# Union Example
__*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 |
+------+
```

------------------------------------------------------------
Observe the execution results and think about which operation overhead expensive?
```
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 |
+------+
```