SQLite Database

SQLite

• Public domain database– Advantages• Small (about 150 KB)

– Used on devices with limited resources

• Each database contained within one file– Can be seen in file explorer

/data/data/packagename/database/

• Self-contained– no special administration needed

SQLite

• Primary components– SQLiteOpenHelper

• android.database.sqlite.SQLiteOpenHelper• manages aspects of the database

– creating or opening database

– SQLiteDatabase• android.database.sqlite.SQLiteDatabase

– methods to interface with database via SQL

– Cursor• android.database.Cursor

– provides access to result set returned by a query

Typical Approaches

• Approach 1: Providing a database with the app– store .db file in Assets folder

• .db file created within Android• .db file created in other SQLite manager, such as Firefox plug-in

– in onCreate() of extended Activity, copy the database if it does not already exist in the device’s internal memory

– onCreate() method in extended SQLiteOpenHelper class typically empty

• Approach 2: Database created when app first run– onCreate() method in extended SQLiteOpenHelper class contains

SQL to create database– will only run if the database does not exist

SQLiteOpenHelper class

SQLiteOpenHelper

• Abstract class– Subclass in a new .java file– Useful methods• onCreate (abstract)

– called when DB first created – table creation done here

• onUpgrade (abstract)– called when DB is changed (i.e. during new app release)– essentially used to drop and recreate database

• getReadableDatabase– called to get a read-only version of database

• getWriteableDatabase– called to get an editable version of database

Sample code - SQLiteOpenHelperpublic class MyClass extends SQLiteOpenHelper { public MyClass (Context ctx) {

//Parameters: Context, file name, CursorFactory factory, database version) //CursorFactory rarely used – only used for customized Cursors super (ctx, "myfilename.db", null, 1);

}

public void onUpgrade (SQLiteDatabase db, int oldVer, int newVer) { db.execSQL("DROP TABLE IF EXISTS TABLE_NAME"); onCreate(db);

}

public void onCreate(SQLiteDatabase db) { //_id field must be included!!!

db.execSQL("CREATE TABLE MYTABLE (_id INTEGER PRIMARY KEY AUTOINCREMENT, ..."); }}

SQLiteDatabase class

SQLiteDatabase

• Concrete class– SQL methods

• Convenience methods– query – returns a Cursor– insert – returns _id of new record or –1 if error occurred– update – returns number of rows affected– delete – number of rows affected (0 if no where clause)

» pass ‘1’ as where clause to get count in this case

• Manual creation of SQL– rawQuery

» can execute any SELECT statement and returns a Cursor– execSQL

» can execute any SQL statement that is not a SELECT• void return type

SQLiteDatabase

• query method– Parameters

• String – table name• String [] – columns to select

– _id field must be included if displaying in List

• String – where clause (without the word WHERE)– null if not wanted– ? for arguments

• String [] – selection clause, to allow arguments for field names in where clause– only if ? in preceding string

• String – group by clause (without the words GROUP BY)• String – having clause (without the word HAVING) • String – order by clause (without the words ORDER BY)

Sample code – query

String [] FROM = {List of desired columns to select};

SQLiteDatabase db = myInstanceOfSQLiteOpenHelper.getReadableDatabase();

Cursor myCursor = db.query(MY_TABLE_NAME, FROM, null, null, null, null, null);

SQLiteDatabase

• insert method– Parameters• String – table name• String – work around to allow insertion of empty row in SQL

– nullColumnHack– SQLLite does not allow:

» INSERT INTO MYTABLE– ContentValues will be null

• ContentValues – Object holding the corresponding field names and values

Sample code – insert

SQLiteDatabase db = myInstanceOfSQLiteOpenHelper.getWritableDatabase();

ContentValues values = new ContentValues();

values.put(COLUMN_NAME, column value);values.put(COLUMN_NAME, column value);...values.put(COLUMN_NAME, column value);

db.insert(MY_TABLE_NAME, null, values);

SQLiteDatabase

• update method– Parameters• String – table name• ContentValues – Object holding the corresponding field names

and values • String – where clause (without the word WHERE)• String [] – selection clause, to allow arguments for field names

in where clause

Sample code – update

SQLiteDatabase db = myInstanceOfSQLiteOpenHelper.getWritableDatabase();

String where = "My WHERE clause (without the word WHERE)";ContentValues values = new ContentValues();

values.put(COLUMN_NAME, column value);values.put(COLUMN_NAME, column value);...values.put(COLUMN_NAME, column value);

db.update(MY_TABLE_NAME, values, where, null);

SQLiteDatabase

• delete method– Parameters• String – table name• String – where clause (without the word WHERE)• String [] – selection clause, to allow arguments for field names

in where clause

Sample code – delete

SQLiteDatabase db = myInstanceOfSQLiteOpenHelper.getWritableDatabase();

String where = "My WHERE clause (without the word WHERE)";

db.delete(MY_TABLE_NAME, where, null);

SQLiteDatabase

• rawQuery method– Parameters• String – SQL statement• String [] – selection clause, to allow arguments for field names

in where clause

Sample code – rawQuery

SQLiteDatabase db = myInstanceOfSQLiteOpenHelper.getWritableDatabase();

Cursor myCursor = db.rawQuery(“SQL statement here”, null);

SQLiteDatabase

• execSQL method– Parameters• String – SQL statement

Sample code – execSQL

SQLiteDatabase db = myInstanceOfSQLiteOpenHelper.getWritableDatabase();

db.execSQL(“SQL statement here”);

Cursor class

Aspects of Cursor class

• SQLiteDatabase query method returns a Cursor– Cursor must be managed by Activity• Cursor class allows manipulation of pointer

– move, moveToFirst, moveToNext, moveToPosition, etc.

• Cursor class allows retrieval of data– getInt, getDouble, getString, etc.– must provide column position

• Cursor can be associated with an Adapter– typically SimpleCursorAdapter

Sample code – Cursor data retrieval

• Retrieving the integer in the 4th row, 3rd column

myCursor.moveToPosition(3);int x = myCursor.getInt(2);

Sample code – populating a Cursor from a Database

private Cursor populateCursor() {String [] FROM = {List of desired DB field names};

SQLiteDatabase db = myInstanceOfSQLiteHelper.getReadableDatabase();

Cursor myCursor = db.query(MY_TABLE_NAME, FROM, null, null, null, null, null);

return myCursor;}

Sample code – displaying Cursor contents in a ListView

private void displayRecords(Cursor c) {String [] from = {List of desired field names to display};int [] to = new int [] {List of TextViews to display field names (i.e. R.id.myTextView)};

ListView lv = (ListView)findViewById(R.id.myListView);

SimpleCursorAdapter records = new SimpleCursorAdapter(this, R.layout.my_file_for_TextViews, c, from, to);

lv.setAdapter(records);}