Inline view(内联视图)

Copy from: http://www.orafaq.com/wiki/Inline_view

An inline view is a SELECT statement in the FROM-clause of another SELECT statement. In-line views are commonly used simplify complex queries by removing join operations and condensing several separate queries into a single query. This feature was introduced in Oracle 7.2.

This feature is commonly referred to in the MSSQL community as a derived table, and in the Postgres community simply refers to it as a subselect (subselects are inline views + subqueries in Oracle nomenclature).

Examples

Example inline view:

SELECT *

  FROM (select deptno, count(*) emp_count

          from emp

         group by deptno) emp,

       dept

 WHERE dept.deptno = emp.depto;

 

Another good example of an inline view is:

SELECT a.last_name, a.salary, a.department_id, b.maxsal

  FROM employees a,

       (SELECT department_id, max(salary)maxsal

          FROM employees

      GROUP BY department_id) b

 WHERE a.department_id = b.department_id

   AND a.salary < b.maxsal;

The above query display the employees who earn the highest salary in each department.