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。
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。