连接查询是数据库查询语句中使用频率很高的查询方式，下面根据Oracle提供的官方文档学习一下连接查询：
1. 连接种类：
1）oracle特有连接种类：
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
......
b. Non-Equijoin 非等值连接
SQL> select e.last_name, e.salary, j.grade
2 from employees e, job_grades j
3 where e.salary between j.lowest_sal and j.highest_sal;
LAST_NAME SALARY G
------------------------- ---------- -
Olson 2100 A
Philtanker 2200 A
Markle 2200 A
...
Feeney 3000 B
Cabrio 3000 B
Fleaur 3100 B
Walsh 3100 B
...
Hartstein 13000 C
Partners 13500 C
Partners 13500 D
Russell 14000 D
De Haan 17000 E
Kochhar 17000 E
c. Outer join 外连接
SQL> select e.last_name, e.department_id, d.department_name
2 from employees e, departments d
3 where e.department_id(+) = d.department_id;
相当于:select e.last_name, e.department_id, d.department_name
from employees e RIGHT JOIN departments d
ON e.department_id = d.department_id;
LAST_NAME DEPARTMENT_ID DEPARTMENT_NAME
------------------------- ------------- ------------------------------
Greenberg 100 Finance
Sciarra 100 Finance
Urman 100 Finance
Popp 100 Finance
Faviet 100 Finance
Gietz 110 Accounting
Higgins 110 Accounting
Treasury
Corporate Tax
Control And Credit
Shareholder Services
...............
注：此处（+）为连接符，放在等号左边代表右连接（相当于RIGHT JOIN），放在等号右边代表左连接（相当于LEFT JOIN）。
此处为右连接，表示查找所有部门表ID和部门名称以及员工表中对应部门ID的员工姓名，没有的返回NULL。
d. Self join 自连接
SQL> select worker.last_name || 'works for '|| manager.last_name
2 from employees worker, employees manager
3 where worker.manager_id = manager.employee_id;
WORKER.LAST_NAME||'WORKSFOR'||MANAGER.LAST_NAME
------------------------------------------------------------
OConnellworks for Mourgos
Grantworks for Mourgos
Whalenworks for Kochhar
Hartsteinworks for King
Fayworks for Hartstein
Mavrisworks for Kochhar
............
2）符合1999规范的连接：
Cross joins 交叉连接
Natural joins 自然连接
Using clause 使用条件
Full or two sided outer joins 完全连接
Arbitrary join conditions for outer joins 为外连接任意加入条件