NULL Column Constraint
Column constraint to allow NULL values
A constraint is used to define rules that help to provide data integrity. Column constraints are specific to the column name specified. You must have ALTER privilege on the table. The table will be locked for EXCLUSIVE use during the operation.
The NULL column constraint is used to specify that the column can contain NULL values. The NOT NULL column constraint can be used to disallow NULL values.
ALTER TABLE customer; ADD COLUMN custref CHAR(5) NULL ALTER TABLE customer; ALTER COLUMN custref NOT NULL