1) What is the purpose of transaction isolated level? 2) Explain the difference between an explicit lock and implicit lock. 3). Explain the relationships of users, groups, permission, and objects for...

1) What is the purpose of transaction isolated level? 2) Explain the difference between an explicit lock and implicit lock. 3). Explain the relationships of users, groups, permission, and objects for a generic database security system. 4). Define the terms roll-back and roll-forward. 5) Why is it important to write to a log before changing the database value?