那么,什么是Key-Preserved Table呢.Oracle给出的定义是:
A table is key preserved if every key of the table can also be a key of the result of the join.
It is not necessary that the key or keys of a table be selected for it to be key preserved.
It is sufficient that if the key or keys were selected, then they would also be key(s) of the result of the join.
如果某一个表的主键可以作为这个join结果(view通常是几个表的join结果)的主键,那么这个表就是key preserved table.
这个表的主键并非一定要出现在select出来的结果集中(select list里面),但是如果其出现在结果集中,那么它必须可以满足作为这个结果集的主键的要求.