`monthname` is not compatible with Mysql

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

select monthname(str_to_date(null, '%m')), monthname(str_to_date(null, '%m')),
       monthname(str_to_date(1, '%m')), monthname(str_to_date(0, '%m'));

2. What did you expect to see?
   In Mysql

mysql> select monthname(str_to_date(null, '%m')), monthname(str_to_date(null, '%m')),
    ->        monthname(str_to_date(1, '%m')), monthname(str_to_date(0, '%m'));
+------------------------------------+------------------------------------+---------------------------------+---------------------------------+
| monthname(str_to_date(null, '%m')) | monthname(str_to_date(null, '%m')) | monthname(str_to_date(1, '%m')) | monthname(str_to_date(0, '%m')) |
+------------------------------------+------------------------------------+---------------------------------+---------------------------------+
| NULL                               | NULL                               | NULL                            | NULL                            |
+------------------------------------+------------------------------------+---------------------------------+---------------------------------+
1 row in set, 2 warnings (0.00 sec)

3. What did you see instead?
   In TiDB

mysql> select monthname(str_to_date(null, '%m')), monthname(str_to_date(null, '%m')),
    ->        monthname(str_to_date(1, '%m')), monthname(str_to_date(0, '%m'));
+------------------------------------+------------------------------------+---------------------------------+---------------------------------+
| monthname(str_to_date(null, '%m')) | monthname(str_to_date(null, '%m')) | monthname(str_to_date(1, '%m')) | monthname(str_to_date(0, '%m')) |
+------------------------------------+------------------------------------+---------------------------------+---------------------------------+
| NULL                               | NULL                               | January                         | NULL                            |
+------------------------------------+------------------------------------+---------------------------------+---------------------------------+
1 row in set, 1 warning (0.00 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                                                     |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v3.0.0-beta-212-g0681b8125
Git Commit Hash: 0681b8125080045ee0d4747ce61923164c6f2dd1
Git Branch: master
UTC Build Time: 2019-03-14 04:39:19
GoVersion: go version go1.12 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+