1、相差年数

1. SELECT
2. NOW() 当前日期,
3. DATE_ADD(NOW(), INTERVAL - 400 DAY) 历史日期,
4. TIMESTAMPDIFF(
5. YEAR,
6. DATE_ADD(NOW(), INTERVAL - 400 DAY),
7. NOW()
8. ) AS 相差年;

2、相差月数 

1. SELECT
2. NOW() 当前日期,
3. DATE_ADD(NOW(), INTERVAL - 400 DAY) 历史日期,
4. TIMESTAMPDIFF(
5. MONTH,
6. DATE_ADD(NOW(), INTERVAL - 400 DAY),
7. NOW()
8. ) AS 相差月;

3、相差周数 

1. SELECT
2. NOW() 当前日期,
3. DATE_ADD(NOW(), INTERVAL - 400 DAY) 历史日期,
4. TIMESTAMPDIFF(
5. WEEK,
6. DATE_ADD(NOW(), INTERVAL - 400 DAY),
7. NOW()
8. ) AS 相差周;

4、相差天数 

整理了一下三种方式：

1. SELECT
2.   NOW() 当前日期,
3.   DATE_ADD(NOW(), INTERVAL - 400 DAY) 历史日期,
4.   TIMESTAMPDIFF(
5.     DAY,
6.     DATE_ADD(NOW(), INTERVAL - 400 DAY),
7.     NOW()
8.   ) AS 相差日;
9. SELECT
10. TO_DAYS('2018-09-10') - TO_DAYS('2017-09-01') AS 相差天数;
11. SELECT
12. DATEDIFF(NOW(), '2018-09-01') AS 相差天数;