1) Can I use a date field as the Primary Key ID? Use the same date in the other WS's as the Foreign Key ID?
Yes, but the same rules apply as to any other primary keys so this will only work if the dates are unique to each row in the primary key table. If any of the dates are repeated, the relationship will not work correctly.
You can see a more in-depth explanation of primary keys and their use in relationships in the video lesson: Expert Skills Lesson 6-1 Understand primary and foreign keys.
2) The WS's are of a different size in both rows and columns. How does this effect or impede the Pivot Table?
This will not be a problem. Pivot tables are designed to summarize large amounts of data so they can work with any number of rows. Different sets of columns will also be no problem as long as relationships are set up appropriately.
Creating relationships is demonstrated in the video tutorial: Expert Skills Lesson 6-2 Create a simple data model.
3) Can you establish a relationship between more than 2 WS's so that you may use fields from 3,4 or more WS's?
Yes you can, and you can see an in-depth example in the video lesson: Expert Skills Lesson 6-10 Create an OLAP pivot table using a many-to-many relationship.
I hope these are the answers that you are looking for.
Note that if you are trying to consolidate the data from your tables (rather than summarize it), a Pivot Table is not the right tool for the job. If you have Excel 2016, there's now a new Get & Transform tool that enables you to consolidate tables of data and perform many other useful operations. We don't currently have video lessons demonstrating this, but the new Get & Transform features are fully covered in our Excel 2016 Expert Skills book, available from our books page.
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As well as extensive Excel knowledge, Jonathan has worked in the IT world for over thirteen years as a programmer, database designer and analyst for some of the world's largest companies.