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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);}
