PERIOD_DIFF改写为 moths_between

mysql> sELECT PERIOD_DIFF(202101,202001);
+----------------------------+
| PERIOD_DIFF(202101,202001) |
+----------------------------+
|                         12 |
+----------------------------+
1 row in set (0.00 sec)

SQL> sELECT months_between(to_date(202101,'yyyymm'),to_date(202001,'yyyymm'));

LINEID     MONTHS_BETWEEN(TO_DATE(202101,'yyyymm'),TO_DATE(202001,'yyyymm'))
---------- -----------------------------------------------------------------
1          12

used time: 0.220(ms). Execute id is 704.
SQL> sELECT DATEDIFF(month,to_date(202101,'yyyymm'),to_date(202001,'yyyymm'));

LINEID     DATEDIFF(MONTH,TO_DATE(202101,'yyyymm'),TO_DATE(202001,'yyyymm'))
---------- -----------------------------------------------------------------
1          -12

used time: 0.197(ms). Execute id is 705.
SQL> sELECT DATEDIFF(month,to_date(202001,'yyyymm'),to_date(202101,'yyyymm'));

LINEID     DATEDIFF(MONTH,TO_DATE(202001,'yyyymm'),TO_DATE(202101,'yyyymm'))
---------- -----------------------------------------------------------------
1          12

used time: 0.732(ms). Execute id is 706.

-- 达梦需要转换为时间类型来进行运算