diff --git a/app/src/main/java/it/niedermann/owncloud/notes/persistence/AbstractNotesDatabase.java b/app/src/main/java/it/niedermann/owncloud/notes/persistence/AbstractNotesDatabase.java
index 5892fedc..6adda673 100644
--- a/app/src/main/java/it/niedermann/owncloud/notes/persistence/AbstractNotesDatabase.java
+++ b/app/src/main/java/it/niedermann/owncloud/notes/persistence/AbstractNotesDatabase.java
@@ -81,6 +81,7 @@ abstract class AbstractNotesDatabase extends SQLiteOpenHelper {
         createNotesTable(db);
     }
 
+    // TODO: Use a foreign key to represent category.
     private void createNotesTable(@NonNull SQLiteDatabase db) {
         db.execSQL("CREATE TABLE " + table_notes + " ( " +
                 key_id + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
@@ -111,6 +112,9 @@ abstract class AbstractNotesDatabase extends SQLiteOpenHelper {
                 key_text_color + " VARCHAR(6) NOT NULL DEFAULT '0082C9')");
         createAccountIndexes(db);
     }
+    private void createCategoryTable(@NonNull SQLiteDatabase db) {
+        // TODO: CreateCategoryTable
+    }
 
     @SuppressWarnings("deprecation")
     @Override
@@ -274,6 +278,7 @@ abstract class AbstractNotesDatabase extends SQLiteOpenHelper {
             db.execSQL("ALTER TABLE " + table_accounts + " ADD COLUMN " + key_text_color + " VARCHAR(6) NOT NULL DEFAULT '0082C9'");
             CapabilitiesWorker.update(context);
         }
+        // TODO: Update database
     }
 
     @Override
@@ -285,6 +290,7 @@ abstract class AbstractNotesDatabase extends SQLiteOpenHelper {
         DatabaseIndexUtil.dropIndexes(db);
         db.execSQL("DROP TABLE IF EXISTS " + table_notes);
         db.execSQL("DROP TABLE IF EXISTS " + table_accounts);
+        // TODO: Drop category
         onCreate(db);
     }
 
@@ -296,5 +302,9 @@ abstract class AbstractNotesDatabase extends SQLiteOpenHelper {
         DatabaseIndexUtil.createIndex(db, table_accounts, key_url, key_username, key_account_name, key_etag, key_modified);
     }
 
+    private static void createCategoryIndexes(@NonNull SQLiteDatabase db) {
+        // TODO: create index
+    }
+
     protected abstract void notifyNotesChanged();
 }
diff --git a/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesDatabase.java b/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesDatabase.java
index ef6699bb..2b5c370e 100644
--- a/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesDatabase.java
+++ b/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesDatabase.java
@@ -67,6 +67,7 @@ public class NotesDatabase extends AbstractNotesDatabase {
     }
 
     public static NotesDatabase getInstance(Context context) {
+        // TODO: Use another way to getInstance to avoid concurrency problem
         if (instance == null)
             return instance = new NotesDatabase(context);
         else
@@ -97,6 +98,7 @@ public class NotesDatabase extends AbstractNotesDatabase {
      * @param note Note to be added. Remotely created Notes must be of type CloudNote and locally created Notes must be of Type DBNote (with DBStatus.LOCAL_EDITED)!
      */
     long addNote(long accountId, CloudNote note) {
+        // TODO: The structure of note table changes. Check it and modify if necessary
         SQLiteDatabase db = this.getWritableDatabase();
         ContentValues values = new ContentValues();
         if (note instanceof DBNote) {
@@ -225,7 +227,7 @@ public class NotesDatabase extends AbstractNotesDatabase {
         validateAccountId(accountId);
         Calendar modified = Calendar.getInstance();
         modified.setTimeInMillis(cursor.getLong(4) * 1000);
-
+        // TODO: cursor.get may throw an exception for the table changes.
         return new DBNote(
                 cursor.getLong(0),
                 cursor.getLong(1),
@@ -282,6 +284,7 @@ public class NotesDatabase extends AbstractNotesDatabase {
     @NonNull
     @WorkerThread
     public List<DBNote> searchNotes(long accountId, @Nullable CharSequence query, @Nullable String category, @Nullable Boolean favorite) {
+        // TODO: 这里没有细看....不知道要不要改
         validateAccountId(accountId);
         List<String> where = new ArrayList<>();
         List<String> args = new ArrayList<>();
@@ -451,6 +454,7 @@ public class NotesDatabase extends AbstractNotesDatabase {
      * @return changed note if differs from database, otherwise the old note.
      */
     public DBNote updateNoteAndSync(SingleSignOnAccount ssoAccount, long accountId, @NonNull DBNote oldNote, @Nullable String newContent, @Nullable ISyncCallback callback) {
+        // TODO: This method may crash for the note table has changed.
         //debugPrintFullDB();
         DBNote newNote;
         if (newContent == null) {
@@ -493,6 +497,7 @@ public class NotesDatabase extends AbstractNotesDatabase {
      * @param forceUnchangedDBNoteState is not null, then the local note is updated only if it was not modified meanwhile
      */
     void updateNote(long id, @NonNull CloudNote remoteNote, @Nullable DBNote forceUnchangedDBNoteState) {
+        // TODO: This method may crash for the note table has changed.
         SQLiteDatabase db = this.getWritableDatabase();
 
         // First, update the remote ID, since this field cannot be changed in parallel, but have to be updated always.
@@ -769,4 +774,8 @@ public class NotesDatabase extends AbstractNotesDatabase {
             throw new IllegalArgumentException("accountId must be greater than 0");
         }
     }
+
+    // TODO: Add a method to update the sorting method in category
+    // TODO: Not sure: Add more methods for category table.
+
 }