What Is Metadata? JDBC

Metadata is data about data (or information about information), which provides structured, descriptive information about other data:

Metadata (Greek: meta-+ Latin: data “information”), literally “data about data”, is information that describes another set of data. A common example is a liary catalog card, which contains data about the contents and location of a book: It is data about the data in the book referred to by the card. Other common contents of metadata include the source or author of the described dataset, how it should be accessed, and its limitations.

The following quote from the NOAA Coastal Services Center, or CSC , illustrates the importance of the concept of metadata:

Imagine trying to find a book in a liary without the help of a card catalog or computerized search interface. Could you do it? Perhaps, but it would be difficult at best. The information contained in such a system is essentially metadata about the books that are housed at that liary or at other liaries. It provides you with vital information to help you find a particular book and aids you in making a decision as to whether that book might fit your needs.Metadata serves a similar purpose for geospatial data.

The NOAA CSC further adds that “metadata is a component of data which describes the data. It is ‘data about data.’” Metadata describes the content, quality, condition, and other characteristics of data. Metadata describes the who, what, when, where, why, and how of a data set. Without proper documentation, a data set is incomplete.

KTWEB ) defines metadata as “data about data, or nformation about information; in practice, metadata comprises a structured set of descriptive elements to describe an information resource or, more generally, any definable entity.” Relational databases (such as MySQL and Oracle) use tables and other means (such as operating system file systems) to store their own data and metadata. Each relational database has its own proprietary methods for storing metadata. Examples of relational database metadata include

  • A list of all the tables in the database, including their names, sizes, and the number of rows
  • A list of the columns in each database, and what tables they are used in, as well as the type of data stored in each column

For example, the Oracle database keeps metadata in several tables (I have listed two here):

  • ALL_TABLES: A list of all tables in the current database
  • ALL_TAB_COLS: A list of all columns in the database

Imagine, at runtime, trying to execute a SQL query in a relational database without knowing the name of tables, columns, or views. Could you do it? Of course not. Metadata helps you to find out what is available in the database and then, with the help of that information (called metadata), you can build proper SQL queries at runtime. Also, having access to structured database metadata relieves a JDBC programmer of having to know the characteristics of relational databases in advance.

Metadata describes the data but is not the actual data itself. For example, the records in a card catalog in a local liary give ief details about the actual book. The card catalog—as metadata—provides enough information to tell you what the book is called, its unique identification number, and how and where you can find it. These details are metadata—in this case, bibliographic elements such as author, title, abstract, publisher, and published date.

In a nutshell, database metadata enables dynamic database access. Typically, most JDBC programmers know their target database’s schema definitions: the names of tables, views, columns, and their associated types. In this case, the JDBC programmer can use the strongly typed JDBC interfaces. However, there is another important class of database access where an application (or an application builder) dynamically (in other words, at runtime) discovers the database schema information and uses that information to perform appropriate dynamic data access. This chapter describes the JDBC support for dynamic access. A dynamic database access application may include building dynamic queries, dynamic owsers, and GUI database adapters, just to mention a few.


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