一个表a有3个字段,year、month、amount
数据如下
| year | month | amount |
| 1991 | 1 | 1.1 |
| 1991 | 2 | 1.2 |
| 1991 | 3 | 1.3 |
| 1991 | 4 | 1.4 |
| 1992 | 1 | 2.1 |
| 1992 | 2 | 2.2 |
| 1992 | 3 | 2.3 |
| 1992 | 4 | 2.4 |
怎么查询出来如下的效果
| year | m1 | m2 | m3 | m4 |
| 1991 | 1.1 | 1.2 | 1.3 | 1.4 |
| 1992 | 2.1 | 2.2 | 2.3 | 2.4 |
一个表a有3个字段,year、month、amount
数据如下
| year | month | amount |
| 1991 | 1 | 1.1 |
| 1991 | 2 | 1.2 |
| 1991 | 3 | 1.3 |
| 1991 | 4 | 1.4 |
| 1992 | 1 | 2.1 |
| 1992 | 2 | 2.2 |
| 1992 | 3 | 2.3 |
| 1992 | 4 | 2.4 |
怎么查询出来如下的效果
| year | m1 | m2 | m3 | m4 |
| 1991 | 1.1 | 1.2 | 1.3 | 1.4 |
| 1992 | 2.1 | 2.2 | 2.3 | 2.4 |