Use the Add clause to add a constraint to an existing table. It is optional.
This command adds a constraint using the same syntax as the Create Table command (see "Create Table").
Syntax
ADD [CONSTRAINT constraint_name] ...
Notes
The only type of constraint you can add is a foreign key constraint.
When adding a foreign key constraint, the table that contains the foreign key must be empty.
Example A
Assuming the current schema is SFORCE, a foreign key constraint is added to the deptId column of the test table, referencing the rowId of the dept table. For the operation to succeed, the dept table must be empty.
ALTER TABLE test ADD FOREIGN KEY (deptId) REFERENCES dept(rowId)
The only type of constraint you can add is a foreign key constraint.