Overview of queries and subqueries

A query is a method for retrieving data from a database. It can be combined with clauses such as WHERE (for specifying conditions) and ORDER BY (for specifying the order of results) to retrieve query results.

A subquery is a query that is nested within another query, which is referred to as the parent query or outer query. The result of the subquery is passed back to the parent query or outer query as input. The parent query then uses this value in its calculations to determine the final output.

SQL allows for multiple levels of nested queries, meaning that a subquery can contain other subqueries. Additionally, subqueries can appear in various clauses of an SQL statement, such as SELECT, FROM, and WHERE.

Common query types in SQL statements include:

• Simple queries

• Hierarchical queries

• Composite queries

• Join queries

• Queries on the DUAL table