Troubleshooting
Problem
This document provides the definition of a relational database.
Resolving The Problem
Short definition: A file is not relational if it is a multi-format logical file or an internally described file.
Long definition and discussion: This document defines how the system determines whether a file is considered to be a relational data base file and the guidelines for this determination. Effectively, any file which is not supported on IBM® OS/400® via SQL requests has a value of DBXREL='N' in the QADBXREF system cross-reference file indicating that the file is not relational.
DBXREL Field in QADBXREF
The only documented values are Y and N. The value will be set to Y unless the file falls into one of the following tests:
o The file is a logical file which is not an SQL view, and the file is a multiple format logical file.
o The file is a physical file which is not an SQL table, and the file is a program-described file (for example, not externally described).
Essentially, the term relational in regards to the system cross-reference files on OS/400 means:
"A relational database is a database that is perceived by the user as a collection of normalized relations of assorted degrees."
Various sources list Ted Codd's "12 rules for determining whether a DBM is relational," which was published in Computerworld, October, 1985.
The summarized rules are:
Rule Zero: For any system claimed to be relational, that system must be able to manage data entirely through its relational capabilities.
| 1. | The information rule: All information is represented at the logical level by values in tables. |
| 2. | Guaranteed Access Rule: Each (atomic) piece of data is accessible by the combination of table name, primary key value, and column name. |
| 3. | Null support: The DBMS systematically supports a distinct null placeholder, regardless of data type. |
| 4. | A catalog: The database description is contained in user-accessible tables. |
| 5. | Comprehensive data sub-language: Must have a text-based language that supports base table and view definitions, data manipulation, integrity constraints, and transaction boundaries. |
| 6. | Updateable views: All views that are theoretically updateable must be updateable. |
| 7. | Set-at-a-time Insert, Update, Delete: The DML must support these operations. |
| 8. | Physical Data Independence: Applications must still work even when changes are made to (internal) data storage or access methods. |
| 9. | Logical Data Independence: Applications must still work even when changes of any kind (for example, adding a column to a base table) are made that theoretically should not impair the application. |
| 10. | Integrity Independence: The DBMS must support the definition and enforcement of entity integrity (primary keys) and referential integrity (foreign keys). |
| 11. | Distribution Independence: Users should not be aware of whether or not a database is distributed. |
| 12. | Nonsubversion: Alternate methods of accessing the data should not be able to bypass integrity constraints. |
Historical Number
8053217
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Document Information
Modified date:
18 December 2019
UID
nas8N1010226