Codd's Twelve Rules - Rule 2 - Guaranteed Access Rule
Rule 2
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Guaranteed Access Rule
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Rule
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Each
and every datum (atomic value) is guaranteed to be logically accessible by
resorting to a combination of table name, primary key value, and column name.
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Description
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We
must be able to access every single piece of information from the tables in
some way. The combination “Table name + Primary key + Column name” must let
us find the information what we need. In the case of multiple records in the
result, the combination of “Table name + Column name” or the “Table name + *
“ would find what we need.
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Example
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SELECT Emp_Salary FROM Employee WHERE ENo = ‘101’;
where, Employee is table name,
Emp_Salary is required field, and ENo is the primary key.
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Some DBMS that fulfills
this property
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SQL in Oracle, Transact SQL in SQL Server, and SQL in MySQL
supports primary keys, and are able to access the data as said above.
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