Every value in SQL belongs to some data type. A data type is associated with each value retrieved from a table, each constant, and each value computed in an
expression.
This chapter discusses data types. The following sections are presented:
A data type defines a set of values. Reference to a previously
defined data type is a convenient way of specifying
the set of values that can occur in some context. For
example, in SQL the type INTEGER is defined as the set
of integers from -2,147,483,648 through +2,147,483,647,
plus the special value NULL. If you define a column with
type INTEGER, each value stored in the column must be
either an integer in the range -2,147,483,648 through
+2,147,483,647, or a null value (if NOT NULL is not specified).