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| Constraints |
| Hierarchic model overview |
The hierarchic model, like the network model was defined by a process of abstraction from implemented systems.
A hierarchic data structure consists of an ordered set of trees - or more precisely, an ordered set consisting of multiple occurrences of a single type of tree.
There are general rules for data manipulation and data integrity contained within the general schema for a hierarchical system, however, these will differ from system to system (e.g. the IMS system contains its own specific IMS data manipulation section etc.). |
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| Enforcing Standards. |
| The DBA must ensure that all applicable standards are observed in the representation of the data in the DBMS, where applicable standards may include any or all of corporate, installation, departmental, industry, national and international standards. Standardizing is essential when many people use the same data and is particularly desirable as an aid to data interchange between systems, but this is only possible when data independence has been achieved. |
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| Maintaining Integrity |
Maintaining database integrity involves ensuring data in the database is accurate. This applies to cases where redundancy appears and also where it does not. Centralized control of the database can help in avoiding this problem, insofar as it can be avoided - by permitting the DBA to do value range checks whenever any update operation is attempted.
Data integrity is even more important in a multi-user database system because the data is shared, for without appropriate controls it would be possible for one user to update the database incorrectly which could cause problems for other users. It is worth noting that most commercial products used to be somewhat weak in their support for integrity controls. More recently however this is changing with more recent versions of DBMS products, ORACLE V7 for example, giving more and more support. This is a consequence of the standardization of SQL where SQL2 does support certain types of integrity. |
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