2003”N 7ŒŽ 1“ú NULL VALUE BACK >> –ÚŽŸ



œ "Null" value

@Let's take an example as below:

@employee: oemployee-NO, employee-name, company-name,...p [ R ]

@The "company-name" is the name of company for which an "mid-career recruiting" employee worked, "null (or nil)" value must be, therefore, observed in the "company-name" of the records for "hiring of new graduates" employees.

@You can create "subsets", "heterotypic" (or "atypical") subsets (of different formats), with the null values as below:

@@employee [ R ]
@@@b
@@@b
@@@~ by null (company-name)
@@@b
@@@bQQQQ hiring of new graduates {employee-NO, employee-name,...}
@@@b
@@@b
@@@bQQQQ mid-career recruting {employee-NO, employee-name, company-name,...}

@
@Subsets can be created through the following two rules:
@@(1) An entity has a "partition-code"
@@(2) An entity has "null value" in its attribute

@The subsets created under the declaration of "partition-code", are said to be "substantial subsets", and the subsets created owing to "null value", are to be "procedural subsets."

@Just ask if the "company-name" must belong to (or an attribute of) "employee", the answer is that it's not.
@It's, therefore, strongly recommended that you should create VE for "company-name" as below:

@employee:
@ oemployee-NO, employee-name,...p [ R ]

@employee. company:
@ oemployee-NO (R), company-name,...p [ VE ]

@



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