Metadata/Forms of Metadata

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:

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.