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NOT NULL is a parameter that does not allow a column field to be left blank. It can be defined for a field or a domain.

It forces a value to be entered into the column. It operates in the same way for tables as for domains. The parameter DEFAULT NULL and NOT NULL cannot be used in the same column definition. The NOT NULL parameter must be specified if the column is to be defined as PRIMARY KEY or UNIQUE.

See also:
Firebird Null Guide: What is null?
Table Editor
SQL Editor
Data types and subtypes
Selecting the right data type to improve database performance
Firebird 2.5 Language Reference Update
Firebird 2.1 Language Reference Update
Firebird 2.0 Language Reference Update
SQL Language Reference
Expressions involving NULL
Passing NULL to UDFs in Firebird 2
Firebird 2 Quick Start Guide: Expressions involving NULL

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