Video Summary

An SQL index is a data structure that improves the performance of queries by reducing the number of rows that need to be scanned. Indexes are created on one or more columns of a table, and they store the values of those columns in a sorted order. When a query is executed, the database engine uses the index to find the rows that match the criteria specified in the query, which can significantly reduce the amount of time it takes to process the query. There are two main types of indexes: clustered and non-clustered. A clustered index is created on the primary key of a table, and it physically orders the rows of the table based on the values in the indexed column. A non-clustered index is created on any column other than the primary key, and it stores the values of the indexed column in a separate structure from the table rows. Indexes can be used to improve the performance of a wide variety of queries, including queries that select rows based on a specific value, queries that order the results of a query, and queries that join two or more tables. However, indexes can also slow down the performance of certain types of queries, such as queries that update or delete rows. It is important to evaluate the potential benefits and risks of using indexes before creating them.