If a foreign key reference a parent key of the same table is called self-referential integrity constraint.
In Circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs , the data for the row is stored in a chain of data block (one or more) reserved for that segment.
An Extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.
A segment is a set of extents allocated for a certain logical structure.
Data Segment, Index Segment, Rollback Segment and Temporary Segment.