oracle连接查询详解

等值连接是条件连接在连接运算符为“=”号时的特例。（相对于非等值连接，如：between…and条件）。它是从关系R与S的广义笛卡尔积中选取A，B属性值相等的那些元组。

SQL> select e.employee_id, e.last_name,
  2                   e.department_id, d.department_id,
  3                   d.location_id
  4  from   employees e, departments d
  5  where  e.department_id = d.department_id;
EMPLOYEE_ID LAST_NAME  DEPARTMENT_ID DEPARTMENT_ID LOCATION_ID
----------- ------------------------- -------------        -------------     -----------
        198              OConnell                              50                           50         1500
        199              Grant                                     50                            50         1500
        200              Whalen                                10                            10          1700
        201              Hartstein                              20                            20         1800
......

自连接(self join)是SQL语句中经常要用的连接方式，使用自连接可以将自身表的一个镜像当作另一个表来对待，从而能够得到一些特殊的数据。

SELECT e1.last_name||' works for '||e2.last_name 
   "Employees and Their Managers"
   FROM employees e1, employees e2 
   WHERE e1.manager_id = e2.employee_id
      AND e1.last_name LIKE 'R%'
   ORDER BY e1.last_name;
 
Employees and Their Managers   
-------------------------------
Rajs works for Mourgos
Raphaely works for King
Rogers works for Kaufling
Russell works for King

   
如果2个表连接查询而没有连接条件，则oracle返回他们的笛卡尔积。即oracle返回一个表里每一行与另一个表每一行的组合（15 X 4）。

SCOTT@orcl> select count(1) from emp;
 
  COUNT(1)
----------
        15
SCOTT@orcl> select count(1) from dept;
 
  COUNT(1)
----------
         4
SCOTT@orcl> select count(1) from emp, dept;
 
  COUNT(1)
----------
        60

内连接也叫简单连接，是2个或更多表的关联并且仅返回那些满足连接条件的行。

select e.employee_id, e.last_name,
                e.department_id, d.department_id,
                  d.location_id
FROM   employees e JOIN departments d
 ON  e.department_id = d.department_id;

简单连接的扩展。分为左外连接（Left  outer joins）、右外连接（Right  outer joins）、全外连接（Full  outer  joins）和Partitioned Outer Joins（分区外连接，用于数据仓库）。

SELECT d.department_id, e.last_name
   FROM departments d LEFT OUTER JOIN employees e
   ON d.department_id = e.department_id
   ORDER BY d.department_id, e.last_name;

另外一种写法：

SELECT d.department_id, e.last_name
   FROM departments d, employees e
   WHERE d.department_id = e.department_id(+)
   ORDER BY d.department_id, e.last_name;

Oracle 官方推荐使用第一种写法。