in line view(内联视图)

如果需要比较多个条件的话，简单的嵌套查询不起作用，就需要用到in line view，比如：

SELECT  a.last_name, a.salary,
       a.department_id, b.salavg
FROM    employees a, (SELECT   department_id,
                     AVG(salary) salavg
                     FROM     employees
                     GROUP BY department_id) b
WHERE   a.department_id = b.department_id
AND     a.salary > b.salavg;