An index at the database level is the same principle as having an index at the end of a book. This is a completely separate section of the content from the rest of the book. So, do you see where I'm going with this? For example, if you are looking for a specific value, you can go to the index and search for it (the indexes are ordered, so finding things in this section is much faster than passing each page of the book).
A database index is an ordered list of relevant information that's used to expedite the search for records that match the criteria.
Specifically, you create an index when you have information that you need to look for frequently and need a quick answer for.
In other words, indexes prevent the DBMS from scanning the entire table looking for matching values, and they also help when you need to sort on a column.
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