What is a Root Segment
In a DL/I hierarchical structure, the top-level parent segment is known as the root segment. There can be only one root segment in any hierarchy.
What is a Database Record
Collectively, all the parent/child occurrences that depend on a given root segment form a DL/I database record. Since there can be only one occurrence of a root segment, the addition of a new root segment occurrence (for example, a new employee) creates a new database record. Database records are variable in size because the number of occurrences for dependent child segments may vary (for example, new skills can be added for a given employee).
A DL/I Physical Database
All of the database records for a particular parent/child hierarchy form a DL/I physical database. Since each child segment can have only one parent segment, the resulting structure resembles an inverted tree, with the root segment at the top. The maximum number of segments in a DL/I structure is 255: one root and up to 254 dependent child segments.
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