The following syntax is used to add a foreign key to an existing table in the database,
ALTER TABLEADD CONSTRAINTS FOREIGN KEY(col_name)REFERENCES pk_table(col_name)
pk_table is the name of the table which will be accessed by the foreign key and that contains the primary key. col_name is the name of the column on which primary key is defined in the primary table.
When a foreign key is added to an existing table, Microsoft SQL
Server 2000 by default checks the existing data in the columns to
ensure that all values except null, exists in the columns of the
refereed primary key.
However SQL Server can be prevented from checking the data in the
column against the new constraints and made to add the new constraint
regardless of data in the column. This option is useful when the rule
requires the constraints to be enforced only from this point forward.