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 of the data are specified in the code of the query.
In general, SELECT queries do not change the data in the database and could be considered as read-only, but there is an exception. If a volatile function is used in the query, then the data...