Oracle中Left Outer Join和外关联(+)的区别

外关联是Oracle数据库的专有语句

Left Outer Join则是SQL-92的标准语句

通常认为这两个SQL是可以等效的，但还是有些细微的差别。

一般说来，外关联的等值条件等效于Left Outer Join中的on语句；两个where中其他语句是一样的。

但是Left Outer Join中的其他条件（非表连接）出现在On关键词后面还是出现在Where关键词后面是不一样的，这样的语句很难用外关联进行同义转义。

 

下面我们先看一个测试数据，和一个测试案例

创建一个部门表和员工表

CREATE TABLE departments
(
 depID  NUMBER(38,0),
 depName VARCHAR2(20),
 delFlag NUMBER(1,0)
);
 
CREATE TABLE employees
(
 empID  NUMBER(38,0),
 empName VARCHAR2(20),
 depID  NUMBER(38,0),
 delFlag NUMBER(1,0)
);
 
INSERT INTO departments VALUES(1,'Finacle',0);
INSERT INTO departments VALUES(2,'Marketing',0);
INSERT INTO departments VALUES(3,'HR',1);
INSERT INTO departments VALUES(4,'IT',0);
 
INSERT INTO employees VALUES(1,'wbq',1,0);
INSERT INTO employees VALUES(2,'czh',2,0);
INSERT INTO employees VALUES(3,'chh',1,0);
INSERT INTO employees VALUES(4,'wal',2,0);
INSERT INTO employees VALUES(5,'ddd',3,0);
 
COMMIT;

　以下为测试例子

--列出部门ID为3的部门和员工信息，不管该部门是否有员工
SELECT d.depID,d.depName,e.empName
 FROM departments d
 LEFT OUTER JOIN employees e
   ON d.depID = e.depID
 WHERE d.depID =3
 ORDER BY d.depID;
--和上面等效
SELECT d.depID,d.depName,e.empName
 FROM departments d, employees e
 WHERE d.depID = e.depID(+)
  AND d.depID =3
 ORDER BY d.depID;
 
--该例子可能不是很恰当，d.depID=3表示首先进行全左连接，然后查找depID=3的纪录，如果该部门中找不到对应的员工，则该部门员工则为NULL，同时都显示部门信息。
SELECT d.depID,d.depName,e.empName,d.delflag
 FROM departments d
 LEFT OUTER JOIN employees e
   ON d.depID = e.depID AND e.depID=3
 ORDER BY d.depID;
--和上面不等价，差别在于能够找到无对应员工的部门和有对应员工的部门，却找不到该部门无员工的部门。
SELECT d.depID,d.depName,e.empName,d.delflag
 FROM departments d, employees e
 WHERE d.depID = e.depID(+)
  AND (e.depID=3 or e.depID is NULL)
 ORDER BY d.depID;