在MySQL中必要时存储冗余信息或连接表是否更好？

Question

I have an online shop where users can have little shops with their own products. Each of this products can have questions associated to it and the owner of the shop has the ability to answer those questions. This information is stored in 3 tables a "Questions"(QuestionID,ProductID,...) table, a "Products"(ProductID,ShopID,...) table and a "Shop"(ShopID,OwnerID,...) table.

Is it better to have a ShopID in the 'Questions' table (to allow a shop owner to view all his questions) or to join those three tables to get Questions matching a certain Shop?

Answer 1

It is almost always better to join and avoid redundant information. You should only denormalize when you must do so in order to meet a performance goal - and you can't know if you need to do this until you try with normalized tables first.

Note that denormalization helps in read performance at the expense of slowing down writes and making it easier for a coding mistake to cause data to be out of sync (since you're storing the same thing in more than one place you now have to be sure to update it all).