Null (SQL): Difference between revisions

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[[File:Db null.png|300px|thumb|right|The Greek lowercase [[Omega|omega (ω)]] character is used to represent Null in [[database theory]].]]

'''Null''' or '''NULL''' is a special marker used in [[SQL|Structured Query Language]] to indicate that a data value does not exist in the [[database]]. Introduced by the creator of the [[Relational model|relational]] database model, [[Edgar F. Codd|E. F. Codd]], SQL Null serves to fulfil the requirement that all ''true relational database management systems ([[Relational database#RDBMS|RDMSRDBMS]])'' 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]]. In SQL, <code>NULL</code> is a [[reserved word]] used to identify this marker.

A null should not be confused with a value of 0. A null value indicates a lack of a value, which is not the same thing as a value of zero. For example, consider the question "How many books does Adam own?" The answer may be "zero" (we ''know'' that he owns ''none'') or "null" (we ''do not know'' how many he owns). In a database table, the [[Column (database)|column]] reporting this answer would start out with no value (marked by Null), and it would not be updated with the value "zero" until we have ascertained that Adam owns no books.