Oracle 内连接,外连接

一般的相等连接:
select * from a, b where a.id = b.id;
这个就属于内连接。
  
对于外连接:
Oracle中可以使用“(+) ”来表示,9i可以使用LEFT/RIGHT/FULL OUTER JOIN  
  
LEFT OUTER JOIN:左外关联  
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);  
等价于  
SELECT e.last_name, e.department_id, d.department_name  
FROM employees e, departments d  
WHERE e.department_id=d.department_id(+)  
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。  
  
RIGHT OUTER JOIN:右外关联  
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);  
等价于  
SELECT e.last_name, e.department_id, d.department_name  
FROM employees e, departments d  
WHERE e.department_id(+)=d.department_id  
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。  
  
FULL OUTER JOIN:全外关联  
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);  
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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