android SQLite数据库的使用

android提供了一个抽象类SQLiteOpenHelper用来管理数据库，因此我们需要创建一个类来继承它，并且重写他的两个方法onCreate()（创建数据库）和onUpgrade()（更新数据库）。他还有两个重要的实例方法，这两个方法都可一创建或打开一个数据库，并且返回一个可读写的数据库对象，区别是如果数据库不可写入时：

getReadableDatabase() 返回一个只可读的

getWriteableDatabase() 出现异常

自定义MyDatabaseHelper

public class MyDatabaseHelper extends SQLiteOpenHelper{

private Context mContext;

public static final String CREATE_BOOK="create table Book("
+ "id integer primary key autoincrement,"
+ "author text,"
+ "price real,"
+ "pages integer,"
+ "name text)";

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

@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(CREATE_BOOK);
Toast.makeText(mContext, "Create Succeeded", Toast.LENGTH_SHORT).show();
}

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

}

}

对数据库进行增删查改(CRUD(Create,Retrieve,Update,Delete))操作，为了给不熟悉sql语句的人方便操作，android提供了insert(),update(),delete(),query()方法进行操作数据库。用法很简单

public class MainActivity extends BaseActivity implements OnClickListener{

private Button mCreateDbButton;

private MyDatabaseHelper mHelper;

private Button mAddDataButton;

private Button mUpdateButton;

private Button mDeleteButton;

private Button mQueryButton;

private static final String TAG="MainActivity";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

//mHelper=new MyDatabaseHelper(this, "BookStore.db", null,1);
mHelper=new MyDatabaseHelper(this, "BookStore.db",null,2);
}

@Override
public void findId() {
mCreateDbButton=(Button) findViewById(R.id.main_createDbButton);
mAddDataButton=(Button) findViewById(R.id.main_addData_Button);
mUpdateButton=(Button) findViewById(R.id.main_update_Button);
mDeleteButton=(Button) findViewById(R.id.main_Delete_Button);
mQueryButton=(Button) findViewById(R.id.main_Query_Button);
}

@Override
public void setListener() {
mCreateDbButton.setOnClickListener(this);
mAddDataButton.setOnClickListener(this);
mUpdateButton.setOnClickListener(this);
mDeleteButton.setOnClickListener(this);
mQueryButton.setOnClickListener(this);
}

@Override
public void onClick(View v) {
SQLiteDatabase db=mHelper.getWritableDatabase();
ContentValues values=new ContentValues();
switch (v.getId()) {
case R.id.main_createDbButton:
mHelper.getWritableDatabase();
break;
case R.id.main_addData_Button:
//db=mHelper.getWritableDatabase();
//values=new ContentValues();
values.put("name", "c语言从入门到放弃");
values.put("author", "c语言");
values.put("pages", 520);
values.put("price", 50.2);
db.insert("Book", null, values);
values.clear();

values.put("name", "java从入门到放弃");
values.put("author", "java");
values.put("pages", 385);
values.put("price", 47.8);
db.insert("Book", null, values);
values.clear();
break;
case R.id.main_update_Button:
values.put("price", 88.8);
db.update("Book", values, "name=?",new String[]{"c语言从入门到放弃"});
values.clear();
break;
case R.id.main_Delete_Button:
db.delete("Book", "pages>?",new String[]{"500"});
break;
case R.id.main_Query_Button:
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"));
Log.i(TAG,name+author+pages+price);

}while(cursor.moveToNext());
}
break;
default:
break;
}
}
}

也可以使用sql语句进行操作，有时候用SQL语句操作更简单。

增删查改

db.execSQL("insert into Book(name,author,pages,price)values(?,?,?,?)",
new String[]{"c#语言","c#","452","12.9"});

db.execSQL("delete from Book where price<?",new String[]{"50"});

Cursor cursor=db.rawQuery("select * from Book where name=?",new String[]{"c#语言"});

db.execSQL("update Book set price=? where name=?",new String[]{"100","c#语言"});

升级数据库的最佳写法,我在第二个版本里面加了一个表Category，第三个版本里面在Book表中加了一个字段category_id
如果是新用户直接安装就执行oncreate()方法，两个表直接创建,如果是第一个版本的用户升级到第三个版本，就执行onUpgrade()方法，先创建第二个表，再在Book表中添加字段的操作。

注意switch case语句不写break

public class MyDatabaseHelper extends SQLiteOpenHelper{

private Context mContext;

public static final String CREATE_BOOK="create table Book("
+ "id integer primary key autoincrement,"
+ "author text,"
+ "price real,"
+ "pages integer,"
+ "name text,"
+ "category_id integer)";

public static final String CREATE_CATEGORY="create table Category("
+ "id integer primary key autoincrement,"
+ "category_name text,"
+ "category_code integer)";

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

@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(CREATE_BOOK);
db.execSQL(CREATE_CATEGORY);
Toast.makeText(mContext, "Create Succeeded", Toast.LENGTH_SHORT).show();
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
switch (oldVersion) {
case 1:
db.execSQL(CREATE_CATEGORY);
case 2:
db.execSQL("alter table Book add column category_id integer");
default:

}
}

}