Null (SQL): Difference between revisions

Revision as of 12:48, 17 November 2021

Revision as of 09:05, 8 January 2022

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|content={{nothing}}

{{(!}} class="wikitable" style="display: inline-table !important"

{{!}}+ ''Emp''

{{!-}}

! Name !! Age

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{{!)}}

{{(!}} class="wikitable" style="display: inline-table !important"

{{!}}+ ''EmpH22''

{{!-}}

! Name !! Age

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{{!)}}

{{(!}} class="wikitable" style="display: inline-table !important"

{{!}}+ ''EmpH37''

{{!-}}

! Name !! Age

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}}

A construct (such as a Codd table) is said to be a ''strong representation'' system (of missing information) if any answer to a query made on the construct can be particularized to obtain an answer for ''any'' corresponding query on the relations it represents, which are seen as [[Structure (mathematical logic)|models]] of the construct. More precisely, if {{mvar|q}} is a query formula in the [[relational algebra]] (of "pure" relations) and if {{mvar|{{overline|q}}}} is its lifting to a construct intended to represent missing information, a strong representation has the property that for any query ''q'' and (table) construct ''T'', {{mvar|{{overline|q}}}} lifts ''all'' the answers to the construct, i.e.:

: <math>\mathop{\mathrm{Models}}(\bar{q}(T)) = \{ q(R)\,| R \in \mathop{\mathrm{Models}}(T) \}</math>

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</syntaxhighlight>

should include the possibility that a relation like EmpH22 may exist. However, Codd tables cannot represent the disjunction "result with possibly 0 or 1 rows". A device, mostly of theoretical interest, called [[conditional table]] (or c-table) can however represent such an answer:

{| class="wikitable"

|+ ''Result''

|-

! Name !! Age !! condition