为什么PowerPivot中的计算字段（或度量）属于哪个表很重要？

Question

I've been using PowerPivot for some time now, and there's still one thing I just don't understand. When you create a measure (except they're now called calculated fields, not measures, in Excel 2013) you can choose which table it belongs to:

I understand that this will affect where I see the calculated field appear, both in Excel:

and also in PowerPivot:

However, any reference I can find suggests that otherwise it makes no odds which table I put the measure in. In the example in this question, there is a direct relationship between the product and transaction table:

So why when I put the calculated field in the Product table does Excel come up with this CREATE button:

Does it matter in which table I put calculated fields, and what are the rules? I've searched the Internet and can't find a good answer! Thanks in advance.

Answer 1

It doesn't matter which table you located your calculations. In fact, some people recommend you make a dummy table and store all of your calculations in one table .

There are a few of things to be aware of:

1. Make sure any references to columns in your calculations contain both the table and column name. Eg, Sum('Transaction'[Quantity]) rather than Sum([Quantity])
2. Make sure your relationships are defined in the correct direction. Power Pivot used to fix this for you, but the December 2014 update caused it to stop doing that . The arrow should point to your dimension/lookup table and the dot should be on the fact/transaction table. It looks like you have this right, but it's something to keep in mind.
3. You can test your relationships by using RELATED(). It looks like you have snowflaked out from the product table to another table, and I can't see how SpeciesName or Year are related to TotalQty. You'll find several references to why snowflakes might be a bad idea , but "If it ain't broke, don't fix it". An easy way to figure out if there is a problem with your relationship is to use the RELATED() function to add a value to the table to which it is linked. For example, let's say you had included ProductName in your pivot table and you wanted to check to see if it was the culprit. For testing purposes, you could add a calculated column to the Transaction table that is =RELATED('Product'[ProductName]). Then you could try using that field to see if it makes a difference. Snowflakes mean you have more complicated relationships than a star schema, but they are perfectly usable as long as you understand how those relationships work.