Merge比传统使用中先进行判断后再插入或更新上的速度要快好多，很多用在数据仓库中．
DB: Oracle 9i

1.MERGE Syntax:
merge [hint] into [schema.] table [t_alias]
using [schema.] {table|view|subquery} [t_alias] on (condition)
when matched then merge_update_clause
when not matched then merge_insert_clause

[@more@]2.Merging into a Table:
Example
The following example creates a bonuses table in the sample schema oe with a default bonus of 100. It then inserts into the bonuses table all employees who made sales, based on the sales_rep_id column of the oe.orders table. Finally, the human resources manager decides that employees with a salary of $8000 or less should receive a bonus. Those who have not made sales get a bonus of 1% of their salary. Those who already made sales get an increase in their bonus equal to 1% of their salary. The MERGE statement implements these changes in one step:

CREATE TABLE bonuses (employee_id NUMBER, bonus NUMBER DEFAULT 100);
INSERT INTO bonuses(employee_id)
(SELECT e.employee_id FROM employees e, orders o
WHERE e.employee_id = o.sales_rep_id
GROUP BY e.employee_id);
SELECT * FROM bonuses;
EMPLOYEE_ID BONUS
----------- ----------
153 100
154 100
155 100
156 100
158 100
159 100
160 100
161 100
163 100
MERGE INTO bonuses D
USING (SELECT employee_id, salary, department_id FROM employees
WHERE department_id = 80) S
ON (D.employee_id = S.employee_id)
WHEN MATCHED THEN UPDATE SET D.bonus = D.bonus + S.salary*.01
DELETE WHERE (S.salary > 8000)
WHEN NOT MATCHED THEN INSERT (D.employee_id, D.bonus)
VALUES (S.employee_id, S.salary*0.1)
WHERE (S.salary <= 8000);
EMPLOYEE_ID BONUS
----------- ----------
153 180
154 175
155 170
159 180
160 175
161 170
179 620
173 610
165 680
166 640
164 720
172 730
167 620
171 740