Show empty channel in imported data as (not set)

lib/plausible/stats/imported/sql/expression.ex

@@ -231,6 +231,18 @@ defmodule Plausible.Stats.Imported.SQL.Expression do
     })
   end
 
+  defp select_group_fields(q, "visit:channel", key, _query) do
+    select_merge_as(q, [i], %{
+      key =>
+        fragment(
+          "if(empty(?), ?, ?)",
+          i.channel,
+          @not_set,
+          i.channel
+        )
+    })
+  end
+
   defp select_group_fields(q, "event:page", key, _query) do
     select_merge_as(q, [i], %{key => i.page, time_on_page: sum(i.time_on_page)})
   end

test/plausible_web/controllers/api/stats_controller/imported_test.exs

@@ -381,6 +381,24 @@ defmodule PlausibleWeb.Api.StatsController.ImportedTest do
                 "totalUsers" => "1",
                 "screenPageViews" => "1"
               }
+            },
+            %{
+              dimensions: %{
+                "sessionManualAdContent" => "",
+                "sessionCampaignName" => "",
+                "date" => "20210101",
+                "sessionGoogleAdsKeyword" => "",
+                "sessionMedium" => "(none)",
+                "sessionSource" => "(direct)",
+                "sessionDefaultChannelGroup" => ""
+              },
+              metrics: %{
+                "bounces" => "1",
+                "userEngagementDuration" => "60",
+                "sessions" => "1",
+                "totalUsers" => "1",
+                "screenPageViews" => "1"
+              }
             }
           ],
           site.id,
@@ -398,6 +416,7 @@ defmodule PlausibleWeb.Api.StatsController.ImportedTest do
           |> Enum.sort()
 
         assert results == [
+                 %{"name" => "(not set)", "visitors" => 1},
                  %{"name" => "Direct", "visitors" => 2},
                  %{"name" => "Organic Search", "visitors" => 3},
                  %{"name" => "Paid Search", "visitors" => 2}