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A One To One Relationship is when one record in one FileMaker table occurrence is related to one and only record in another FileMaker table occurrence. I don’t think I’ve ever used a one to one relationship in the real world. Normally, if you have a one to one relationship, you would simply have all the data from the two entities in the same table. I guess you could use it as a security feature, using relational key to filter whether the related data can be seen or not. Another use might be for temporary data about an entity. You could put all the temporary data in one related table under a one to one relationship. When the temporary data is no longer useful, you could delete all of it in one click of the mouse.
I’ll take that back, I have used a one to one relationship but the reason I did it … wouldn’t make sense with FileMaker 7. It is a cute little trick, in case you are still supporting any FileMaker 6 networked systems. In FileMaker 6 and before, you cannot define fields while the database is being shared ( in FileMaker 7 you can ). You can however create a file with just one field, link it relationally using a one to one relationship[ to your main system, make sure the relation is set to create new fields automatically, upload the new file and place it’s field on the layout of the main shared database. FileMaker would then see the main field, be able to collect data and this would be transparent to the user. Later on, you would merge this related data back in a field in the main system, when the database wasn’t sharing any other users. I learned this technique at one of the FileMaker developers conferences and I believe it was referred to as ” a live splice.”
NOTE: Another one to one example could be a customer that ordered only one product from you and never placed a repeat order. This one customer – one invoice would be a one to many relationship in waiting … waiting for another invoice! So if you were looking at this in an ER diagram, it would probably be viewed as a one to many relationship.
Normally, you don’t strike out to make a relation of the one to one variety. However, it can be done via validation. You can make sure that both the parent and child keys have the field validation set to unique. This means the information in the key field can only appear once or validation fails. This would lock in a one to one relationship.