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Hierarchical databases work on the relationships between two or more segments. The following example shows how segments are related to each other in the IMS database structure.
Root Segment
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The segment that lies at the top of the hierarchy is called the root segment.
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The root segment is the only segment through which all dependent segments are accessed.
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The root segment is the only segment in the database which is never a child segment.
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There can be only one root segment in the IMS database structure.
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For example, ”A” is the root segment in the above example.
Parent Segment
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A parent segment has one or more dependent segments directly below it.
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For example, ”A”, ”B”, and ”E” are the parent segments in the above example.
Dependent Segment
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All segments other than the root segment are known as dependent segments.
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Dependent segments depend on one or more segments to present complete meaning.
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For example, ”B”, ”C1”, ”C2”, ”D”, ”E”, ”F1” and ”F2” are dependent segments in our example.
Child Segment
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Any segment having a segment directly above it in the hierarchy is known as a child segment.
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Each dependent segment in the structure is a child segment.
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For example, ”B”, ”C1”, ”C2”, ”D”, ”E”, ”F1” and ”F2” are child segments.
Twin Segments
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Two or more segment occurrences of a particular segment type under a single parent segment are called twin segments.
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For example, ”C1” and ”C2” are twin segments, so do ”F1” and ”F2” are.
Sibling Segment
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Sibling segments are the segments of different types and the same parent.
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For example, ”B” and ”E” are sibling segments. Similarly, ”C1”, ”C2”, and ”D” are sibling segments.
Database Record
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Each occurrence of the root segment, plus all the subordinate segment occurrences make one database record.
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Every database record has only one root segment but it may have any number of segment occurrences.
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In standard file processing, a record is a unit of data that an application program uses for certain operations. In DL/I, that unit of data is known as a segment. A single database record has many segment occurrences.
Database Path
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A path is the series of segments that starts from the root segment of a database record to any specific segment occurrence.
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A path in the hierarchy structure need not be complete to the lowest level. It depends on how much information we require about an entity.
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A path must be continuous and we cannot skip intermediate levels in the structure.
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In the following figure, the child records in dark grey color show a path which starts from ”A” and goes through ”C2”.
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