Why use In/Not in Results instead of adding the Data Extension to my Selected Data Extensions?

Sometimes you might want to filter using data from a Data Extension that you do not want to add to your Selected Data Extensions.

Let's see an example to understand why this could be the case. Suppose you have an Orders Data Extension and a Customers Data Extension with the records shown below:

Because the CustomerId column in the Orders Data Extension is not unique when you try to join those two Data Extensions, the result would look like this:

Because there is a one-to-many relationship between those two tables duplicate values on the Email column show up.

Depending on the use case, the Email duplicate values that show up might be unwanted.

To avoid such a scenario it is recommended, when possible, to use In/Not in results filter instead of adding to your Selected Data Extensions.

To sum up: