Key-Preserved Tables:

A concept of a key-preserved table is fundamental to understanding the restrictions on changing the value of join views. A table is known as key preserved if every key of the table can also be a key of the result of the join. Then a key-preserved table has its keys preserved by a join.

Note: It is not must in which the key or keys of a table be selected for it to be key preserved. It is sufficient in which if the key or keys were selected and then they would also be key(s) of the conclusion of the join.

Attention: The key-preserving property of a table does not depend on the real data in the table. It is quite, a property of its schema and not of the data in the table. For instance, if in the EMP table there was at most one employee in every department then DEPT. the DEPTNO will be unique in the result of a join of EMP and DEPT, but DEPT would since not be a key- preserved table.

Dropping Views:

The Dropping Views can be done by using the statement:

DROP VIEW <viewname>;