我有看起来像这样的数据：
+----+------+-------+
| ID | YEAR | MONTH |
+----+------+-------+
| A | 2017 | 1 |
| B | 2017 | 2 |
| C | 2017 | 3 |
| D | 2017 | 4 |
| E | 2017 | 5 |
| F | 2017 | 6 |
+----+------+-------+
我想添加一个名为的新列DATE，它存储由YEAR和MONTH列的日期对象组成的新列。像这样的东西：
+----+------+-------+------------+
| ID | YEAR | MONTH | DATE |
+----+------+-------+------------+
| A | 2017 | 1 | 2017-01-01 | …