请问各位老师，对于面板数据stkcd year，如何从第一个表变为第二个表
| stkcd | year | var1 | var2 |
| 1 | 2011 | 34 | . |
| 1 | 2012 | 34 | . |
| 1 | 2013 | 34 | . |
| 1 | 2014 | . | 25 |
| 1 | 2015 | . | 25 |
| 1 | 2016 | . | 25 |
| 2 | 2011 | 7 | . |
| 2 | 2012 | 7 | . |
| 2 | 2013 | 7 | . |
| 2 | 2014 | . | 98 |
| 2 | 2015 | . | 98 |
| 2 | 2016 | . | 98 |
| 3 | 2011 | 65 | . |
| 3 | 2012 | 65 | . |
| 3 | 2013 | 65 | . |
| 3 | 2014 | . | 112 |
| 3 | 2015 | . | 112 |
| 3 | 2016 | . | 112 |
| stkcd | year | var1 | var2 |
| 1 | 2011 | 34 | 25 |
| 1 | 2012 | 34 | 25 |
| 1 | 2013 | 34 | 25 |
| 1 | 2014 | 34 | 25 |
| 1 | 2015 | 34 | 25 |
| 1 | 2016 | 34 | 25 |
| 2 | 2011 | 7 | 98 |
| 2 | 2012 | 7 | 98 |
| 2 | 2013 | 7 | 98 |
| 2 | 2014 | 7 | 98 |
| 2 | 2015 | 7 | 98 |
| 2 | 2016 | 7 | 98 |
| 3 | 2011 | 65 | 112 |
| 3 | 2012 | 65 | 112 |
| 3 | 2013 | 65 | 112 |
| 3 | 2014 | 65 | 112 |
| 3 | 2015 | 65 | 112 |
| 3 | 2016 | 65 | 112 |
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