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 "Constraint Definition for Local Tables").
Syntax
ADD [CONSTRAINT constraint_name] ...
Notes
You cannot add a Unique constraint if one is already assigned to the same column list. A Unique constraint works only if the values of the columns in the constraint columns list for the existing rows are unique or include a Null value.
Adding a foreign key constraint to the table fails if, for each existing row in the referring table, a matching row (with equal values for the column list) is not found in the referenced table.
Example A
Assuming the current schema is PUBLIC, this example adds a foreign key constraint to the deptId column of the test table that references the rowId of the dept table.
ALTER TABLE test ADD CONSTRAINT test_fk FOREIGN KEY (deptId) REFERENCES dept(id)
You cannot add a Unique constraint if one is already assigned to the same column list. A Unique constraint works only if the values of the columns in the constraint columns list for the existing rows are unique or include a Null value.