sql 用inner join取代in和exists

使用表连接替换EXISTS                                               
一般情况下，使用表连接比EXISTS更高效                                                            

-->低效:                                                    
SELECT *                                                                                                       
FROM  t1 e                                                                                         
WHERE  EXISTS                                                                                                      
          (SELECT 1                                                                       
           FROM   t2                                                              
           WHERE  department_id = e.department_id AND department_name = 'IT');                                 
                                                                                    
-->高效:                  
SELECT *              -->经测试此写法SQLplus下比上面的写法多一次逻辑读，而在Toad下两者结果一致               
FROM   employees e INNER JOIN departments d ON d.department_id = e.department_id              
WHERE  d.department_name = 'IT';