An example: a customer's current balance is calculated by taking adding all their charges and subtracting all their payments. If you have dynamically executed miscalculated values, you have reported incorrect results. A well-designed database 'just works'. Thanks @Gaurav, Is it better to store calculated values or recalculate them on request? Update the question so it focuses on one problem only by editing this post. for a long period of time it could get quite unwieldy. I can either calculate the portion energy based on the corresponding food and store the energy of each portion in the portions table OR I can calculate from the join with the corresponding food every time. Secrets of Access 2013 Database Design. An additional consideration: some values are calculated from a continually increasing number of other values. Good points, I was thinking of a similar hybrid approach using materialized views with a trigger, but recalculating the values within the rows themselves would be another alternative as well. The primary concern with database design in this case is security. Does my concept for light speed travel pass the "handwave test"? Updating the derived values via triggers may mitigate having to add this logic to many places in your code, but be aware that triggers have their own set of potential gotchas (unexpected locking issues, code in the business logic layer not expecting values that it doesn't explicitly reference to update, triggers can be disabled, and so on). Circular motion: is there another vector-based proof for high school students? I don't understand the bottom number in a time signature. Is it bad practice to calculate within the insert query? Examples would include: The extended cost on an invoice line that carries price and quantity fields. Calculate time data 4. Once, while I was trying to fix it I saw that some fields where calculated on the fly and this made the db very slow performing simple select querys. I think the latter is better, but is there a general rule of thumb? Rule 10: Centralize name value table design. Access provides a variety of aggregate functions, including Sum, Count, Avg (for computing averages), Min and Max. What if a live customer updates the data the computations depend on during the recalculation (e.g. How to prevent guerrilla warfare from existing. How would I connect multiple ground wires in this case (replacing ceiling pendant lights)? Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. MOSFET blowing when soft starting a motor. Break the rule, and you have to worry about how to maintain the calculation correctly in every possible circumstance. @AdamThompson I can’t help you there. Is this recomendend? Entity–attribute–value model (EAV) is a data model to encode, in a space-efficient manner, entities where the number of attributes (properties, parameters) that can be used to describe them is potentially vast, but the number that will actually apply to a given entity is relatively modest. site design / logo © 2020 Stack Exchange Inc; user contributions licensed under cc by-sa. Normally, when you create a database, you should only have to enter the information you need and not worry about data or values that Access calculates based on information already in the database. In addition to the sorting problem, there are several other problems with Decimal types, including: Decimal types use more storage space than other numeric field types. It will always be "it depends". The contacts example introducing subkeys is also an excellent illustration of another problem that is found in many database designs: repeated attributes.. Obviously, the contacts database will need to store phone numbers in addition to addresses. Once, while I was trying to fix it I saw that some fields where calculated on the fly and this made the db very slow performing simple select querys. If your data is not in a sufficiently normal form inconsistencies can creep in and cause trouble. An experienced designer can make a trade-off, based on an informed judgment of the specific requirements. All the same concerns (time/resources to compute on the fly, vs. the recalculation cost if there's a problem) still apply, of course. In my opinion if you can precalculate some data that speeds up some frequent and heavy queries - precalculate them. Calculated columns are part of life on a spreadsheet, but do not belong in a database table. It depends. If you watch the data closely they actually only have a key and value. There is one key problem with denormalising your data in this way though: you have to ensure that it is not possible for the derived value to become out-of-date undetected. If you just store the invoice total as calculated at the time of transaction, you no longer need this useless data. The following review questions require the database provided at the beginning of the article. Compare date data 5. Any ideas on what caused my engine failure? One of the frequent exceptions to this 'rule' is with invoice values, due to prices fluctuating over time and the calculated result being impossible, or sometimes just very difficult, to reproduce reliably. Is it bad practice to store calculated data in each row, or is it better to calculate at the application layer with every read from the database. Never store a value that is dependent on other fields: it's a basic rule of normalization. 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