What is an index in SQL?

Question

What is an index in SQL? Can you explain or reference to understand clearly?

Where should I use an index?

Answer 1

Indexes are all about finding data quickly.

Indexes in a database are analogous to indexes that you find in a book. If a book has an index, and I ask you to find a chapter in that book, you can quickly find that with the help of the index. On the other hand, if the book does not have an index, you will have to spend more time looking for the chapter by looking at every page from the start to the end of the book.

In a similar fashion, indexes in a database can help queries find data quickly. If you are new to indexes, the following videos, can be very useful. In fact, I have learned a lot from them.

Index Basics
Clustered and Non-Clustered Indexes
Unique and Non-Unique Indexes
Advantages and disadvantages of indexes