android数据存储（三）之SQLiteDatabase

 

1.定义：SQLite是一款轻量级的关系型数据库，运行快，占用资源少。

2.创建数据库

  *android为方便管理数据库，专门提供了SQLiteOpenHelper帮助类可以简单地对数据库进行创建和升级。

  *SQLiteOpenHelper是一个抽象类，有两个抽象方法，分别是onCreate()、onUpgrade()负责数据库的创建和升级。

  *SQLiteOpenHelper中还有两个非常重要的实例方法：getReadableDatabase()和getWritableDatabase()。这两个

  方法都可以创建或打开一个现有的数据库（如数据库已存在则直接打开，否则创建一个新的数据库），并返回一

  个可对数据库进行读写操作的对象。不同的是，当数据库不可写入时（如磁盘已满），getReadableDatabase()

  方法返回的对象将以只读的方式打开数据库，而getWritableDatabase()方法则将出现异常。

  *SQLiteOpenHelper中有两个构造方法可供重写，一般使用参数少的构造方法即可。这个构造方法接收四个参数，

  第一个参数是Context,第二个参数是数据库名，第三个参数允许我们在查询数据的时候返回一个自定义的Cursor,

  一般都传入null，第四个参数表示数据库的版本号。数据库文件存放在/data/data/<package name>/databases/目录

  下。

  步骤：首先书写创建语句，然后自定义一个类继承SQLiteOpenHelper然后执行。

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">
    <Button
        android:id="@+id/create_database"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:text="创建数据库"/>
    <Button
        android:id="@+id/add_data"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:text="添加数据"/>

    <Button
        android:id="@+id/update_data"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:text="更新数据"/>

    <Button
        android:id="@+id/delete_data"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:text="删除数据"/>

    <Button
        android:id="@+id/query_data"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:text="查询数据"/>
</LinearLayout>

public class MyDatabaseHelper extends SQLiteOpenHelper {
    //创建数据库
    public static final String CREATE_BOOK="create table Book("
            +"id integer primary key autoincrement,"
            +"author text,"
            +"price real,"
            +"pages integer,"
            +"name text)";
  
    private Context mContext;

    public MyDatabaseHelper(Context context,String name,SQLiteDatabase.CursorFactory 
            factory, int version) {
        super(context, name, factory, version);
        mContext=context;
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        //创建数据库
        sqLiteDatabase.execSQL(CREATE_BOOK);
        Toast.makeText(mContext,"Created successed",Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int oldVersion, int newVersion) 
    {
        
    }
}

public class MainActivity extends AppCompatActivity {
    Button createDatabase;
    Button addData;
    Button updateData;
    Button deleteData;
    Button queryData;
    private MyDatabaseHelper dbHelper;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        createDatabase=findViewById(R.id.create_database);
        addData=findViewById(R.id.add_data);
        updateData=findViewById(R.id.update_data);
        deleteData=findViewById(R.id.delete_data);
        queryData=findViewById(R.id.query_data);

        //1创建数据库
        dbHelper=new MyDatabaseHelper(this,"BookStore.db",null,1);
        createDatabase.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                dbHelper.getWritableDatabase();
            }
        });
    }
}

3.升级数据库

public class MyDatabaseHelper extends SQLiteOpenHelper {
    //创建数据库
    public static final String CREATE_BOOK="create table Book("
            +"id integer primary key autoincrement,"
            +"author text,"
            +"price real,"
            +"pages integer,"
            +"name text)";
    //省级数据库
    public static final String CREATE_CATEGORY="create table Category("
            +"id integer primary key autoincrement,"
            +"category_name text,"
            +"category_code integer)";
    private Context mContext;

    public MyDatabaseHelper(Context context,String name,SQLiteDatabase.CursorFactory
            factory, int version) {
        super(context, name, factory, version);
        mContext=context;
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        //创建数据库
        sqLiteDatabase.execSQL(CREATE_BOOK);
        //升级数据库
        sqLiteDatabase.execSQL(CREATE_CATEGORY);
        Toast.makeText(mContext,"Created successed",Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int oldVersion, int newVersion) {
        sqLiteDatabase.execSQL("drop table if exists Book");
        sqLiteDatabase.execSQL("drop table if exists Category");
        onCreate(sqLiteDatabase);
    }
}

public class MainActivity extends AppCompatActivity {
    Button createDatabase;
    Button addData;
    Button updateData;
    Button deleteData;
    Button queryData;
    private MyDatabaseHelper dbHelper;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        createDatabase=findViewById(R.id.create_database);
        addData=findViewById(R.id.add_data);
        updateData=findViewById(R.id.update_data);
        deleteData=findViewById(R.id.delete_data);
        queryData=findViewById(R.id.query_data);

        //1创建数据库
//        dbHelper=new MyDatabaseHelper(this,"BookStore.db",null,1);
        //升级数据库
        dbHelper=new MyDatabaseHelper(this,"BookStore.db",
                null,2);
    }
}

4.添加数据

public class MainActivity extends AppCompatActivity {
    Button createDatabase;
    Button addData;
    Button updateData;
    Button deleteData;
    Button queryData;
    private MyDatabaseHelper dbHelper;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        createDatabase=findViewById(R.id.create_database);
        addData=findViewById(R.id.add_data);
        updateData=findViewById(R.id.update_data);
        deleteData=findViewById(R.id.delete_data);
        queryData=findViewById(R.id.query_data);

        //1创建数据库
//        dbHelper=new MyDatabaseHelper(this,"BookStore.db",null,1);
        //升级数据库
        dbHelper=new MyDatabaseHelper(this,"BookStore.db",
                null,2);
        createDatabase.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                dbHelper.getWritableDatabase();
            }
        });
        //添加数据
        addData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SQLiteDatabase db=dbHelper.getWritableDatabase();
                ContentValues values=new ContentValues();
                values.put("name","The Da Vinci Code");
                values.put("author","Dan Brown");
                values.put("pages",454);
                values.put("price",15.34);
                //插入一条数据
                db.insert("Book",null,values);
                values.clear();
                //开始第二组数据
                values.put("name","The Lost Symbol");
                values.put("author","Dan Brown");
                values.put("pages",510);
                values.put("price",23.43);
                //插入第二条数据
                db.insert("Book",null,values);
            }
        });
    }
}

5.更新数据

        //更新数据
        updateData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SQLiteDatabase db=dbHelper.getWritableDatabase();
                ContentValues values=new ContentValues();
                values.put("price",12.23);
                db.update("Book",values,"name=?",new String[]{
                        "The Da Vinci Code"
                });
            }
        });

6.删除数据

        //删除数据
        deleteData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SQLiteDatabase db=dbHelper.getWritableDatabase();
                db.delete("Book","pages>?",new String[]{"500"});
            }
        });

7.查询数据

        //查询数据
        queryData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SQLiteDatabase db=dbHelper.getWritableDatabase();
                Cursor cursor=db.query("Book",null,null,
                        null,null,null,null);
                if (cursor.moveToFirst()){
                    do {
                        String name=cursor.getString(cursor.getColumnIndex("name"));
                        String author=cursor.getString(cursor.getColumnIndex("author"));
                        int pages=cursor.getInt(cursor.getColumnIndex("pages"));
                        double price=cursor.getDouble(cursor.getColumnIndex("price"));
                    }while (cursor.moveToNext());
                }
                cursor.close();
            }
        });