Oracle中的外连接简单介绍

冷不防 2022-08-27 08:54 140阅读 0赞

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在讲外连接之前，先举例介绍内连接，也就是一般的相等连接。  
  
select \* from a, b where a.id = b.id;  
  
对于外连接，Oracle中可以使用“(+)”来表示，9i可以使用LEFT/RIGHT/FULL OUTER JOIN，下面将配合实例一一介绍。  
  
1. LEFT OUTER JOIN：左外关联

<table> 
 <tbody> 
  <tr> 
   <td style="FONT-SIZE: 9pt"><pre>
       
        SELECT e.last_name, e.department_id, d.department_name

FROM employees e

LEFT OUTER JOIN departments d

ON (e.department_id = d.department_id);
       </pre></td> 
  </tr> 
 </tbody> 
</table>

等价于

<table> 
 <tbody> 
  <tr> 
   <td style="FONT-SIZE: 9pt"><pre>
       
        SELECT e.last_name, e.department_id, d.department_name

FROM employees e, departments d

WHERE e.department_id=d.department_id(+);
       </pre></td> 
  </tr> 
 </tbody> 
</table>

结果为：所有员工及对应部门的记录，包括没有对应部门编号department\_id的员工记录。  
  
2. RIGHT OUTER JOIN：右外关联

<table> 
 <tbody> 
  <tr> 
   <td style="FONT-SIZE: 9pt"><pre>
       
        SELECT e.last_name, e.department_id, d.department_name

FROM employees e

RIGHT OUTER JOIN departments d

ON (e.department_id = d.department_id);
       </pre></td> 
  </tr> 
 </tbody> 
</table>

等价于

<table> 
 <tbody> 
  <tr> 
   <td style="FONT-SIZE: 9pt"><pre>
       
        SELECT e.last_name, e.department_id, d.department_name

FROM employees e, departments d

WHERE e.department_id(+)=d.department_id;
       </pre></td> 
  </tr> 
 </tbody> 
</table>

结果为：所有员工及对应部门的记录，包括没有任何员工的部门记录。  
  
3. FULL OUTER JOIN：全外关联

<table> 
 <tbody> 
  <tr> 
   <td style="FONT-SIZE: 9pt"><pre>
       
        SELECT e.last_name, e.department_id, d.department_name

FROM employees e

FULL OUTER JOIN departments d

ON (e.department_id = d.department_id);
       </pre></td> 
  </tr> 
 </tbody> 
</table>

结果为：所有员工及对应部门的记录，包括没有对应部门编号department\_id的员工记录和没有任何员工的部门记录。  
  
  
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