basics/data-storage/databases.jd

page.title=Saving Data in SQL Databases
parent.title=Data Storage
parent.link=index.html

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previous.title=Saving Data in Files
previous.link=files.html

@jd:body


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<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#DefineContract">Define a Schema and Contract</a></li>
  <li><a href="#DbHelper">Create a Database Using a SQL Helper</a></li>
  <li><a href="#WriteDbRow">Put Information into a Database</a></li>
  <li><a href="#ReadDbRow">Read Information from a Database</a></li>
  <li><a href="#DeleteDbRow">Delete Information from a Database</a></li>
  <li><a href="#UpdateDbRow">Update a Database</a></li>
</ol>

<h2>You should also read</h2>
<ul>
  <li><a href="{@docRoot}guide/topics/data/data-storage.html#db">Using Databases</a></li>
</ul>

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<p>Saving data to a database is ideal for repeating or structured data,
such as contact information. This class assumes that you are
familiar with SQL databases in general and helps you get started with
SQLite databases on Android. The APIs you'll need to use a database
on Android are available in the  {@link android.database.sqlite} package.</p>


<h2 id="DefineContract">Define a Schema and Contract</h2>

<p>One of the main principles of SQL databases is the schema: a formal
declaration of how the database is organized. The schema is reflected in the SQL
statements that you use to create your database.  You may find it helpful to
create a companion class, known as a <em>contract</em> class, which explicitly specifies
the layout of your schema in a systematic and self-documenting way.</p>

<p>A contract class is a container for constants that define names for URIs,
tables, and columns. The contract class allows you to use the same constants
across all the other classes in the same package. This lets you change a column
name in one place and have it propagate throughout your code.</p>

<p>A good way to organize a contract class is to put definitions that are
global to your whole database in the root level of the class. Then create an inner
class for each table that enumerates its columns.</p>

<p class="note"><strong>Note:</strong> By implementing the {@link
android.provider.BaseColumns} interface, your inner class can inherit a primary
key field called {@code _ID} that some Android classes such as cursor adaptors
will expect it to have.  It's not required, but this can help your database
work harmoniously with the Android framework.</p>

<p>For example, this snippet defines the table name and column names for a
single table:</p>


<pre>
public static abstract class FeedEntry implements BaseColumns {
    public static final String TABLE_NAME = &quot;entry&quot;;
    public static final String COLUMN_NAME_ENTRY_ID = &quot;entryid&quot;;
    public static final String COLUMN_NAME_TITLE = &quot;title&quot;;
    public static final String COLUMN_NAME_SUBTITLE = &quot;subtitle&quot;;
    ...
}
</pre>


<p>To prevent someone from accidentally instantiating the contract class, give
it an empty constructor.  </p>

<pre>
// Prevents the FeedReaderContract class from being instantiated.
private FeedReaderContract() {}
</pre>


<h2 id="DbHelper">Create a Database Using a SQL Helper</h2>

<p>Once you have defined how your database looks, you should implement methods
that create and maintain the database and tables.  Here are some typical
statements that create and delete a table:</P>

<pre>
private static final String TEXT_TYPE = &quot; TEXT&quot;;
private static final String COMMA_SEP = &quot;,&quot;;
private static final String SQL_CREATE_ENTRIES =
    &quot;CREATE TABLE &quot; + FeedReaderContract.FeedEntry.TABLE_NAME + &quot; (&quot; +
    FeedReaderContract.FeedEntry._ID + &quot; INTEGER PRIMARY KEY,&quot; +
    FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID + TEXT_TYPE + COMMA_SEP +
    FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE + TEXT_TYPE + COMMA_SEP +
    ... // Any other options for the CREATE command
    &quot; )&quot;;

private static final String SQL_DELETE_ENTRIES =
    &quot;DROP TABLE IF EXISTS &quot; + TABLE_NAME_ENTRIES;
</pre>

<p>Just like files that you save on the device's <a
href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">internal
storage</a>, Android stores your database in private disk space that's associated
application. Your data is secure, because by default this area is not
accessible to other applications.</p>

<p>A useful set of APIs is available in the {@link
android.database.sqlite.SQLiteOpenHelper} class.
When you use this class to obtain references to your database, the system
performs the potentially
long-running operations of creating and updating the database only when
needed and <em>not during app startup</em>. All you need to do is call
{@link android.database.sqlite.SQLiteOpenHelper#getWritableDatabase} or
{@link android.database.sqlite.SQLiteOpenHelper#getReadableDatabase}.</p>

<p class="note"><strong>Note:</strong> Because they can be long-running,
be sure that you call {@link
android.database.sqlite.SQLiteOpenHelper#getWritableDatabase} or {@link
android.database.sqlite.SQLiteOpenHelper#getReadableDatabase} in a background thread,
such as with {@link android.os.AsyncTask} or {@link android.app.IntentService}.</p>

<p>To use {@link android.database.sqlite.SQLiteOpenHelper}, create a subclass that
overrides the {@link
android.database.sqlite.SQLiteOpenHelper#onCreate onCreate()}, {@link
android.database.sqlite.SQLiteOpenHelper#onUpgrade onUpgrade()} and {@link
android.database.sqlite.SQLiteOpenHelper#onOpen onOpen()} callback methods. You may also
want to implement {@link android.database.sqlite.SQLiteOpenHelper#onDowngrade onDowngrade()},
but it's not required.</p>

<p>For example, here's an implementation of {@link
android.database.sqlite.SQLiteOpenHelper} that uses some of the commands shown above:</p>

<pre>
public class FeedReaderDbHelper extends SQLiteOpenHelper {
    // If you change the database schema, you must increment the database version.
    public static final int DATABASE_VERSION = 1;
    public static final String DATABASE_NAME = &quot;FeedReader.db&quot;;

    public FeedReaderDbHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(SQL_CREATE_ENTRIES);
    }
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // This database is only a cache for online data, so its upgrade policy is
        // to simply to discard the data and start over
        db.execSQL(SQL_DELETE_ENTRIES);
        onCreate(db);
    }
    public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        onUpgrade(db, oldVersion, newVersion);
    }
}
</pre>

<p>To access your database, instantiate your subclass of {@link
android.database.sqlite.SQLiteOpenHelper}:</p>

<pre>
FeedReaderDbHelper mDbHelper = new FeedReaderDbHelper(getContext());
</pre>


<h2 id="WriteDbRow">Put Information into a Database</h2>

<p>Insert data into the database by passing a {@link android.content.ContentValues}
object to the {@link android.database.sqlite.SQLiteDatabase#insert insert()} method:</p>

<pre>
// Gets the data repository in write mode
SQLiteDatabase db = mDbHelper.getWritableDatabase();

// Create a new map of values, where column names are the keys
ContentValues values = new ContentValues();
values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID, id);
values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE, title);
values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_CONTENT, content);

// Insert the new row, returning the primary key value of the new row
long newRowId;
newRowId = db.insert(
         FeedReaderContract.FeedEntry.TABLE_NAME,
         FeedReaderContract.FeedEntry.COLUMN_NAME_NULLABLE,
         values);
</pre>

<p>The first argument for {@link android.database.sqlite.SQLiteDatabase#insert insert()}
is simply the table name. The second argument provides
the name of a column in which the framework can insert NULL in the event that the
{@link android.content.ContentValues} is empty (if you instead set this to {@code "null"},
then the framework will not insert a row when there are no values).</p>


<h2 id="ReadDbRow">Read Information from a Database</h2>

<p>To read from a database, use the {@link android.database.sqlite.SQLiteDatabase#query query()}
method, passing it your selection criteria and desired columns.
The method combines elements of {@link android.database.sqlite.SQLiteDatabase#insert insert()}
and {@link android.database.sqlite.SQLiteDatabase#update update()}, except the column list
defines the data you want to fetch, rather than the data to insert. The results of the query
are returned to you in a {@link android.database.Cursor} object.</p>

<pre>
SQLiteDatabase db = mDbHelper.getReadableDatabase();

// Define a <em>projection</em> that specifies which columns from the database
// you will actually use after this query.
String[] projection = {
    FeedReaderContract.FeedEntry._ID,
    FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE,
    FeedReaderContract.FeedEntry.COLUMN_NAME_UPDATED,
    ...
    };

// How you want the results sorted in the resulting Cursor
String sortOrder =
    FeedReaderContract.FeedEntry.COLUMN_NAME_UPDATED + " DESC";

Cursor c = db.query(
    FeedReaderContract.FeedEntry.TABLE_NAME,  // The table to query
    projection,                               // The columns to return
    selection,                                // The columns for the WHERE clause
    selectionArgs,                            // The values for the WHERE clause
    null,                                     // don't group the rows
    null,                                     // don't filter by row groups
    sortOrder                                 // The sort order
    );
</pre>

<p>To look at a row in the cursor, use one of the {@link android.database.Cursor} move
methods, which you must always call before you begin reading values. Generally, you should start
by calling {@link android.database.Cursor#moveToFirst}, which places the "read position" on the
first entry in the results. For each row, you can read a column's value by calling one of the
{@link android.database.Cursor} get methods, such as {@link android.database.Cursor#getString
getString()} or {@link android.database.Cursor#getLong getLong()}. For each of the get methods,
you must pass the index position of the column you desire, which you can get by calling
{@link android.database.Cursor#getColumnIndex getColumnIndex()} or
{@link android.database.Cursor#getColumnIndexOrThrow getColumnIndexOrThrow()}.
For example:</p>

<pre>
cursor.moveToFirst();
long itemId = cursor.getLong(
    cursor.getColumnIndexOrThrow(FeedReaderContract.FeedEntry._ID)
);
</pre>


<h2 id="DeleteDbRow">Delete Information from a Database</h2>

<p>To delete rows from a table, you need to provide selection criteria that
identify the rows. The database API provides a mechanism for creating selection
criteria that protects against SQL injection. The mechanism divides the
selection specification into a selection clause and selection arguments. The
clause defines the columns to look at, and also allows you to combine column
tests. The arguments are values to test against that are bound into the clause.
Because the result isn't handled the same as a regular SQL statement, it is
immune to SQL injection.</p>

<pre>
// Define 'where' part of query.
String selection = FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID + &quot; LIKE ?&quot;;
// Specify arguments in placeholder order.
String[] selelectionArgs = { String.valueOf(rowId) };
// Issue SQL statement.
db.delete(table_name, selection, selectionArgs);
</pre>


<h2 id="UpdateDbRow">Update a Database</h2>

<p>When you need to modify a subset of your database values, use the {@link
android.database.sqlite.SQLiteDatabase#update update()} method.</p>

<p>Updating the table combines the content values syntax of {@link
android.database.sqlite.SQLiteDatabase#insert insert()}  with the {@code where} syntax
of {@link android.database.sqlite.SQLiteDatabase#delete delete()}.</p>

<pre>
SQLiteDatabase db = mDbHelper.getReadableDatabase();

// New value for one column
ContentValues values = new ContentValues();
values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE, title);

// Which row to update, based on the ID
String selection = FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID + &quot; LIKE ?&quot;;
String[] selelectionArgs = { String.valueOf(rowId) };

int count = db.update(
    FeedReaderDbHelper.FeedEntry.TABLE_NAME,
    values,
    selection,
    selectionArgs);
</pre>