SQL (Structured Query Language) is a standard language used to manage and manipulate relational databases. It is used to create, read, update, and delete (CRUD) data, as well as to manage database structure and permissions.

WHERE is used to filter rows before any groupings are made with GROUP BY. HAVING is used to filter groups after GROUP BY.

An index is a database object that improves the speed of data retrieval. Indexes are created on one or more columns of a table and help to quickly locate rows. However, they can slow down INSERT, UPDATE, and DELETE operations due to the additional overhead of maintaining the index.