DROP

PRIMARY KEY
Drops the definition of the primary key and all referential constraints in which the primary key is a parent key. The table must have a primary key.
FOREIGN KEY constraint-name
Drops the referential constraint constraint-name. The constraint-name must identify a referential constraint in which the table is a dependent.
UNIQUE constraint-name
Drops the unique constraint constraint-name and all referential constraints dependent on this unique constraint. The constraint-name must identify a unique constraint on the table. DROP UNIQUE will not drop a PRIMARY KEY unique constraint.
CHECK constraint-name
Drops the check constraint constraint-name. The constraint-name must identify a check constraint on the table.
CONSTRAINT constraint-name
Drops the constraint constraint-name. The constraint-name must identify a unique, referential, or check constraint on the table. If the constraint is a PRIMARY KEY or UNIQUE constraint, all referential constraints in which the primary key or unique key is a parent are also dropped.
CASCADE
Specifies for unique constraints that any referential constraints that are dependent on the constraint being dropped are also dropped.
RESTRICT
Specifies for unique constraints that the constraint cannot be dropped if any referential constraints are dependent on the constraint.