How to use "WHERE expr" in mysql?

[aaron chan]

I found "show databases" and "show character set" both support the "WHERE expr".but something differs slightly and very strange.
1)mysql> show character set where Description='DEC West European';
+---------+-------------------+-------------------+--------+
| Charset | Description | Default collation | Maxlen |
+---------+-------------------+-------------------+--------+
| dec8 | DEC West European | dec8_swedish_ci | 1 |
+---------+-------------------+-------------------+--------+
1 row in set (0.00 sec)
2)mysql> show databases where Database='mysql';
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '='mysql'' at line 1

why the first one can use the "Description" column in the where expr but the second one can not?