#  UNION 示例

__*UNION*__

```
tidb> SELECT id FROM test.a 
   -> UNION
   -> SELECT id FROM test.b;
+------+
| id   |
+------+
|    2 |
|    1 |
|    3 |
+------+
tidb> SELECT id FROM test.a 
   -> UNION ALL
   -> SELECT id FROM test.b;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    1 |
|    2 |
|    3 |
+------+
```
------------------------------------------------------
请观察执行结果,并思考一下,哪个操作的开销更大？
```
tidb> SELECT id FROM test.a 
   -> UNION
   -> SELECT id FROM test.b;
+------+
| id   |
+------+
|    2 |
|    1 |
|    3 |
+------+
tidb> SELECT id FROM test.a 
   -> UNION ALL
   -> SELECT id FROM test.b;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    1 |
|    2 |
|    3 |
+------+
```