MySQL If Database Not Exists Create DB - Else Alter Table

I'm using MySQL 5.5.15. I have a .sql file I'm using as part of an installer.  I need to check whether the database exists or not.

If it doesn't exist then create a new database and tables.
If it DOES exist then update the database tables with additional columns.

I'd like to check this with a simple IF statement:

If NOT EXISTS(SELECT SCHEMA_NAME from INFORMATION_SCHEMA.SCHEMATA WHERE SCHEMA_NAME = 'MyDatabase')
THEN
     Create MyDatabase;
     Create Tables;
Else
    Alter Table1;
    Alter Table2;
    ...
    Alter TableN;
End If;

Why doesn't this work?  The problem is the IF statement. It seems logical to me but I'm obviously thinking differently than MySQL.