1.创建合同类(contract class)---是一个常量的容器,定义表、列、URIs的名字。
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";
...
}
}
在合同类中为每张表创建内部类并列举其列。通过实现BaseColumns接口,该内部类会继承一个主键字段"_ID"。它不是必须的,但可以帮你的数据库与Android框架和谐的工作。
2.构造创建表和删除表的语句
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;
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);
}
}
访问数据库前,先实例化
SQLiteOpenHelper子类:
FeedReaderDbHelper mDbHelper = new FeedReaderDbHelper(getContext());
4.向数据库中插入数据
// 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(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 row
long newRowId;
newRowId = db.insert(
FeedEntry.TABLE_NAME,
FeedEntry.COLUMN_NAME_NULLABLE,
values);
第二个参数提供当ContentValues是空的情况,框架(framework)可以插入NULL的列的名称。当你设置为null的时候,当ContentValues没有任何值时框架不会插入数据。
5.查询数据库中的数据
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 Cursor
String 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
);
查询结果返回一个Cursor(游标),调用Cursor的方法moveToFirst()移动游标到第一个查询出来的行,获得各个列的值。
cursor.moveToFirst();
long itemId = cursor.getLong(
cursor.getColumnIndexOrThrow(FeedEntry._ID)
);
6.删除数据库中的数据。
// 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);
7.更新数据库中的数据。
SQLiteDatabase db = mDbHelper.getReadableDatabase();
// New value for one column
ContentValues values = new ContentValues();
values.put(FeedEntry.COLUMN_NAME_TITLE, title);
// Which row to update, based on the ID
String selection = FeedEntry.COLUMN_NAME_ENTRY_ID + " LIKE ?";
String[] selectionArgs = { String.valueOf(rowId) };
int count = db.update(
FeedReaderDbHelper.FeedEntry.TABLE_NAME,
values,
selection,
selectionArgs);
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