When two or more user try to update same type of data in a table then Database Concurrency helps in this situation.
There are 2 types of Database Concurrency
Pessimistic: When one user try to change the data with pessimistic concurrency then a lock is placed on the data in a table so that another user cannot change data. when one user finishes then other user can able to change.
Optimistic: When two user works on the same data in a table and one change that data first then second user cannot change that same data becasue the which he have using is allready changed so he cannot do the change becasue change apply to another data that is changed by first user.