SQL Index Types: Clustered & Nonclustered

Instructor: David Gloag

David has over 40 years of industry experience in software development and information technology and a bachelor of computer science

Databases are used everywhere these days, and the ability to find things quickly within them is a must. In this lesson, we'll take a look at SQL, and both clustered and non-clustered indexes.

The Need for Access

We want access to information. We want to know the particulars of a product we're interested in buying, we want to search online for our favorite songs, and we want to find information for an article or paper we're writing. In addition, we don't want it to take all day. We want the information the instant we ask for it. But isn't that a little unrealistic? Surely, we're expecting too much? The reality is that we are not. Current technologies can provide the information we want in the time frames we need. What are those technologies? SQL and indexes, among others.

What is SQL?

Structured Query Language (SQL) is a language used to view, update, and maintain a database. Developed by IBM in the early 1970's, it got its commercial start from Oracle in 1979. The language is robust, containing a number of commands to perform its intended use. Examples of the commands include:

  • SELECT - a command for retrieving information in a database.
  • INSERT - a command for adding information to a database.
  • UPDATE - a command for changing information that already exists in a database.
  • DELETE - a command for removing information from a database.
  • CREATE - a command for generating a new object in a database.

Taken together, SQL commands provide an organized approach to information manipulation, and management.

What is an Index?

An index is a list of values that imply an ordering to the elements in a database. We say imply, because the ordering is not necessarily physical. The elements of the database retain the order they were initially given. Instead, by accessing the information through an index, we can traverse (step through) the elements in an order that is implied by the index. For example, say we have the list of physical database elements Zoe, Dave, Mark, Janet, and Karen. Then an alphabetical index based on name, traversed in ascending order, would yield Dave, Janet, Karen, Mark, and Zoe. Similar indexes can be created for other attributes.

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