NOT NULL Column Constraint
Column constraint to disallow 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 NOT NULL column constraint is used to specify that the column cannot contain NULL values. The NULL column constraint can be used to allow NULL values.
ALTER TABLE customer; ADD COLUMN custref CHAR(5) NULL ALTER TABLE customer; ALTER COLUMN custref NOT NULL