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一 Equijoins 等值連接

等值連接是條件連接在連接運算符為“=”號時的特例。（相對于非等值連接，如：between...and條件）。它是從關系R與S的廣義笛卡爾積中選取A，B屬性值相等的那些元組。

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
......

二 Self joins 自連接

自連接(self join)是SQL語句中經常要用的連接方式，使用自連接可以將自身表的一個鏡像當作另一個表來對待，從而能夠得到一些特殊的數據。

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

三 Cartesian Products 笛卡爾積

如果2個表連接查詢而沒有連接條件，則oracle返回他們的笛卡爾積。即oracle返回一個表里每一行與另一個表每一行的組合（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

四 Inner Joins 內連接

內連接也叫簡單連接，是2個或更多表的關聯并且僅返回那些滿足連接條件的行。

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;

五 Outer joins 外連接

簡單連接的擴展。分為左外連接（Left outer joins）、右外連接（Right outer joins）、全外連接（Full outer joins）和Partitioned Outer Joins（分區外連接，用于數據倉庫）。

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;

另外一種寫法：

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 官方推薦使用第一種寫法。

https://www.linuxprobe.com/oracle-connection-query.html