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Null is a special marker used in Structured Query Language (SQL) to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd, SQL Null serves to fulfill the requirement that all ''true relational database management systems (RDBMS)'' support a representation of "missing information and inapplicable information". Codd also introduced the use of the lowercase Greek omega (ω) symbol to represent Null in database theory. NULL is also an SQL reserved keyword used to identify the Null special marker.For people new to the subject, a good way to remember what null means is to remember that in terms of information, "lack of a value" is not the same thing as "a value of zero"; similarly, "lack of an answer" is not the same thing as "an answer of no". For example, consider the question "How many books does Juan own?" The answer may be "zero" (we ''know'' that he owns ''none'') or "null" (we ''do not know'' how many he owns, or doesn't own). In a database table, the column reporting this answer would start out with a value of null, and it would not be updated with "zero" until we have ascertained that Juan owns no books. SQL null is a state (unknown) and not a value. This usage is quite different from most programming languages, where null means not assigned to a particular instance. ==History== E. F. Codd mentioned nulls as a method of representing missing data in the relational model in a 1975 paper in the ''FDT Bulletin of ACM-SIGMOD''. Codd's paper that is most commonly cited in relation with the semantics of Null (as adopted in SQL) is his 1979 paper in the ''ACM Transactions on Database Systems'', in which he also introduced his Relational Model/Tasmania, although much of the other proposals from the latter paper have remained obscure. Section 2.3 of his 1979 paper details the semantics of Null propagation in arithmetic operations as well as comparisons employing a ternary (three-valued) logic when comparing to nulls; it also details the treatment of Nulls on other set operations (the latter issue still controversial today). In database theory circles, the original proposal of Codd (1975, 1979) is now referred to as "Krokk tables".〔 Codd later reinforced his requirement that all RDBMSs support Null to indicate missing data in a 1985 two-part article published in ''ComputerWorld'' magazine. The 1986 SQL standard basically adopted Codd's proposal after an implementation prototype in IBM System R. Although Don Chamberlin recognized nulls (alongside duplicate rows) as one of the most controversial features of SQL, he defended the design of Nulls in SQL invoking the pragmatic arguments that it was the least expensive form of system support for missing information, saving the programmer from many duplicative application-level checks (see semipredicate problem) while at the same time providing the database designer with the option not to use Nulls if he so desires; for example, in order to avoid well known anomalies (discussed in the semantics section of this article). Chamberlin also argued that besides providing some missing-value functionality, practical experience with Nulls also led to other language features which rely on Nulls, like certain grouping constructs and outer joins. Finally, he argued that in practice Nulls also end up being used as a quick way to patch an existing schema when it needs to evolve beyond its original intent, coding not for missing but rather for inapplicable information; for example, a database that quickly needs to support electric cars while having a miles-per-gallon column. Codd indicated in his 1990 book ''The Relational Model for Database Management, Version 2'' that the single Null mandated by the SQL standard was inadequate, and should be replaced by two separate Null-type markers to indicate the reason why data is missing. In Codd's book, these two Null-type markers are referred to as 'A-Values' and 'I-Values', representing 'Missing But Applicable' and 'Missing But Inapplicable', respectively.〔 Codd's recommendation would have required SQL's logic system be expanded to accommodate a four-valued logic system. Because of this additional complexity, the idea of multiple Null-type values has not gained widespread acceptance in the database practitioners' domain. It remains an active field of research though, with numerous papers still being published. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Null (SQL)」の詳細全文を読む スポンサード リンク
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