在講外連接之前，先舉例介紹內連接，也就是一般的相等連接。
select * from a, b where a.id = b.id;
對于外連接，Oracle中可以使用“(+)”來表示，9i可以使用LEFT/RIGHT/FULL OUTER JOIN，下面將配合實例一一介紹。
1. LEFT OUTER JOIN：左外關聯

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);
?

等價于
SELECT e.last_name, e.department_id, d.department_name?
FROM employees e, departments d?
WHERE e.department_id=d.department_id(+);
?

結果為：所有員工及對應部門的記錄，包括沒有對應部門編號department_id的員工記錄。
2. RIGHT OUTER JOIN：右外關聯
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);
?

等價于
SELECT e.last_name, e.department_id, d.department_name?
FROM employees e, departments d?
WHERE e.department_id(+)=d.department_id;
?

結果為：所有員工及對應部門的記錄，包括沒有任何員工的部門記錄。
3. FULL OUTER JOIN：全外關聯
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);
?

結果為：所有員工及對應部門的記錄，包括沒有對應部門編號department_id的員工記錄和沒有任何員工的部門記錄。