Metadata/Forms of Metadata

Structural 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:

This establishes a schema -- a set of constraints -- for our database table. Here are some of the constraints:
 * The  must be in SQL's  . The database will throw an error if somebody tries to enter   for a transaction's , but will accept.
 * Customers cannot spend a negative amount of money during a purchase, because the  column only allows values with an   type, which must be positive.
 * Customers with very long names may find their names shortened in the database, since the  type only allows 50 characters of data in the   column.

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

By themselves, the values,  ,  , and   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.