fix: Repo.exists? generates valid SQL with SELECT 1

lib/ecto/adapters/libsql/connection.ex

@@ -976,6 +976,8 @@ defmodule Ecto.Adapters.LibSql.Connection do
     ["SELECT ", select_fields(select, sources, query), ?\s]
   end
 
+  defp select_fields(%{fields: []}, _sources, _query), do: "1"
+
   defp select_fields(%{fields: fields}, sources, query) do
     intersperse_map(fields, ", ", fn
       {:&, _, [idx]} ->

test/exists_query_test.exs

@@ -0,0 +1,135 @@
+defmodule EctoLibSql.ExistsQueryTest do
+  @moduledoc """
+  Tests for Repo.exists?/2 functionality.
+
+  This test reproduces a bug where `Repo.exists?` generated invalid SQL
+  with an empty SELECT clause: `SELECT  FROM "users"` instead of
+  `SELECT 1 FROM "users"`.
+
+  The fix adds a clause to handle empty field lists:
+  `defp select_fields(%{fields: []}, _sources, _query), do: "1"`
+  """
+
+  use ExUnit.Case, async: false
+
+  import Ecto.Query
+
+  defmodule TestRepo do
+    use Ecto.Repo,
+      otp_app: :ecto_libsql,
+      adapter: Ecto.Adapters.LibSql
+  end
+
+  defmodule User do
+    use Ecto.Schema
+
+    schema "users" do
+      field(:name, :string)
+      field(:age, :integer)
+    end
+  end
+
+  @test_db "z_ecto_libsql_test-exists_query.db"
+
+  setup_all do
+    {:ok, _} = TestRepo.start_link(database: @test_db)
+
+    Ecto.Adapters.SQL.query!(TestRepo, """
+    CREATE TABLE IF NOT EXISTS users (
+      id INTEGER PRIMARY KEY AUTOINCREMENT,
+      name TEXT NOT NULL,
+      age INTEGER
+    )
+    """)
+
+    on_exit(fn ->
+      EctoLibSql.TestHelpers.cleanup_db_files(@test_db)
+    end)
+
+    :ok
+  end
+
+  setup do
+    Ecto.Adapters.SQL.query!(TestRepo, "DELETE FROM users")
+    :ok
+  end
+
+  describe "Repo.exists?/2" do
+    test "returns false when no records match the query" do
+      result = TestRepo.exists?(from(u in User, where: u.age > 25))
+      assert result == false
+    end
+
+    test "returns true when records match the query" do
+      Ecto.Adapters.SQL.query!(TestRepo, "INSERT INTO users (name, age) VALUES ('Alice', 30)")
+
+      result = TestRepo.exists?(from(u in User, where: u.age > 25))
+      assert result == true
+    end
+
+    test "works with simple table query without where clause" do
+      assert TestRepo.exists?(User) == false
+
+      Ecto.Adapters.SQL.query!(TestRepo, "INSERT INTO users (name, age) VALUES ('Bob', 20)")
+
+      assert TestRepo.exists?(User) == true
+    end
+
+    test "works with complex where clauses" do
+      Ecto.Adapters.SQL.query!(TestRepo, "INSERT INTO users (name, age) VALUES ('Alice', 30)")
+      Ecto.Adapters.SQL.query!(TestRepo, "INSERT INTO users (name, age) VALUES ('Bob', 20)")
+      Ecto.Adapters.SQL.query!(TestRepo, "INSERT INTO users (name, age) VALUES ('Charlie', 35)")
+
+      # Multiple conditions
+      result =
+        TestRepo.exists?(
+          from(u in User,
+            where: u.age > 25 and u.name != "Alice"
+          )
+        )
+
+      assert result == true
+
+      # No matches
+      result =
+        TestRepo.exists?(
+          from(u in User,
+            where: u.age > 100
+          )
+        )
+
+      assert result == false
+    end
+
+    test "works inside transactions" do
+      TestRepo.transaction(fn ->
+        assert TestRepo.exists?(User) == false
+
+        Ecto.Adapters.SQL.query!(TestRepo, "INSERT INTO users (name, age) VALUES ('Dave', 25)")
+
+        assert TestRepo.exists?(User) == true
+        assert TestRepo.exists?(from(u in User, where: u.age == 25)) == true
+        assert TestRepo.exists?(from(u in User, where: u.age == 30)) == false
+      end)
+    end
+  end
+
+  describe "workaround pattern (for documentation)" do
+    test "count-based alternative approach still works" do
+      Ecto.Adapters.SQL.query!(TestRepo, "INSERT INTO users (name, age) VALUES ('Alice', 30)")
+
+      # The workaround pattern
+      has_records? =
+        User
+        |> where([u], u.age > 25)
+        |> select([u], count(u.id))
+        |> TestRepo.one()
+        |> Kernel.>(0)
+
+      assert has_records? == true
+
+      # Should match exists? behaviour
+      assert TestRepo.exists?(from(u in User, where: u.age > 25)) == has_records?
+    end
+  end
+end