Enforcing unique records on a table
In my database users create parent/child relationships on a join table using a script. The script loops through a found set of records and makes a set of records on the join table to "combine" them with a new record. I would like for the script to also check if a current join record exists on the table already so that, for example, record ID #4 could not be joined to record ID #25 more than once. I would also like users to get a message displayed when a certain record (or records) already existed and they tried to create the join. So this essentially is enforcing unique records (which on this table are simply two field joins of a parent and child record ID) on the join table. How would I go about doing this?