Codd's Twelve Rules - Rule 3 - Systematic treatment of NULL values
Rule 3
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Systematic treatment of NULL values
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Rule
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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 RDBMS for representing missing information in a systematic way,
independent of data type.
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Description
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This
rule demands that we must be able to use a NULL placeholder irrespective of
data type used. NULLs are distinct from an empty character string or any
other number, and they are always to be considered as unknown values. This rule
insists provisions for manipulating NULL values in all the possible ways.
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Example
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SELECT Emp_Salary FROM Employee WHERE EPhone IS NULL;
This query should return Emp_Salary
if EPhone value is NULL/Not known.
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Some DBMS that fulfills
this property
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Oracle, SQL Server, and MySQL supports NULL values to be
handled. They provide mechanisms to handle NULL values like other values.
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