How many tables? How complex is the business (e.g. are there divisions within the company) and how much control do you want to exert to promote accuracy?
Let's say you purchase a new piece of equipment and are going to set it up and you allow free form entry for equipment type. Later, you create a query wherein the query uses the type field, but you don't get the new piece in the results. Why? Because the free form entry became loadre when it should have been loader. You avoid this by having an equipment type table and force the db user to pick from a list. That's just one example of a possible extra table.
I was an equipment reliability specialist and diesel mechanic for a time and am probably rusty on the theory of db design for that, but can vouch for how complex the design could be. Here it seems the scope of this db is much simpler. On that note, to perform a service, you would need to join each filter table in a query to get a list that includes each type, but you would need the equipment id in each table to be able to associate a filter type with the equipment. Not only that, you need to join every table each time, so why split them? So you usually cannot escape duplicating data, but you do it in a way that supports normalization. Any time you need a new table or field in a table to support a new type of an entity (not talking about an attribute that was overlooked) you have usually made a design error.
Then there is the probability that you will need substitute filters due to pricing or availability, so you want to be able to pick the same filter type for the same machine, but be able to buy Cat one time and Donaldson the next time. Or maybe you want to allow for different suppliers for the same item in some cases (like batteries). Taking all the above into consideration, you should be able to see that data duplication is pretty much unavoidable, but it needs to be done properly and with sound reasoning.
Last edited by Micron; 02-28-2018 at 03:01 PM.
Reason: clarification
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.