How to check if a column is already a foreign key?

Question

I have table named Person and column named ID how to check if ID is already FOREIGN KEY cause I want to make it with this code:

ALTER TABLE Person ADD FOREIGN KEY(ID) REFERENCES Job(ID)
ON DELETE CASCADE ON UPDATE CASCADE

but if ID is already a FOREIGN KEY it gives me the following error "may cause cycles or multiple cascade paths" because of the condition with two cascades... How to check if this field is FOREIGN KEY to avoid this error?

Answer 1

You'd want to look in the INFORMATION SCHEMA views

Though it's not as complete as it should be. This is the final query you'd want:

SELECT 
     KCU1.CONSTRAINT_NAME AS 'FK_CONSTRAINT_NAME'
   , KCU1.TABLE_NAME AS 'FK_TABLE_NAME'
   , KCU1.COLUMN_NAME AS 'FK_COLUMN_NAME'
   , KCU1.ORDINAL_POSITION AS 'FK_ORDINAL_POSITION'
   , KCU2.CONSTRAINT_NAME AS 'UQ_CONSTRAINT_NAME'
   , KCU2.TABLE_NAME AS 'UQ_TABLE_NAME'
   , KCU2.COLUMN_NAME AS 'UQ_COLUMN_NAME'
   , KCU2.ORDINAL_POSITION AS 'UQ_ORDINAL_POSITION'
FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS RC
JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE KCU1
ON KCU1.CONSTRAINT_CATALOG = RC.CONSTRAINT_CATALOG 
   AND KCU1.CONSTRAINT_SCHEMA = RC.CONSTRAINT_SCHEMA
   AND KCU1.CONSTRAINT_NAME = RC.CONSTRAINT_NAME
JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE KCU2
ON KCU2.CONSTRAINT_CATALOG = 
RC.UNIQUE_CONSTRAINT_CATALOG 
   AND KCU2.CONSTRAINT_SCHEMA = 
RC.UNIQUE_CONSTRAINT_SCHEMA
   AND KCU2.CONSTRAINT_NAME = 
RC.UNIQUE_CONSTRAINT_NAME
   AND KCU2.ORDINAL_POSITION = KCU1.ORDINAL_POSITION

See here for more information

http://msdn.microsoft.com/en-us/library/aa175805(v=sql.80).aspx

Answer 2

Here is a simple little version

SELECT TOP(1) a.COLUMN_NAME FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS b JOIN 
                INFORMATION_SCHEMA.KEY_COLUMN_USAGE a ON a.CONSTRAINT_CATALOG = b.CONSTRAINT_CATALOG AND a.CONSTRAINT_NAME = b.CONSTRAINT_NAME WHERE a.COLUMN_NAME = *your column*)

You can easily add table name and DB name in the where clause as well