Question: Are UNIONs supported in DESelect Segment?
Answer: Yes, in DESelect Segment you can work with Append Data Extensions (using different tabs as sources). Each tab is a query, and the different tabs are added together with one UNIONs statement. Create a new tab by clicking on the + icon in the tab bar on the Selection Criteria screen.
Example: You can use Append Data Extension if you want to add rows of multiple Data Extensions together. For instance: you have 4 Data Extensions with contact information (first name, last name, email), and want to create one new Data Extension that contains the contacts in these 4 Data Extensions. You will create a tab for each of the Data Extensions, and your Target Data Extension will contain the rows of all 4 Data Extensions matching the Filters in the respective tabs.
More details on how this functionality works can be found below: