Add --orphan filtering flag

adapter/sqlite/note_dao.go

@@ -410,6 +410,11 @@ func (d *NoteDAO) findRows(opts note.FinderOpts) (*sql.Rows, error) {
 
 			whereExprs = append(whereExprs, expr)
 
+		case note.OrphanFilter:
+			whereExprs = append(whereExprs, `n.id NOT IN (
+				SELECT target_id FROM links WHERE target_id IS NOT NULL
+			)`)
+
 		case note.DateFilter:
 			value := "?"
 			field := "n." + dateField(filter)

adapter/sqlite/note_dao_test.go

@@ -527,6 +527,15 @@ func TestNoteDAOFindNotLinkingTo(t *testing.T) {
 	)
 }
 
+func TestNoteDAOFindOrphan(t *testing.T) {
+	testNoteDAOFindPaths(t,
+		note.FinderOpts{
+			Filters: []note.Filter{note.OrphanFilter{}},
+		},
+		[]string{"ref/test/b.md", "f39c8.md", "log/2021-02-04.md", "index.md"},
+	)
+}
+
 func TestNoteDAOFindCreatedOn(t *testing.T) {
 	testNoteDAOFindPaths(t,
 		note.FinderOpts{

cmd/finder_opts.go

@@ -23,6 +23,7 @@ type Filtering struct {
 	LinkingTo      []string `help:"Only the notes linking to the given notes" placeholder:"<path>" short:"L"`
 	NotLinkedBy    []string `help:"Only the notes not linked by the given notes" placeholder:"<path>"`
 	NotLinkingTo   []string `help:"Only the notes not linking to the given notes" placeholder:"<path>"`
+	Orphan         bool     `help:"Only the notes which don't have any other note linking to them"`
 	Exclude        []string `help:"Excludes notes matching the given file path pattern from the list" short:"x" placeholder:"<glob>"`
 	Interactive    bool     `help:"Further filter the list of notes interactively" short:"i"`
 }
@@ -154,6 +155,10 @@ func NewFinderOpts(zk *zk.Zk, filtering Filtering, sorting Sorting) (*note.Finde
 		})
 	}
 
+	if filtering.Orphan {
+		filters = append(filters, note.OrphanFilter{})
+	}
+
 	if filtering.Interactive {
 		filters = append(filters, note.InteractiveFilter(true))
 	}

core/note/find.go

@@ -52,6 +52,9 @@ type LinkingToFilter struct {
 	Negate bool
 }
 
+// OrphanFilter is a note filter used to select notes having no other notes linking to them.
+type OrphanFilter struct{}
+
 // DateFilter can be used to filter notes created or modified before, after or on a given date.
 type DateFilter struct {
 	Date      time.Time
@@ -67,6 +70,7 @@ func (f PathFilter) sealed()        {}
 func (f ExcludePathFilter) sealed() {}
 func (f LinkedByFilter) sealed()    {}
 func (f LinkingToFilter) sealed()   {}
+func (f OrphanFilter) sealed()      {}
 func (f DateFilter) sealed()        {}
 func (f InteractiveFilter) sealed() {}