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NULL

Database Default "NULL" allowed in Definition Notes
Access 2000 NULL yes
DB2 8.1 NULL no
Firebird 1.5 NULL no
FrontBase 4.1 NULL no
MySQL 4.0 – 4.1 NULL yes
Oracle 9.2 NULL yes Empty strings considered NULL
PostgreSQL 7.4 NULL yes
SQL Server 2000 NOT NULL yes
SQLite 2.8 NULL yes
Sybase ASE 12.5 NOT NULL yes

Output formatting handled by PEAR::DB's DB_PORTABILITY_NULL_TO_EMPTY configuration option.

Default
The default nullability state when a NOT NULL constraint is not used
eg: CREATE TABLE t (i INT)
"NULL" allowed in Definition
Can the column be defined as being nullable by using "NULL"
eg: CREATE TABLE t (i INT NULL)