Which of the following is used to give a default value to a column if the value for that column is
unknown?

A.
Concurrency

B.
DEFAULT constraint

C.
Nested transaction

D.
Transaction

Explanation:
The DEFAULT constraint is a constraint in which a default value is given to a column if the value for
that column is unknown. If a user does not provide any value for that column, the default value is
automatically inserted. If a default value is not provided, then NULL is inserted. If a column does not
allow NULL value and a default value is also not assigned for that column, an error is sent by the
database engine.
Answer D is incorrect. A transaction is a logical unit or work, where related DML statements are
grouped together. When a transaction is active, either all the statements in the transaction will
complete successfully or none of them will run. This means that if any of the statements in the
transaction fails due to any reason, the whole transaction will be rolled back. Transactions are ended
by using the COMMIT or ROLLBACK statement. A perfect example would be a bank transaction,
where a check deposit should cause a debit in the check issuer’s account and a credit in the
depositor’s account. If any one operation fails, the bank should reverse the whole transaction, thus
maintaining correct balances in both the accounts.
Answer C is incorrect. A nested transaction occurs when a new transaction is started by an
instruction that is already inside an existing transaction. The new transaction is said to be nested
within the existing transaction.Nested transactions are implemented differently in databases.
However, these nested transactions have in common that the changes are not made visible to any
unrelated transactions until the outermost transaction has committed. This means that a commit in
an inner transaction does not necessary persist updates to the database.
Answer A is incorrect. Concurrency is the ability of several users to access data from the database
simultaneously. Database concurrency increases when the number of simultaneous operations that
the database engine can support is large. In Microsoft SQL Server 2008 Compact 3.5, concurrency
control is achieved by using locks to protect data. The locks control how multiple users can access
and modify shared data simultaneously without conflicting with each other.