P1 and P2 should be in the format YYMM or YYYYMM.
The period arguments P1 and P2 are not date values.
mysql>mysql> SELECT PERIOD_DIFF(9803,199803);+--------------------------+| PERIOD_DIFF(9803,199803) |+--------------------------+| 0 |+--------------------------+1 row in set (0.00 sec)mysql>
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