Null (SQL), special marker and keyword in SQL.

null,SQL null

Null or NULL is a special marker used in Structured Query Language 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 fulfil the requirement that all true relational database management systems support a representation of "missing information and inapplicable information". Codd also introduced the use of the lowercase Greek omega symbol to represent Null in database theory. In SQL, NULL 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" or "null" . In a database table, the column reporting this answer would start out with no value , and it would not be updated with the value "zero" until we have ascertained that Adam owns no books.

SQL null is a state, not a value. This usage is quite different from most programming languages, where null value of a reference means it is not pointing to any object.

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