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Employee
Employee

Are you stuck with a complex data model? Do you need some tips to resolve the linkage of different fact tables? Does your model have data knots, circular references or synthetic keys?

If you have ever faced any of these situations you've probably heard about Link Tables and Concatenate as methods that can help you to put some light in your data model. It’s hard to find a general rule to determine when you should Concatenate rather than joining tables by a Link Table so today we will highlight the main differences of both methods by reviewing usage case scenarios.

Lets start with some basic examples:

Concatenate

“This statement forces concatenation with an existing named table or the latest previously created Logical Table. A concatenation is in principle the same as the SQL UNION statement, but with two differences: first that Concatenate prefix can be used no matter if the tables have identical field names or not; and secondly that no removals of identical records are made”

Source: QlikView help

Most basic example of Concatenate usage could be when you need to merge two or more tables that have identical structures, let say you have a historical data warehouse with sales from 2005 to 2012 and then another table with 2013 sales live in you transactional database. To create a visualization of sales trends over the years you will want to have everything normalized in one fact table, Sales table, with the data from 2005 to 2013.

concatenate.png

Link Table(s)

“(…) a junction table is a database table that contains common fields from two or more other database tables within the same database. It is on the many side of a one-to-many relationship with each of the other tables. Junction tables are known under many names, among them cross-reference table, bridge table, join table, map table, intersection table, linking table, many-to-many resolver, Link Table, pairing table, pivot table, transition table, or association table. (…).”

Source: Wikipedia

This definition is mostly about relational SQL databases so if we adapt it for QlikView it could be something like the following.

Link Table: It´s a table that contains common fields from two or more tables (within the same database or not). Easy, right?

The most common scenario for using Link Tables will be to replace synthetic keys and to avoid circular references by joining two or more fact tables against a common set of dimensions.

linktable.png

See much more in the attached files.

AMZ

Enjoy Qliking!

23 Comments
Master
Master

Hi Sir

Nice article. I happen to stuck with how to add sales table to my existing GL_TABLE , i cannot able to get it work. may be you can advise me where i go wrong ?

http://community.qlik.com/message/348362#348362

12,787 Views
Luminary
Luminary

My rule of thumb is:

  • Use a link table when possible, as it keeps your data model more understandable and creates less 'hassle' on the front-end

  • Move to a concatenated fact table when data volumes increase to a level where performance becomes an issue.
12,787 Views
Not applicable

Hi, friend.

I'm starting developing with QlikView, and I have a little doubt for which you may give me a hand.

Suppose that we have two tables(t1, t2), and that they have a column called in the same way: 'ID', so they are going to be automatically linked by QlikView on Column ID, right?

So then, I create a table selecting the field ID (that has been automatically linked), and the question is: what IDs are going to be shown? I see that it's not an union between the ID of the two tables; just appear the IDs of one table, but I don't understand why.

Thanks in advance,

Lautaro.

12,787 Views
Not applicable

Hi,

Why not just join the two fact tables directly?

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12,787 Views
Not applicable

"so they are going to be automatically linked". Try to go to the table viewer and see what happens

12,787 Views
Specialist III
Specialist III

This as a great article that helped me a lot in the past

12,787 Views