Answer
Data Source Designer only lets you reference a single view in its View section — if what you actually need requires a join between views (which is often the real reason a seemingly-correct aggregate/GROUP BY doesn't behave as expected — the join condition isn't actually being applied the way you'd expect from plain SQL), that join can't be expressed directly through the designer's UI. Two practical workarounds: create your own standalone database view with the necessary join baked into its definition, then reference that single view from Data Source Designer instead of trying to express the join in the designer itself; or restructure your query to avoid needing a join at all, working entirely from one view's own columns with adjusted WHERE and GROUP BY conditions instead. It helps to keep in mind that Data Source Designer effectively assembles your entries into a fixed SELECT-FROM-WHERE-GROUP BY-ORDER BY structure behind the scenes (columns section, one view, conditions section, group-by section, order-by section) — understanding that fixed shape makes it clearer why certain multi-view SQL patterns don't translate directly into what the designer can express.