Enhance distinct error

lib/ecto/adapters/myxql/connection.ex

@@ -347,7 +347,7 @@ if Code.ensure_loaded?(MyXQL) do
     defp distinct(%ByExpr{expr: false}, _sources, _query), do: []
 
     defp distinct(%ByExpr{expr: exprs}, _sources, query) when is_list(exprs) do
-      error!(query, "DISTINCT with multiple columns is not supported by MySQL")
+      error!(query, "To apply DISTINCT to multiple columns at once, use distinct: true")
     end
 
     defp select([], _sources, _query),

test/ecto/adapters/myxql_test.exs

@@ -385,7 +385,7 @@ defmodule Ecto.Adapters.MyXQLTest do
     assert all(query) == ~s{SELECT s0.`x`, s0.`y` FROM `schema` AS s0}
 
     assert_raise Ecto.QueryError,
-                 ~r"DISTINCT with multiple columns is not supported by MySQL",
+                 ~r"To apply DISTINCT to multiple columns at once, use distinct: true",
                  fn ->
                    query =
                      Schema |> distinct([r], [r.x, r.y]) |> select([r], {r.x, r.y}) |> plan()
@@ -394,7 +394,7 @@ defmodule Ecto.Adapters.MyXQLTest do
                  end
 
     assert_raise Ecto.QueryError,
-                 ~r"DISTINCT with multiple columns is not supported by MySQL",
+                 ~r"To apply DISTINCT to multiple columns at once, use distinct: true",
                  fn ->
                    query =
                      from(row in Schema, as: :r, select: row.x)