I have a master/detail lists in SharePoint. In my workflow, I need to fetch all details based on master item ID, and then to iterate trough those items and based on some field values to send some emails and so on.
What is the best way to achieve this? I checked query list action, but there I can specify a collection variable and then to store concrete list item column value into this collection (which is not enough in my case, I need multiple values).
You are already on the right track. You will still want to use the Query List action, but think of each column as needing to be represented by a "Collection" variable. Therefore if you want to retrieve 3 columns of information, you'll need 3 collections. You can then iterate through one of these collections, and use the "Collection Operation" (one for each collection) within the loop to retrieve info from the other related collections.