Every constant, variable, and parameter has a data type (also called a type) that determines its storage format, constraints, valid range of values, and operations that can be performed on it. PL/SQL provides many predefined data types and subtypes, and lets you define your own PL/SQL subtypes.
A subtype is a subset of another data type, which is called its base type. A subtype has the same valid operations as its base type, but only a subset of its valid values. Subtypes can increase reliability, provide compatibility with ANSI/ISO types, and improve readability by indicating the intended use of constants and variables.
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