SELECT statements or
SELECT queries or just queries are used to retrieve data from the database.
SELECT queries can have different sources: tables, views, functions or the
VALUES command. All of them are relations or can be treated as relations or return relations, which functions can do. The output of
SELECT is also a relation which in general can have multiple columns and contain many rows. Since the result and the source of the query have the same nature in SQL, it is possible to use one
SELECT query as a source for another statement. But in this case, both queries are considered as parts of one bigger query. The source of the data, output format, filters, grouping, ordering and required transformations ...