builtin function WEIGHT_STRING() not support

[winkyao]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

   use mysql builtin function WEIGHT_STRING(), but it is not supported by TiDB yet.

2. What did you expect to see?

mysql> SELECT HEX(WEIGHT_STRING('ab' AS CHAR(4)));
+-------------------------------------+
| HEX(WEIGHT_STRING('ab' AS CHAR(4))) |
+-------------------------------------+
| 0041004200200020                    |
+-------------------------------------+
1 row in set (0.03 sec)

3. What did you see instead?

mysql> SELECT HEX(WEIGHT_STRING('ab' AS CHAR(4)));
ERROR 1105 (HY000): line 0 column 25 near "'ab' AS CHAR(4)))" (total length 42)

4. What version of TiDB are you using (tidb-server -V)?

[wink@localhost tidb]$ bin/tidb-server -V
Git Commit Hash: 677fd05c82517e20c60345202e8dc612b1942615
UTC Build Time:  2017-06-29 08:03:08