多表关联查询
如果未指定连接条件，则结果返回是个笛卡尔乘积
比如：
select employee_id, department_id, location_id from employees, departments;
大多数情况下，笛卡尔乘积不是我们想要的结果，我们一般要在Where子句中提供链接条件，对于链接，通常又包括多种类型：
不同的数据库厂商对链接类型有不同的定义，国际上有个凌驾于各个厂商的工业标准定义（SQL1999）。
Oracle定义的链接类型：
1、等于链接
语法：select table1.column, table2.column from table1, table2 where table1.column1 = table2.column2;
举例：select employees.employee_id, employees.last_name, employees.department_id,
departments.department_id, departments.location_id
FROM employees, departments
WHERE employees.department_id = departments.department_id;
2、不等链接
语法：select table1.column, table2.column from table1, table2 where table1.column1 > table2.column2;
举例：select e.last_name, e.salary, j.grade_level from employees e, job_grades j where e.salary
between j.lowest_sal and j.highest_sal;
3、外链接（左外链接、右外链接）
语法：(左外链接)select table1.column, table2.column from table1, table2 where table1.column(+) = table2.column;
(右外链接)select table1.column, table2.column from table1, table2 where table1.column = table2.column(+)
小规律：没有括号的外联有括号的，即当没有共同的column的时候，没有括号的数据依然会显示出来。
举例：select e.last_name, e.department_id, d.department_name from employees e, departments d
where e.department_id(+) = d.department_id;
4、自链接(即某个table和自己本身链接)
语法：select table1.column, table2.column from table1, table1 table2 where table1.column1 = table2.column2
举例：选择某个员工和经理的从属关系
select worker.last_name || ‘ works for ‘ || manager.last_name
from employees worker, employees manager
where worker.manager_id = manager.employee_id;
SQL1999定义的链接类型(Oracle从9i开始提供对SQL1999的兼容支持)：
SQL1999的语法如下(一句SQL代码概括)：
select table1.column, table2.column
from table1
[cross join table2] |
[natural join table2] |
[join table2 using (column_name)] |
[join table2
on (table1.column_name = table2.column_name)] |
[left | right | full outer join table2
on (table1.column_name = table2.column_name)];
1、交叉链接
相当于没有链接条件的多表关联查询，结果相当于笛卡尔乘积，实际工作中很少用到
举例：select last_name, department_name from employees cross join departments;
2、自然链接
相当于Oracle的“等于链接”，只不过是让系统自己去找两张表中字段名相同的字段作为“等于链接”的条件；
（PS:如果两个表中有相同的列名，但字段类型不一样，就会引发错误）
举例：select department_id, department_name, location_id, city
from departments
natural join locations;
相当于：select department_id, department_name, location_id, city
from departments, locations
where departments.location_id = locations.location_id;
3、Using子句
算是自然链接的一种补充功能，自然链接会让系统自动查找两张表中所有列名相同的字段，并试图建立”等于链接“；
但有的时候我们不期望这么做，而是只期望某个特定的字段作为”等于链接“的条件，这种情况下可以使用Using子句来做限制。
举例：想建立链接，又想控制只使用department_id作为链接条件
select e.employee_id, e.last_name, d.location_id
from employee e join departments d
using (department_id);
4、内链接
相当于Oracle的”等于链接“，关键字：inner join，可省略inner
举例：select employee_id, city, department_name
from employees e
inner join departments d on d.department_id = e.department_id
inner join locations l on d.location_id = l.location_id;
5、外链接（全外链接、左外链接、右外链接）
举例：
select e.last_name, e.department_id, d.department_name
from employees e
left outer join departments d
on (e.department_id = d.department_id);
相当于：select e.last_name, e.department_id, d.department_name
from employees e, departments d
where e.department_id(+)=d.department_id
union
select e.last_name, e.department_id, d.department_name
from employees e, departments d
where e.department_id=d.department_id(+);
上个例子是左外链接的例子，右外连接、全外链接同理。