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;

--当join on 后有过滤条件时，是先执行该过滤，然后将过滤后的结果再外连接

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;

--等效的oracle SQL:

SELECT d.depID,d.depName,e.empName
FROM departments d, (select * from employees where depID =3 )e
WHERE d.depID = e.depID(+)
ORDER BY d.depID;

--where后的条件是在连接之后才进行的

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;

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