数据存储（3）---数据库存储

定义一个模式和合同

合同是一个都是常数的类，它定义的是URI的名称，表名，列名。

比如这样：

public final class FeedReaderContract {    // To prevent someone from accidentally instantiating the contract class,    // give it an empty constructor.    public FeedReaderContract() {}    /* Inner class that defines the table contents */    public static abstract class FeedEntry implements BaseColumns {        public static final String TABLE_NAME = "entry";        public static final String COLUMN_NAME_ENTRY_ID = "entryid";        public static final String COLUMN_NAME_TITLE = "title";        public static final String COLUMN_NAME_SUBTITLE = "subtitle";        ...    }}

创建一个数据库使用SQLHelper

private static final String TEXT_TYPE = " TEXT";private static final String COMMA_SEP = ",";private static final String SQL_CREATE_ENTRIES =    "CREATE TABLE " + FeedEntry.TABLE_NAME + " (" +    FeedEntry._ID + " INTEGER PRIMARY KEY," +    FeedEntry.COLUMN_NAME_ENTRY_ID + TEXT_TYPE + COMMA_SEP +    FeedEntry.COLUMN_NAME_TITLE + TEXT_TYPE + COMMA_SEP +    ... // Any other options for the CREATE command    " )";private static final String SQL_DELETE_ENTRIES =    "DROP TABLE IF EXISTS " + FeedEntry.TABLE_NAME;

SQLiteOpenHelper子类：

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 = "FeedReader.db";    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);    }}

初始化数据库帮助类：

FeedReaderDbHelper mDbHelper = new FeedReaderDbHelper(getContext());

插入数据库：

// Gets the data repository in write modeSQLiteDatabase db = mDbHelper.getWritableDatabase();// Create a new map of values, where column names are the keysContentValues values = new ContentValues();values.put(FeedEntry.COLUMN_NAME_ENTRY_ID, id);values.put(FeedEntry.COLUMN_NAME_TITLE, title);values.put(FeedEntry.COLUMN_NAME_CONTENT, content);// Insert the new row, returning the primary key value of the new rowlong newRowId;newRowId = db.insert(         FeedEntry.TABLE_NAME,         FeedEntry.COLUMN_NAME_NULLABLE,         values);

读取数据库：

SQLiteDatabase db = mDbHelper.getReadableDatabase();// Define a projection that specifies which columns from the database// you will actually use after this query.String[] projection = {    FeedEntry._ID,    FeedEntry.COLUMN_NAME_TITLE,    FeedEntry.COLUMN_NAME_UPDATED,    ...    };// How you want the results sorted in the resulting CursorString sortOrder =    FeedEntry.COLUMN_NAME_UPDATED + " DESC";Cursor c = db.query(    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    );

把Cursor移到第一个位置：

cursor.moveToFirst();long itemId = cursor.getLong(    cursor.getColumnIndexOrThrow(FeedEntry._ID));

删除：

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

更改：

SQLiteDatabase db = mDbHelper.getReadableDatabase();// New value for one columnContentValues values = new ContentValues();values.put(FeedEntry.COLUMN_NAME_TITLE, title);// Which row to update, based on the IDString selection = FeedEntry.COLUMN_NAME_ENTRY_ID + " LIKE ?";String[] selectionArgs = { String.valueOf(rowId) };int count = db.update(    FeedReaderDbHelper.FeedEntry.TABLE_NAME,    values,    selection,    selectionArgs);