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.
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. Computer science professor Ron van der Meyden summarized the various issues as: "The inconsistencies in the SQL standard mean that it is not possible to ascribe any intuitive logical semantics to the treatment of nulls in SQL." Although various proposals have been made for resolving these issues, the complexity of the alternatives has prevented their widespread adoption.
Read more about Null (SQL): History, Comparisons With NULL and The Three-valued Logic (3VL), Analysis of SQL Null Missing-value Semantics, Check Constraints and Foreign Keys, Outer Joins, Aggregate Functions, When Two Nulls Are Equal: Grouping, Sorting, and Some Set Operations, Effect On Index Operation, Null-handling Functions, Data Typing of Null and Unknown, BOOLEAN Data Type
Famous quotes containing the word null:
“A strong person makes the law and custom null before his own will.”
—Ralph Waldo Emerson (18031882)