Question: Are UNIONs supported in DESelect?
Answer: Yes, in DESelect you can work with tabs. 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 this function 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.