## period_diff，extract
### 说明
| 函数 | 说明 |
| --- | --- |
| period_diff(p1,p2) | 返回周期p1和p2之间的月数，p1和p2格式为YYMM或YYYYMM |
| extract(unit FROM date) | 函数用于返回日期/时间的单独部分 |
### 例子：查询宝宝月龄
```sql
SELECT
id,
baby_name,
baby_birthday,
period_diff(
extract(YEAR_MONTH FROM curdate()),
extract(YEAR_MONTH FROM baby_birthday)
) AS baby_monthage
FROM
baby;
```
*****
### 附录
#### extract函数中unit的值
| Unit 值 |
| --- |
| MICROSECOND |
| SECOND |
| MINUTE |
| HOUR |
| DAY |
| WEEK |
| MONTH |
| QUARTER |
| YEAR |
| SECOND\_MICROSECOND |
| MINUTE\_MICROSECOND |
| MINUTE\_SECOND |
| HOUR\_MICROSECOND |
| HOUR\_SECOND |
| HOUR\_MINUTE |
| DAY\_MICROSECOND |
| DAY\_SECOND |
| DAY\_MINUTE |
| DAY\_HOUR |
| YEAR\_MONTH |
