一、语法
query { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] query
二、参数
| 子句/关键字 | 说明 |
| UNION | 返回两个查询结果的并集。默认行为等价于 UNION DISTINCT，即自动去重。 |
| INTERSECT | 返回两个查询结果的交集。默认行为等价于 INTERSECT DISTINCT，即自动去重。 |
| EXCEPT | 返回在第一个查询结果中存在、但在第二个查询结果中不存在的行。默认行为等价于 EXCEPT DISTINCT。 |
| ALL | 保留重复行，不进行去重。 |
| DISTINCT | 对结果进行去重，只返回唯一的行。默认行为，可省略不写。 |
三、示例
— UNION ALLSELECT 1 AS id UNION ALL SELECT 2 AS id UNION ALL SELECT 1 AS id– UNION DISTINCTSELECT 1 AS id UNION SELECT 2 AS id UNION SELECT 1 AS id– INTERSECTSELECT 1 AS id UNION ALL SELECT 2 INTERSECT SELECT 2– EXCEPTSELECT 1 UNION ALL SELECT 2 EXCEPT SELECT 1