar	2784	2020-02-14 09:22:15
2	Hong Gil dong	3516	2020-02-14 09:30:02
3	Joe Borg	499	2020-02-14 09:31:43
4	Israel Israeli	2495	2020-02-15 09:00:04

By themselves, the values 499, Hong Gil dong, 4, and 2020-02-14 09:22:15 are meaningless. However, the structural metadata we entered allows us to define, interpret, maintain, and efficiently store the data in the table.

Confusingly, structural metadata has a second definition. In the digital library community, the term structural metadata is frequently used to refer to a specific type of descriptive metadata, namely metadata that describes the physical and/or logical structure of a particular communication. For example, if a digital library project had created digital images of all the pages in a book, structural metadata would document the order of the pages, and perhaps which page each chapter or section began with.