ru.website

 
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 From : Dmitry Olyenyov                      2:5020/400     20 Mar 2001  09:17:14
 To : Serge Shikov
 Subject : Re: Хoчу заделать WebServer
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 Привет, Serge!
 
 >>>>> "SS" == Serge Shikov пишет:
 AN> давай уже коддовское определение кидай, да?
 >> А я уже кинул, да? Буквально недавно.
 >> 5. Comprehensive data sublanguage--At least one supported
 SS> ....
 >> 10. Integrity independence--The database language must be
 SS> ....
 SS> Блин, ну киньте уже кто-нибудь все пункты сразу, и закончим на этом с
 SS> определением, да? Пусть наконец оно у всех будет. А еще лучше - URL.
 
 Первая же ссылка на www.google.com
 
 http://luna.pepperdine.edu/gsbm/class/ckettemborough/Codd12R.html
 
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                 "CODDs 12 RULES" for a FULLY RELATIONAL DBMS
                                       
    
    INTRODUCTION
    
    According to Elmasri and Navathe (1994), Dr. E. F. Codd, the
    originator of the relational data model, published a two-part article
    in ComputerWorld (Codd, 1985) that lists 12 rules for how to determine
    whether a DBMS is relational and to what extent it is relational.
    These rules provide a very useful yardstick for evaluating a
    relational system. Codd also mentions that, according to these rules,
    no fully relational system is available yet. In particular, rules 6,
    9, 10, 11, and 12 are difficult to satisfy.
    
    THE 12 RULES
    
    Rule 1: The Information Rule
    
    All information in a relational database is represented explicitly at
    the logical level in exactly one wayby values in tables.
    
    Rule 2: Guaranteed Access Rule
    
    Each and every datum (atomic value) in a relational database is
    guaranteed to be logically accessible by resorting to a table name,
    primary key value, and column name.
    
    Rule 3: Systematic Treatment of Null Values
    
    Null values (distinct from empty character string or a string of blank
    characters and distinct from zero or any other number) are supported
    in the fully relational DBMS for representing missing information in a
    systematic way, independent of data type.
    
    Rule 4: Dynamic On-line Catalog Based on the Relational Model
    
    The database description is represented at the logical level in the
    same way as ordinary data, so authorized users can apply the same
    relational language to its interrogation as they apply to regular
    data.
    
    Rule 5: Comprehensive Data Sublanguage Rule
    
    A relational system may support several languages and various modes of
    terminal use (for example, the fill-in-blanks mode). However, there
    must be at least one language whose statements are expressible, per
    some well-defined syntax, as character strings and whose ability to
    support all of the following is comprehensible: data definition, view
    definition, data manipulation (interactive and by program), integrity
    constraints, and transaction boundaries (begin, commit, and rollback).
    
    Rule 6: View Updating Rule
    
    All views that are theoretically updateable are also updateable by the
    system.
    
    Rule 7: High-level Insert, Update, and Delete
    
    The capability of handling a base relation or a derived relation as a
    single operand applies nor only to the retrieval of data but also to
    the insertion, update, and deletion of data.
    
    Rule 8: Physical Data Independence
    
    Application programs and terminal activities remain logically
    unimpaired whenever any changes are made in either storage
    representation or access methods.
    
    Rule 9: Logical Data Independence
    
    Application programs and terminal activities remain logically
    unimpaired when information preserving changes of any kind that
    theoretically permit unimpairment are made to the base tables.
    
    Rule 10: Integrity Independence
    
    Integrity constraints specific to a particular relational database
    must be definable in the relational data sublanguage and storable in
    the catalog, not in the application programs.
    
    A minimum of the following two integrity constraints must be
    supported:
    1. Entity integrity: No components of a primary key is allowed to have
        a null value.
        2. Referential integrity: For each distinct nonnull foreign key
        value in a relational database, there must exist a matching
        primary key value from the same domain.
        
    Rule 11: Distribution Independence
    
    A relational DBMS has distribution independence. Distribution
    independence implies that users should not have to be aware of whether
    a database is distributed.
    
    Rule 12: Nonsubversion Rule
    
    If a relational system has a low-level (single-record-at-a-time)
    language, that low-level language cannot be used to subvert or bypass
    the integrity rules or constraints expressed in the higher-level
    (multiple-records-at-a-time) relational language.
    
    Note: There is a rider to these 12 rules known as Rule Zero: "For any
    system that is claimed to be a relational database management system,
    that system must be able to manage data entirely through its
    relational capabilities."
    
    On the basis of the above rules, there is no fully relational DBMS
    available today.
    
    REFERENCES
    
    Codd, E. (1985). "Is Your DBMS Really Relational?" and "Does Your DBMS
    Run By the Rules?" ComputerWorld, October 14 and October 21.
    
    Elmasri, R., & Navathe, S. (1994). Fundamentals of Database Systems.
    2nd ed. Redwood City, CA: The Benjamin/Cummings Publishing Co. pp. 283
    285.
 
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 -- 
   С уважением, Дмитрий.
 
 Hе надо заменять приятное полезным...
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