Add CollectExactlyOneRow function

conn.go

@@ -99,8 +99,12 @@ func (ident Identifier) Sanitize() string {
 	return strings.Join(parts, ".")
 }
 
-// ErrNoRows occurs when rows are expected but none are returned.
-var ErrNoRows = errors.New("no rows in result set")
+var (
+	// ErrNoRows occurs when rows are expected but none are returned.
+	ErrNoRows = errors.New("no rows in result set")
+	// ErrTooManyRows occurs when more rows than expected are returned.
+	ErrTooManyRows = errors.New("too many rows in result set")
+)
 
 var errDisabledStatementCache = fmt.Errorf("cannot use QueryExecModeCacheStatement with disabled statement cache")
 var errDisabledDescriptionCache = fmt.Errorf("cannot use QueryExecModeCacheDescribe with disabled description cache")

rows.go

@@ -48,7 +48,7 @@ type Rows interface {
 	// Callers should check rows.Err() after rows.Next() returns false to detect
 	// whether result-set reading ended prematurely due to an error. See
 	// Conn.Query for details.
-        //
+	//
 	// For simpler error handling, consider using the higher-level pgx v5
 	// CollectRows() and ForEachRow() helpers instead.
 	Next() bool
@@ -465,6 +465,39 @@ func CollectOneRow[T any](rows Rows, fn RowToFunc[T]) (T, error) {
 	return value, rows.Err()
 }
 
+// CollectExactlyOneRow calls fn for the first row in rows and returns the result.
+//   - If no rows are found returns an error where errors.Is(ErrNoRows) is true.
+//   - If more than 1 row is found returns an error where errors.Is(ErrTooManyRows) is true.
+func CollectExactlyOneRow[T any](rows Rows, fn RowToFunc[T]) (T, error) {
+	defer rows.Close()
+
+	var (
+		err   error
+		value T
+	)
+
+	if !rows.Next() {
+		if err = rows.Err(); err != nil {
+			return value, err
+		}
+
+		return value, ErrNoRows
+	}
+
+	value, err = fn(rows)
+	if err != nil {
+		return value, err
+	}
+
+	if rows.Next() {
+		var zero T
+
+		return zero, ErrTooManyRows
+	}
+
+	return value, rows.Err()
+}
+
 // RowTo returns a T scanned from row.
 func RowTo[T any](row CollectableRow) (T, error) {
 	var value T

rows_test.go

@@ -274,6 +274,45 @@ func TestCollectOneRowPrefersPostgreSQLErrorOverErrNoRows(t *testing.T) {
 	})
 }
 
+func TestCollectExactlyOneRow(t *testing.T) {
+	defaultConnTestRunner.RunTest(context.Background(), t, func(ctx context.Context, t testing.TB, conn *pgx.Conn) {
+		rows, _ := conn.Query(ctx, `select 42`)
+		n, err := pgx.CollectExactlyOneRow(rows, func(row pgx.CollectableRow) (int32, error) {
+			var n int32
+			err := row.Scan(&n)
+			return n, err
+		})
+		assert.NoError(t, err)
+		assert.Equal(t, int32(42), n)
+	})
+}
+
+func TestCollectExactlyOneRowNotFound(t *testing.T) {
+	defaultConnTestRunner.RunTest(context.Background(), t, func(ctx context.Context, t testing.TB, conn *pgx.Conn) {
+		rows, _ := conn.Query(ctx, `select 42 where false`)
+		n, err := pgx.CollectExactlyOneRow(rows, func(row pgx.CollectableRow) (int32, error) {
+			var n int32
+			err := row.Scan(&n)
+			return n, err
+		})
+		assert.ErrorIs(t, err, pgx.ErrNoRows)
+		assert.Equal(t, int32(0), n)
+	})
+}
+
+func TestCollectExactlyOneRowExtraRows(t *testing.T) {
+	defaultConnTestRunner.RunTest(context.Background(), t, func(ctx context.Context, t testing.TB, conn *pgx.Conn) {
+		rows, _ := conn.Query(ctx, `select n from generate_series(42, 99) n`)
+		n, err := pgx.CollectExactlyOneRow(rows, func(row pgx.CollectableRow) (int32, error) {
+			var n int32
+			err := row.Scan(&n)
+			return n, err
+		})
+		assert.ErrorIs(t, err, pgx.ErrTooManyRows)
+		assert.Equal(t, int32(0), n)
+	})
+}
+
 func TestRowTo(t *testing.T) {
 	defaultConnTestRunner.RunTest(context.Background(), t, func(ctx context.Context, t testing.TB, conn *pgx.Conn) {
 		rows, _ := conn.Query(ctx, `select n from generate_series(0, 99) n`)