Manual:MySQL/Start

mysql -u <wikiuser> -p

then enter your password, then,

 USE <wikidb>

Example

aaron@escargot ~ $ mysql -u wikiuser -p
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 816 to server version: 5.5.20

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> USE wikidb
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql>