Create a table to save user's role information. There are three types of roles: 'normal', 'admin', 'guest'. 

1. . The table should have two fields: "username", "user_roles".
2. . The "user_roles" column can be any of the roles or any combination. 
3. . The "user_roles" column is not allowed to save any other values.

INSERT few users with different roles and combinations of roles.

2. Create tables to save orders and its items for a store. Then write the SQLs in a transation with SERIALIZABLE isolation level to save a order which has 2 items. You are free to create the fields as long as they are reasonable.