It can depend on what makes them "similar" if similar means the records have a value in a field in common the linking might be something that can be made automatic. If not a user has to link the similar items to each other
This method illustrates a setup where a user selects items to link them as similar to each other.
Do you know how to create new table occurrences so that you can have more than one "box" in manage | database | relationships that refers to the same table?
Here's one way to link any record in your table to any number of other records in the same table:
MainTable----<JoinTable>----MainTable 2 (---< means one to many and MainTable 2 is a second occurrence of MainTable.)
MainTable::__pk_MainTableID = JoinTable::_fk_MainTableParentID
MainTable 2::__pk_MainTableID = JoinTable::_fk_MainTableChildID
This is a many to many self join relationship. Enable "allow creation of records via this relationship" for the JoinTable in the relationship linking MainTable to JoinTable.
You can place a portal to the JoinTable on your MainTable layout with _fk_MainTableChildID formatted as a drop down list of values from MainTable::__pk_MainTableID. This drop down list will have the ID number specified as field 1 and a name or description field as field 2 so the user can use the name/description to select an item, but the value list enters the ID number. Selecting a value from the list in this portal creates a record in the join table and links a Record in MainTable to a different Record in MainTable.
This will be a user guided link. What id we're connecting 2 items in the same item's database that already exist? Then i wouldn't want to create new records based on the relationship. I would just like to link them somehow so that if you are lookling at the record for item A, there would be a portal showing the linked items.
There are several approaches that can be used to keep from linking the same two items a second time. This many to many demo file illustrates three different methods for dealing with that issue: