半链接和关联转换

SQL> select  department_name
from hr.departments dept
where department_id IN (select department_id from hr.employees emp);
  2    3  
DEPARTMENT_NAME
------------------------------
Administration
Marketing
Purchasing
Human Resources
Shipping
IT
Public Relations
Sales
Executive
Finance
Accounting

已选择11行。

SQL> select   department_name
 from hr.departments dept, hr.employees emp
 where dept.department_id = emp.department_id;  2    3  

DEPARTMENT_NAME
------------------------------
Administration
Marketing
Marketing
Purchasing
Purchasing
Purchasing
Purchasing
Purchasing
Purchasing
Human Resources
Shipping

DEPARTMENT_NAME
------------------------------
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping

DEPARTMENT_NAME
------------------------------
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping

DEPARTMENT_NAME
------------------------------
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping

DEPARTMENT_NAME
------------------------------
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping
Shipping

DEPARTMENT_NAME
------------------------------
IT
IT
IT
IT
IT
Public Relations
Sales
Sales
Sales
Sales
Sales

DEPARTMENT_NAME
------------------------------
Sales
Sales
Sales
Sales
Sales
Sales
Sales
Sales
Sales
Sales
Sales

DEPARTMENT_NAME
------------------------------
Sales
Sales
Sales
Sales
Sales
Sales
Sales
Sales
Sales
Sales
Sales

DEPARTMENT_NAME
------------------------------
Sales
Sales
Sales
Sales
Sales
Sales
Sales
Executive
Executive
Executive
Finance

DEPARTMENT_NAME
------------------------------
Finance
Finance
Finance
Finance
Finance
Accounting
Accounting

已选择106行。


SQL> select  distinct department_name
 from hr.departments dept, hr.employees emp
 where dept.department_id = emp.department_id;
  2    3  
DEPARTMENT_NAME
------------------------------
Administration
Accounting
Purchasing
Human Resources
IT
Public Relations
Executive
Shipping
Sales
Finance
Marketing

已选择11行。

IN半链接改成关联,就得去从


那从2个表返回数据呢？能改写成半链接吗？
SQL> select distinct dept.department_name,emp.department_id
  from hr.departments dept, hr.employees emp
 where dept.department_id = emp.department_id;  2    3  

DEPARTMENT_NAME 	       DEPARTMENT_ID
------------------------------ -------------
Marketing				  20
Accounting				 110
Human Resources 			  40
IT					  60
Administration				  10
Executive				  90
Public Relations			  70
Finance 				 100
Purchasing				  30
Shipping				  50
Sales					  80

已选择11行。

SQL> select dept.department_name, emp.department_id
  from hr.departments dept
 where dept.department_id in
       (select emp.department_id from hr.employees emp)  2    3    4  ;
select dept.department_name, emp.department_id
                             *
第 1 行出现错误:
ORA-00904: "EMP"."DEPARTMENT_ID": 标识符无效

从多个表返回数据的关联,无法改成半链接。