一般的相等连接：
select * from a, b where a.id = b.id;
这个就属于内连接。

对于外连接：
Oracle中可以使用“(+) ”来表示，9i可以使用LEFT/RIGHT/FULL OUTER JOIN;;

LEFT OUTER JOIN：左外关联;;
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);;;
等价于;;
SELECT e.last_name, e.department_id, d.department_name;;
FROM employees e, departments d;;
WHERE e.department_id=d.department_id(+);;
结果为：所有员工及对应部门的记录，包括没有对应部门编号department_id的员工记录。;;

RIGHT OUTER JOIN：右外关联;;
SELECT e.last_name, e.department_id, d.department_name;;
FROM employees e;;
RIGHT OUTER JOIN departments d;;
ON (e.department_id = d.department_id);;;
等价于;;
SELECT e.last_name, e.department_id, d.department_name;;
FROM employees e, departments d;;
WHERE e.department_id(+)=d.department_id;;
结果为：所有员工及对应部门的记录，包括没有任何员工的部门记录。;;

FULL OUTER JOIN：全外关联;;
SELECT e.last_name, e.department_id, d.department_name;;
FROM employees e;;
FULL OUTER JOIN departments d;;
ON (e.department_id = d.department_id);;;
结果为：所有员工及对应部门的记录，包括没有对应部门编号department_id的员工记录和没有任何员工的部门记录。

ORACLE8i是不直接支持完全外连接的语法，也就是说不能在左右两个表上同时加上(+)，下面是在ORACLE8i可以参考的完全外连接语法
select t1.id,t2.id from table1 t1,table t2 where t1.id=t2.id(+)
union
select t1.id,t2.id from table1 t1,table t2 where t1.id(+)=t2.id