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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;
Oracle中Left Outer Join和外关联(+)的区别
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原文地址:http://www.cnblogs.com/dar521lin/p/5443179.html