Add a Foreign Key Constraint to Existing Table 5/8/2010
Here is the syntax for adding a foreign key constraint (a relationship) on an existing column.
ALTER TABLE TableName
ADD CONSTRAINT ConstraintName
FOREIGN KEY (ForeignKeyField)
REFERENCES ForeignKeyTable (ForeignKeyTableField)
If you want to specify that deleting the parent will cascade down to the child table, add at the very end:
On Delete Cascade
If you want to specify that deleting the parent will set the child table field to null, change the above line to:
On Delete Set NULL