SQLite: Android Sqlite Database With Multiple Tables

 

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Use Case: Todo Application

To make it easier for you to understand, I am taking a real use case example of TODO Application database schema in this tutorial. This article doesn’t covers how to design the application, but explains the database design, preparing database helper classes and models.

Database Design

I considered a basic Todo Application with minimal functionality like creating a todo note and assigning it under a tag(s) (category). So for this we just need three tables in the database.
The three tables are
todos – to store all todo notes
tags – to store list of tags
todo_tags – to store the tags which are assigned to a todo


Check the below diagram that explains the table structure and the relationship between tables

Let’s start a new Project

So let’s start by creating a new project in Eclipse IDE
1. Create a new project in Eclipse from File ⇒ New ⇒ Android ⇒ Application Project. I named my package name as info.androidhive.sqlite and left the main activity name as MainActivity.java
2. We need two more packages to keep helpers and model classes. Right Clicking on src ⇒ New ⇒ Package and name them as info.androidhive.sqlite.helper and info.androidhive.sqlite.model

Creating Model Class for Tables

Next step is to create model classes for our database tables just to make single row as an object. We need only two models for todos and tags. For todo_tags we don’t need a model class.
3. Create a new class file under info.androidhive.sqlite.helper package named Todo.java and type the code like below. This is the model class for todos table

package info.androidhive.sqlite.model;

public class Todo {

 int id;
 String note;
 int status;
 String created_at;

 // constructors
 public Todo() {
 }

 public Todo(String note, int status) {
  this.note = note;
  this.status = status;
 }

 public Todo(int id, String note, int status) {
  this.id = id;
  this.note = note;
  this.status = status;
 }

 // setters
 public void setId(int id) {
  this.id = id;
 }

 public void setNote(String note) {
  this.note = note;
 }

 public void setStatus(int status) {
  this.status = status;
 }
 
 public void setCreatedAt(String created_at){
  this.created_at = created_at;
 }

 // getters
 public long getId() {
  return this.id;
 }

 public String getNote() {
  return this.note;
 }

 public int getStatus() {
  return this.status;
 }
}

4. Create one more model class for tags table named Tag.java under the same package.

package info.androidhive.sqlite.model;

public class Tag {

 int id;
 String tag_name;

 // constructors
 public Tag() {

 }

 public Tag(String tag_name) {
  this.tag_name = tag_name;
 }

 public Tag(int id, String tag_name) {
  this.id = id;
  this.tag_name = tag_name;
 }

 // setter
 public void setId(int id) {
  this.id = id;
 }

 public void setTagName(String tag_name) {
  this.tag_name = tag_name;
 }

 // getter
 public int getId() {
  return this.id;
 }

 public String getTagName() {
  return this.tag_name;
 }
}

Database Helper Class

Database helper class contains all the methods to perform database operations like opening connection, closing connection, insert, update, read, delete and other things. As this class is helper class, place this under helper package.
5. So create another class named DatabaseHelper.java under info.androidhive.sqlite.helper package and extend the class from SQLiteOpenHelper

public class DatabaseHelper extends SQLiteOpenHelper {

6. Add required variables like database name, database version, column names. I also executed table create statements in onCreate() method. Type the following code in DatabaseHelper.java class

public class DatabaseHelper extends SQLiteOpenHelper {

 // Logcat tag
 private static final String LOG = "DatabaseHelper";

 // Database Version
 private static final int DATABASE_VERSION = 1;

 // Database Name
 private static final String DATABASE_NAME = "contactsManager";

 // Table Names
 private static final String TABLE_TODO = "todos";
 private static final String TABLE_TAG = "tags";
 private static final String TABLE_TODO_TAG = "todo_tags";

 // Common column names
 private static final String KEY_ID = "id";
 private static final String KEY_CREATED_AT = "created_at";

 // NOTES Table - column nmaes
 private static final String KEY_TODO = "todo";
 private static final String KEY_STATUS = "status";

 // TAGS Table - column names
 private static final String KEY_TAG_NAME = "tag_name";

 // NOTE_TAGS Table - column names
 private static final String KEY_TODO_ID = "todo_id";
 private static final String KEY_TAG_ID = "tag_id";

 // Table Create Statements
 // Todo table create statement
 private static final String CREATE_TABLE_TODO = "CREATE TABLE "
   + TABLE_TODO + "(" + KEY_ID + " INTEGER PRIMARY KEY," + KEY_TODO
   + " TEXT," + KEY_STATUS + " INTEGER," + KEY_CREATED_AT
   + " DATETIME" + ")";

 // Tag table create statement
 private static final String CREATE_TABLE_TAG = "CREATE TABLE " + TABLE_TAG
   + "(" + KEY_ID + " INTEGER PRIMARY KEY," + KEY_TAG_NAME + " TEXT,"
   + KEY_CREATED_AT + " DATETIME" + ")";

 // todo_tag table create statement
 private static final String CREATE_TABLE_TODO_TAG = "CREATE TABLE "
   + TABLE_TODO_TAG + "(" + KEY_ID + " INTEGER PRIMARY KEY,"
   + KEY_TODO_ID + " INTEGER," + KEY_TAG_ID + " INTEGER,"
   + KEY_CREATED_AT + " DATETIME" + ")";

 public DatabaseHelper(Context context) {
  super(context, DATABASE_NAME, null, DATABASE_VERSION);
 }

 @Override
 public void onCreate(SQLiteDatabase db) {

  // creating required tables
  db.execSQL(CREATE_TABLE_TODO);
  db.execSQL(CREATE_TABLE_TAG);
  db.execSQL(CREATE_TABLE_TODO_TAG);
 }

 @Override
 public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
  // on upgrade drop older tables
  db.execSQL("DROP TABLE IF EXISTS " + TABLE_TODO);
  db.execSQL("DROP TABLE IF EXISTS " + TABLE_TAG);
  db.execSQL("DROP TABLE IF EXISTS " + TABLE_TODO_TAG);

  // create new tables
  onCreate(db);
 }

CRUD (Create, Read, Update and Delete) Operations

From now on we are going to add one by one method into DatabaseHelper.class

1. Creating a Todo

The function will create a todo item in todos table. In this same function we are assigning the todo to a tag name which inserts a row in todo_tags table.

 /*
  * Creating a todo
  */
 public long createToDo(Todo todo, long[] tag_ids) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TODO, todo.getNote());
  values.put(KEY_STATUS, todo.getStatus());
  values.put(KEY_CREATED_AT, getDateTime());

  // insert row
  long todo_id = db.insert(TABLE_TODO, null, values);

  // assigning tags to todo
  for (long tag_id : tag_ids) {
   createTodoTag(todo_id, tag_id);
  }

  return todo_id;
 }

2. Fetching a Todo

Following will fetch a todo from todos table.

 /*
  * get single todo
  */
 public Todo getTodo(long todo_id) {
  SQLiteDatabase db = this.getReadableDatabase();

  String selectQuery = "SELECT  * FROM " + TABLE_TODO + " WHERE "
    + KEY_ID + " = " + todo_id;

  Log.e(LOG, selectQuery);

  Cursor c = db.rawQuery(selectQuery, null);

  if (c != null)
   c.moveToFirst();

  Todo td = new Todo();
  td.setId(c.getInt(c.getColumnIndex(KEY_ID)));
  td.setNote((c.getString(c.getColumnIndex(KEY_TODO))));
  td.setCreatedAt(c.getString(c.getColumnIndex(KEY_CREATED_AT)));

  return td;
 }

3. Fetching all Todos

Fetching all todos involves reading all todo rows and adding them to a list array.

  /*
  * getting all todos
  * */
 public List<Todo> getAllToDos() {
  List<Todo> todos = new ArrayList<Todo>();
  String selectQuery = "SELECT  * FROM " + TABLE_TODO;

  Log.e(LOG, selectQuery);

  SQLiteDatabase db = this.getReadableDatabase();
  Cursor c = db.rawQuery(selectQuery, null);

  // looping through all rows and adding to list
  if (c.moveToFirst()) {
   do {
    Todo td = new Todo();
    td.setId(c.getInt((c.getColumnIndex(KEY_ID))));
    td.setNote((c.getString(c.getColumnIndex(KEY_TODO))));
    td.setCreatedAt(c.getString(c.getColumnIndex(KEY_CREATED_AT)));

    // adding to todo list
    todos.add(td);
   } while (c.moveToNext());
  }

  return todos;
 }

4. Fetching all Todos under a Tag name

This is also same as reading all the rows but it filters the todos by tag name. Check the following select query which fetches the todos under Watchlist tag name.

  /*
  * getting all todos under single tag
  * */
 public List<Todo> getAllToDosByTag(String tag_name) {
  List<Todo> todos = new ArrayList<Todo>();

  String selectQuery = "SELECT  * FROM " + TABLE_TODO + " td, "
    + TABLE_TAG + " tg, " + TABLE_TODO_TAG + " tt WHERE tg."
    + KEY_TAG_NAME + " = '" + tag_name + "'" + " AND tg." + KEY_ID
    + " = " + "tt." + KEY_TAG_ID + " AND td." + KEY_ID + " = "
    + "tt." + KEY_TODO_ID;

  Log.e(LOG, selectQuery);

  SQLiteDatabase db = this.getReadableDatabase();
  Cursor c = db.rawQuery(selectQuery, null);

  // looping through all rows and adding to list
  if (c.moveToFirst()) {
   do {
    Todo td = new Todo();
    td.setId(c.getInt((c.getColumnIndex(KEY_ID))));
    td.setNote((c.getString(c.getColumnIndex(KEY_TODO))));
    td.setCreatedAt(c.getString(c.getColumnIndex(KEY_CREATED_AT)));

    // adding to todo list
    todos.add(td);
   } while (c.moveToNext());
  }

  return todos;
 }

5. Updating a Todo

Following function will update a todo. It will update Todo values only, not the tag name.

  /*
  * Updating a todo
  */
 public int updateToDo(Todo todo) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TODO, todo.getNote());
  values.put(KEY_STATUS, todo.getStatus());

  // updating row
  return db.update(TABLE_TODO, values, KEY_ID + " = ?",
    new String[] { String.valueOf(todo.getId()) });
 }

6. Deleting a Todo

Pass todo id to the following function to delete the todo from db.

  /*
  * Deleting a todo
  */
 public void deleteToDo(long tado_id) {
  SQLiteDatabase db = this.getWritableDatabase();
  db.delete(TABLE_TODO, KEY_ID + " = ?",
    new String[] { String.valueOf(tado_id) });
 }

Until now we are done creating the CRUD methods onto todos table. Now we can start the methods required on tags table.

7. Creating Tag

Following method will insert a row into tags table.

  /*
  * Creating tag
  */
 public long createTag(Tag tag) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TAG_NAME, tag.getTagName());
  values.put(KEY_CREATED_AT, getDateTime());

  // insert row
  long tag_id = db.insert(TABLE_TAG, null, values);

  return tag_id;
 }

8. Fetching all Tag names

Performing select all statement on tags table will give you list of tag names.

  /**
  * getting all tags
  * */
 public List<Tag> getAllTags() {
  List<Tag> tags = new ArrayList<Tag>();
  String selectQuery = "SELECT  * FROM " + TABLE_TAG;

  Log.e(LOG, selectQuery);

  SQLiteDatabase db = this.getReadableDatabase();
  Cursor c = db.rawQuery(selectQuery, null);

  // looping through all rows and adding to list
  if (c.moveToFirst()) {
   do {
    Tag t = new Tag();
    t.setId(c.getInt((c.getColumnIndex(KEY_ID))));
    t.setTagName(c.getString(c.getColumnIndex(KEY_TAG_NAME)));

    // adding to tags list
    tags.add(t);
   } while (c.moveToNext());
  }
  return tags;
 }

9. Updating Tags

Following method will update tag.

 /*
  * Updating a tag
  */
 public int updateTag(Tag tag) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TAG_NAME, tag.getTagName());

  // updating row
  return db.update(TABLE_TAG, values, KEY_ID + " = ?",
    new String[] { String.valueOf(tag.getId()) });
 }

10. Deleting Tag and Todos under the Tag name

Following method will delete a tag from db. This also will delete all the todos under the tag name, but this is optional.
should_delete_all_tag_todos = Passing true will delete all the todos under the tag name

  /*
  * Deleting a tag
  */
 public void deleteTag(Tag tag, boolean should_delete_all_tag_todos) {
  SQLiteDatabase db = this.getWritableDatabase();

  // before deleting tag
  // check if todos under this tag should also be deleted
  if (should_delete_all_tag_todos) {
   // get all todos under this tag
   List<Todo> allTagToDos = getAllToDosByTag(tag.getTagName());

   // delete all todos
   for (Todo todo : allTagToDos) {
    // delete todo
    deleteToDo(todo.getId());
   }
  }

  // now delete the tag
  db.delete(TABLE_TAG, KEY_ID + " = ?",
    new String[] { String.valueOf(tag.getId()) });
 }

Below are the methods to access the rows from todo_tags table

11. Assigning a Tag to Todo

Following method will assign a todo under a tag name. You can also assign multiple tags to a todo by calling this function multiple times.

/*
  * Creating todo_tag
  */
 public long createTodoTag(long todo_id, long tag_id) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TODO_ID, todo_id);
  values.put(KEY_TAG_ID, tag_id);
  values.put(KEY_CREATED_AT, getDateTime());

  long id = db.insert(TABLE_TODO_TAG, null, values);

  return id;
 }

12. Removing Tag of Todo

Following method will remove the tag assigned to a todo

  /*
  * Updating a todo tag
  */
 public int updateNoteTag(long id, long tag_id) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TAG_ID, tag_id);

  // updating row
  return db.update(TABLE_TODO, values, KEY_ID + " = ?",
    new String[] { String.valueOf(id) });
 }

13. Changing the tag of todo

Following simply replaces the tag name of a todo

  /*
  * Updating a todo tag
  */
 public int updateNoteTag(long id, long tag_id) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TAG_ID, tag_id);

  // updating row
  return db.update(TABLE_TODO, values, KEY_ID + " = ?",
    new String[] { String.valueOf(id) });
 }

14. Closing Database Connection

Importantly don’t forget to close the database connection once you done using it. Call following method when you don’t need access to db anymore.

// closing database
 public void closeDB() {
  SQLiteDatabase db = this.getReadableDatabase();
  if (db != null && db.isOpen())
   db.close();
 }

How to Use / Testing

As this tutorial already seems lengthy I am not considering giving an example with a sample application. In upcoming tutorial I will give you a simple todo application which will give you complete picture of using multiple SQLite tables in your android apps.
For now we will test the class just by printing the data to Logcat.
Open your main activity class and type the following. In the below I just created sample tags and todo data and performed the all the operations by calling the methods which we prepared in DatabaseHelper class.

package info.androidhive.sqlite;

import info.androidhive.sqlite.helper.DatabaseHelper;
import info.androidhive.sqlite.model.Tag;
import info.androidhive.sqlite.model.Todo;

import java.util.List;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;

public class MainActivity extends Activity {

 // Database Helper
 DatabaseHelper db;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);

  db = new DatabaseHelper(getApplicationContext());

  // Creating tags
  Tag tag1 = new Tag("Shopping");
  Tag tag2 = new Tag("Important");
  Tag tag3 = new Tag("Watchlist");
  Tag tag4 = new Tag("Androidhive");

  // Inserting tags in db
  long tag1_id = db.createTag(tag1);
  long tag2_id = db.createTag(tag2);
  long tag3_id = db.createTag(tag3);
  long tag4_id = db.createTag(tag4);

  Log.d("Tag Count", "Tag Count: " + db.getAllTags().size());

  // Creating ToDos
  Todo todo1 = new Todo("iPhone 5S", 0);
  Todo todo2 = new Todo("Galaxy Note II", 0);
  Todo todo3 = new Todo("Whiteboard", 0);

  Todo todo4 = new Todo("Riddick", 0);
  Todo todo5 = new Todo("Prisoners", 0);
  Todo todo6 = new Todo("The Croods", 0);
  Todo todo7 = new Todo("Insidious: Chapter 2", 0);

  Todo todo8 = new Todo("Don't forget to call MOM", 0);
  Todo todo9 = new Todo("Collect money from John", 0);

  Todo todo10 = new Todo("Post new Article", 0);
  Todo todo11 = new Todo("Take database backup", 0);

  // Inserting todos in db
  // Inserting todos under "Shopping" Tag
  long todo1_id = db.createToDo(todo1, new long[] { tag1_id });
  long todo2_id = db.createToDo(todo2, new long[] { tag1_id });
  long todo3_id = db.createToDo(todo3, new long[] { tag1_id });

  // Inserting todos under "Watchlist" Tag
  long todo4_id = db.createToDo(todo4, new long[] { tag3_id });
  long todo5_id = db.createToDo(todo5, new long[] { tag3_id });
  long todo6_id = db.createToDo(todo6, new long[] { tag3_id });
  long todo7_id = db.createToDo(todo7, new long[] { tag3_id });

  // Inserting todos under "Important" Tag
  long todo8_id = db.createToDo(todo8, new long[] { tag2_id });
  long todo9_id = db.createToDo(todo9, new long[] { tag2_id });

  // Inserting todos under "Androidhive" Tag
  long todo10_id = db.createToDo(todo10, new long[] { tag4_id });
  long todo11_id = db.createToDo(todo11, new long[] { tag4_id });

  Log.e("Todo Count", "Todo count: " + db.getToDoCount());

  // "Post new Article" - assigning this under "Important" Tag
  // Now this will have - "Androidhive" and "Important" Tags
  db.createTodoTag(todo10_id, tag2_id);

  // Getting all tag names
  Log.d("Get Tags", "Getting All Tags");

  List<Tag> allTags = db.getAllTags();
  for (Tag tag : allTags) {
   Log.d("Tag Name", tag.getTagName());
  }

  // Getting all Todos
  Log.d("Get Todos", "Getting All ToDos");

  List<Todo> allToDos = db.getAllToDos();
  for (Todo todo : allToDos) {
   Log.d("ToDo", todo.getNote());
  }

  // Getting todos under "Watchlist" tag name
  Log.d("ToDo", "Get todos under single Tag name");

  List<Todo> tagsWatchList = db.getAllToDosByTag(tag3.getTagName());
  for (Todo todo : tagsWatchList) {
   Log.d("ToDo Watchlist", todo.getNote());
  }

  // Deleting a ToDo
  Log.d("Delete ToDo", "Deleting a Todo");
  Log.d("Tag Count", "Tag Count Before Deleting: " + db.getToDoCount());

  db.deleteToDo(todo8_id);

  Log.d("Tag Count", "Tag Count After Deleting: " + db.getToDoCount());

  // Deleting all Todos under "Shopping" tag
  Log.d("Tag Count",
    "Tag Count Before Deleting 'Shopping' Todos: "
      + db.getToDoCount());

  db.deleteTag(tag1, true);

  Log.d("Tag Count",
    "Tag Count After Deleting 'Shopping' Todos: "
      + db.getToDoCount());

  // Updating tag name
  tag3.setTagName("Movies to watch");
  db.updateTag(tag3);

  // Don't forget to close database connection
  db.closeDB();
  
 }
}

Run the application and the check the Logcat.

Complete Code of DatabaseHelper.java Class

package info.androidhive.sqlite.helper;

import info.androidhive.sqlite.model.Tag;
import info.androidhive.sqlite.model.Todo;

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.Locale;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class DatabaseHelper extends SQLiteOpenHelper {

 // Logcat tag
 private static final String LOG = DatabaseHelper.class.getName();

 // Database Version
 private static final int DATABASE_VERSION = 1;

 // Database Name
 private static final String DATABASE_NAME = "contactsManager";

 // Table Names
 private static final String TABLE_TODO = "todos";
 private static final String TABLE_TAG = "tags";
 private static final String TABLE_TODO_TAG = "todo_tags";

 // Common column names
 private static final String KEY_ID = "id";
 private static final String KEY_CREATED_AT = "created_at";

 // NOTES Table - column nmaes
 private static final String KEY_TODO = "todo";
 private static final String KEY_STATUS = "status";

 // TAGS Table - column names
 private static final String KEY_TAG_NAME = "tag_name";

 // NOTE_TAGS Table - column names
 private static final String KEY_TODO_ID = "todo_id";
 private static final String KEY_TAG_ID = "tag_id";

 // Table Create Statements
 // Todo table create statement
 private static final String CREATE_TABLE_TODO = "CREATE TABLE "
   + TABLE_TODO + "(" + KEY_ID + " INTEGER PRIMARY KEY," + KEY_TODO
   + " TEXT," + KEY_STATUS + " INTEGER," + KEY_CREATED_AT
   + " DATETIME" + ")";

 // Tag table create statement
 private static final String CREATE_TABLE_TAG = "CREATE TABLE " + TABLE_TAG
   + "(" + KEY_ID + " INTEGER PRIMARY KEY," + KEY_TAG_NAME + " TEXT,"
   + KEY_CREATED_AT + " DATETIME" + ")";

 // todo_tag table create statement
 private static final String CREATE_TABLE_TODO_TAG = "CREATE TABLE "
   + TABLE_TODO_TAG + "(" + KEY_ID + " INTEGER PRIMARY KEY,"
   + KEY_TODO_ID + " INTEGER," + KEY_TAG_ID + " INTEGER,"
   + KEY_CREATED_AT + " DATETIME" + ")";

 public DatabaseHelper(Context context) {
  super(context, DATABASE_NAME, null, DATABASE_VERSION);
 }

 @Override
 public void onCreate(SQLiteDatabase db) {

  // creating required tables
  db.execSQL(CREATE_TABLE_TODO);
  db.execSQL(CREATE_TABLE_TAG);
  db.execSQL(CREATE_TABLE_TODO_TAG);
 }

 @Override
 public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
  // on upgrade drop older tables
  db.execSQL("DROP TABLE IF EXISTS " + TABLE_TODO);
  db.execSQL("DROP TABLE IF EXISTS " + TABLE_TAG);
  db.execSQL("DROP TABLE IF EXISTS " + TABLE_TODO_TAG);

  // create new tables
  onCreate(db);
 }

 // ------------------------ "todos" table methods ----------------//

  /**
  * Creating a todo
  */
 public long createToDo(Todo todo, long[] tag_ids) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TODO, todo.getNote());
  values.put(KEY_STATUS, todo.getStatus());
  values.put(KEY_CREATED_AT, getDateTime());

  // insert row
  long todo_id = db.insert(TABLE_TODO, null, values);

  // insert tag_ids
  for (long tag_id : tag_ids) {
   createTodoTag(todo_id, tag_id);
  }

  return todo_id;
 }

 /**
  * get single todo
  */
 public Todo getTodo(long todo_id) {
  SQLiteDatabase db = this.getReadableDatabase();

  String selectQuery = "SELECT  * FROM " + TABLE_TODO + " WHERE "
    + KEY_ID + " = " + todo_id;

  Log.e(LOG, selectQuery);

  Cursor c = db.rawQuery(selectQuery, null);

  if (c != null)
   c.moveToFirst();

  Todo td = new Todo();
  td.setId(c.getInt(c.getColumnIndex(KEY_ID)));
  td.setNote((c.getString(c.getColumnIndex(KEY_TODO))));
  td.setCreatedAt(c.getString(c.getColumnIndex(KEY_CREATED_AT)));

  return td;
 }

 /**
  * getting all todos
  * */
 public List<Todo> getAllToDos() {
  List<Todo> todos = new ArrayList<Todo>();
  String selectQuery = "SELECT  * FROM " + TABLE_TODO;

  Log.e(LOG, selectQuery);

  SQLiteDatabase db = this.getReadableDatabase();
  Cursor c = db.rawQuery(selectQuery, null);

  // looping through all rows and adding to list
  if (c.moveToFirst()) {
   do {
    Todo td = new Todo();
    td.setId(c.getInt((c.getColumnIndex(KEY_ID))));
    td.setNote((c.getString(c.getColumnIndex(KEY_TODO))));
    td.setCreatedAt(c.getString(c.getColumnIndex(KEY_CREATED_AT)));

    // adding to todo list
    todos.add(td);
   } while (c.moveToNext());
  }

  return todos;
 }

 /**
  * getting all todos under single tag
  * */
 public List<Todo> getAllToDosByTag(String tag_name) {
  List<Todo> todos = new ArrayList<Todo>();

  String selectQuery = "SELECT  * FROM " + TABLE_TODO + " td, "
    + TABLE_TAG + " tg, " + TABLE_TODO_TAG + " tt WHERE tg."
    + KEY_TAG_NAME + " = '" + tag_name + "'" + " AND tg." + KEY_ID
    + " = " + "tt." + KEY_TAG_ID + " AND td." + KEY_ID + " = "
    + "tt." + KEY_TODO_ID;

  Log.e(LOG, selectQuery);

  SQLiteDatabase db = this.getReadableDatabase();
  Cursor c = db.rawQuery(selectQuery, null);

  // looping through all rows and adding to list
  if (c.moveToFirst()) {
   do {
    Todo td = new Todo();
    td.setId(c.getInt((c.getColumnIndex(KEY_ID))));
    td.setNote((c.getString(c.getColumnIndex(KEY_TODO))));
    td.setCreatedAt(c.getString(c.getColumnIndex(KEY_CREATED_AT)));

    // adding to todo list
    todos.add(td);
   } while (c.moveToNext());
  }

  return todos;
 }

 /**
  * getting todo count
  */
 public int getToDoCount() {
  String countQuery = "SELECT  * FROM " + TABLE_TODO;
  SQLiteDatabase db = this.getReadableDatabase();
  Cursor cursor = db.rawQuery(countQuery, null);

  int count = cursor.getCount();
  cursor.close();

  // return count
  return count;
 }

 /**
  * Updating a todo
  */
 public int updateToDo(Todo todo) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TODO, todo.getNote());
  values.put(KEY_STATUS, todo.getStatus());

  // updating row
  return db.update(TABLE_TODO, values, KEY_ID + " = ?",
    new String[] { String.valueOf(todo.getId()) });
 }

 /**
  * Deleting a todo
  */
 public void deleteToDo(long tado_id) {
  SQLiteDatabase db = this.getWritableDatabase();
  db.delete(TABLE_TODO, KEY_ID + " = ?",
    new String[] { String.valueOf(tado_id) });
 }

 // ------------------------ "tags" table methods ----------------//

 /**
  * Creating tag
  */
 public long createTag(Tag tag) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TAG_NAME, tag.getTagName());
  values.put(KEY_CREATED_AT, getDateTime());

  // insert row
  long tag_id = db.insert(TABLE_TAG, null, values);

  return tag_id;
 }

 /**
  * getting all tags
  * */
 public List<Tag> getAllTags() {
  List<Tag> tags = new ArrayList<Tag>();
  String selectQuery = "SELECT  * FROM " + TABLE_TAG;

  Log.e(LOG, selectQuery);

  SQLiteDatabase db = this.getReadableDatabase();
  Cursor c = db.rawQuery(selectQuery, null);

  // looping through all rows and adding to list
  if (c.moveToFirst()) {
   do {
    Tag t = new Tag();
    t.setId(c.getInt((c.getColumnIndex(KEY_ID))));
    t.setTagName(c.getString(c.getColumnIndex(KEY_TAG_NAME)));

    // adding to tags list
    tags.add(t);
   } while (c.moveToNext());
  }
  return tags;
 }

 /**
  * Updating a tag
  */
 public int updateTag(Tag tag) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TAG_NAME, tag.getTagName());

  // updating row
  return db.update(TABLE_TAG, values, KEY_ID + " = ?",
    new String[] { String.valueOf(tag.getId()) });
 }

 /**
  * Deleting a tag
  */
 public void deleteTag(Tag tag, boolean should_delete_all_tag_todos) {
  SQLiteDatabase db = this.getWritableDatabase();

  // before deleting tag
  // check if todos under this tag should also be deleted
  if (should_delete_all_tag_todos) {
   // get all todos under this tag
   List<Todo> allTagToDos = getAllToDosByTag(tag.getTagName());

   // delete all todos
   for (Todo todo : allTagToDos) {
    // delete todo
    deleteToDo(todo.getId());
   }
  }

  // now delete the tag
  db.delete(TABLE_TAG, KEY_ID + " = ?",
    new String[] { String.valueOf(tag.getId()) });
 }

 // ------------------------ "todo_tags" table methods ----------------//

 /**
  * Creating todo_tag
  */
 public long createTodoTag(long todo_id, long tag_id) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TODO_ID, todo_id);
  values.put(KEY_TAG_ID, tag_id);
  values.put(KEY_CREATED_AT, getDateTime());

  long id = db.insert(TABLE_TODO_TAG, null, values);

  return id;
 }

 /**
  * Updating a todo tag
  */
 public int updateNoteTag(long id, long tag_id) {
  SQLiteDatabase db = this.getWritableDatabase();

  ContentValues values = new ContentValues();
  values.put(KEY_TAG_ID, tag_id);

  // updating row
  return db.update(TABLE_TODO, values, KEY_ID + " = ?",
    new String[] { String.valueOf(id) });
 }

 /**
  * Deleting a todo tag
  */
 public void deleteToDoTag(long id) {
  SQLiteDatabase db = this.getWritableDatabase();
  db.delete(TABLE_TODO, KEY_ID + " = ?",
    new String[] { String.valueOf(id) });
 }

 // closing database
 public void closeDB() {
  SQLiteDatabase db = this.getReadableDatabase();
  if (db != null && db.isOpen())
   db.close();
 }

 /**
  * get datetime
  * */
 private String getDateTime() {
  SimpleDateFormat dateFormat = new SimpleDateFormat(
    "yyyy-MM-dd HH:mm:ss", Locale.getDefault());
  Date date = new Date();
  return dateFormat.format(date);
 }
}