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.
Sursă: Wikipedia engleză