Which statements are true regarding the FOR UPDATE clau…

Which statements are true regarding the FOR UPDATE clause in a SELECT statement? (Choose all that
apply.)

A.
It locks only the columns specified in the SELECT list.

B.
It locks the rows that satisfy the condition in the SELECT statement.

C.
It can be used only in SELECT statements that are based on a single table.

D.
It can be used in SELECT statements that are based on a single or multiple tables.

E.
After it is enforced by a SELECT statement, no other query can access the same rows until a COMMIT or
ROLLBACK is issued.

Explanation:
FOR UPDATE Clause in a SELECT Statement
Locks the rows in the EMPLOYEES table where job_id is SA_REP.
Lock is released only when you issue a ROLLBACK or a COMMIT.
If the SELECT statement attempts to lock a row that is locked by another user, the database waits until the row
is available, and then returns the results of the SELECT statement.
FOR UPDATE Clause in a SELECT Statement
When you issue a SELECT statement against the database to query some records, no locks are placed on the
selected rows. In general, this is required because the number of records locked at any given time is (by
default) kept to the absolute minimum: only those records that have been changed but not yet committed are
locked. Even then, others will be able to read those records as they appeared before the change (the “before
image” of the data). There are times, however, when you may want to lock a set of records even before you
change them in your program.
Oracle offers the FOR UPDATE clause of the SELECT statement to perform this locking. When you issue a
SELECT…FOR UPDATE statement, the relational database management system (RDBMS) automatically
obtains exclusive row-level locks on all the rows identified by the SELECT statement, thereby holding the
records “for your changes only.” No one else will be able to change any of these records until you perform a
ROLLBACK or a COMMIT.
You can append the optional keyword NOWAIT to the FOR UPDATE clause to tell the Oracle server not to wait
if the table has been locked by another user. In this case, control will be returned immediately to your program
or to your SQL Developer environment so that you can perform other work, or simply wait for a period of time
before trying again. Without the NOWAIT clause, your process will block until the table is available, when the
locks are released by the other user through the issue of a COMMIT or a ROLLBACK command.