`FIND_IN_SET` is not compatible with MySQL

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

mysql> SELECT FIND_IN_SET(' ', '  , , ,');
+-----------------------------+
| FIND_IN_SET(' ', '  , , ,') |
+-----------------------------+
|                           2 |
+-----------------------------+
1 row in set (0.06 sec)

3. What did you see instead (Required)

mysql> SELECT FIND_IN_SET(' ', '  , , ,');
+-----------------------------+
| FIND_IN_SET(' ', '  , , ,') |
+-----------------------------+
|                           1 |
+-----------------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

[zanmato1984]

/severity moderate