This can Lead to Unanticipated Results

Null has been the focus of controversy and a source of debate because of its associated three-valued logic (3VL), special requirements for its use in SQL joins, and the special handling required by aggregate functions and SQL grouping operators. 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. It remains an active field of research though, with numerous papers still being published. 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).

However, certain operations on Null can return values if the absent value is not relevant to the outcome of the operation. In this case, the fact that the value on the left of OR is unknowable is irrelevant, because the outcome of the OR operation would be True regardless of the value on the left. Though this behavior is not defined by the ISO SQL standard many DBMS vendors treat this operation similarly. 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. 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 they so desire; for example, in order to avoid well known anomalies (discussed in the semantics section of this article).

Codd's recommendation would have required SQL's logic system be expanded to accommodate a four-valued logic system. 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 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. In database theory circles, the original proposal of Codd (1975, 1979) is now referred to as "Codd tables". Codd also introduced the use of the lowercase Greek omega (ω) symbol to represent null in database theory. A null should not be confused with a value of 0. A null indicates a lack of a value, which is not the same thing as a zero value. Because Null is not a data value, but a marker for an absent value, using mathematical operators on Null gives an unknown result, Machine learning eBooks for developers which is represented by Null. Codd later reinforced his requirement that all RDBMSs support Null to indicate missing data in a 1985 two-part article published in Computerworld magazine.

Introduced by the creator of the relational database model, E. F. Codd, SQL null serves to fulfil the requirement that all true relational database management systems (RDBMS) support a representation of "missing information and inapplicable information". String concatenation operations, which are common in SQL, also result in Null when one of the operands is Null. In an Oracle RDBMS for example NULL and the empty string are considered the same thing and therefore 'Fish ' || NULL || 'Chips' results in 'Fish Chips'. Since Null is not a member of any data domain, it is not considered a "value", but rather a marker (or placeholder) indicating the undefined value. In a database table, the column reporting this answer would start out with no value (marked by null), and it would not be updated with the value zero until it is ascertained that Adam owns no books. In SQL, null is a marker, not a value. This usage is quite different from most programming languages, where a null value of a reference means it is not pointing to any object.

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