Add test for #15863 (#19896)

* Add test for #15863

* Remove dead code

test/metabase/query_processor_test/explicit_joins_test.clj

@@ -688,3 +688,41 @@
                     ["2016-06-01T00:00:00Z" 82 nil nil]]
                    (mt/formatted-rows [str int str int]
                      (qp/process-query query))))))))))
+
+(deftest join-against-multiple-saved-questions-with-same-column-test
+  (testing "Should be able to join multiple against saved questions on the same column (#15863)"
+    (mt/dataset sample-dataset
+      (let [q1         (mt/mbql-query products {:breakout [$category], :aggregation [[:count]]})
+            q2         (mt/mbql-query products {:breakout [$category], :aggregation [[:sum $price]]})
+            q3         (mt/mbql-query products {:breakout [$category], :aggregation [[:avg $rating]]})
+            metadata   (fn [query]
+                         {:post [(some? %)]}
+                         (-> query qp/process-query :data :results_metadata :columns))
+            query-card (fn [query]
+                         {:dataset_query query, :result_metadata (metadata query)})]
+        (mt/with-temp* [Card [{card-1-id :id} (query-card q1)]
+                        Card [{card-2-id :id} (query-card q2)]
+                        Card [{card-3-id :id} (query-card q3)]]
+          (let [query (mt/mbql-query products
+                        {:source-table (format "card__%d" card-1-id)
+                         :joins        [{:fields       :all
+                                         :source-table (format "card__%d" card-2-id)
+                                         :condition    [:=
+                                                        $category
+                                                        &Q2.category]
+                                         :alias        "Q2"}
+                                        {:fields       :all
+                                         :source-table (format "card__%d" card-3-id)
+                                         :condition    [:=
+                                                        $category
+                                                        &Q3.category]
+                                         :alias        "Q3"}]})]
+            (mt/with-native-query-testing-context query
+              (let [results (qp/process-query query)]
+                (is (= ["Category" "Count" "Q2 → Category" "Q2 → Sum" "Q3 → Category" "Q3 → Avg"]
+                       (map :display_name (get-in results [:data :results_metadata :columns]))))
+                (is (= [["Doohickey" 42 "Doohickey" 2185.89 "Doohickey" 3.73]
+                        ["Gadget"    53 "Gadget"    3019.2  "Gadget"    3.43]
+                        ["Gizmo"     51 "Gizmo"     2834.88 "Gizmo"     3.64]
+                        ["Widget"    54 "Widget"    3109.31 "Widget"    3.15]]
+                       (mt/formatted-rows [str int str 2.0 str 2.0] results)))))))))))