Basic use and upgrade of Android native database

android Native databases use
SQLiteOpenHelper： Database creation 、 Updated operation object
SQLiteDatabase： The operation object of data addition, deletion, modification and query
SQLiteStatement：SQL The operation object executed

sqlite Internal implementation
Insert ：SQLiteStatement.executeInsert
to update 、 Delete ：SQLiteStatement.executeUpdateDelete
Inquire about ：SQLiteCursorDriver.query

performance optimization
1、 precompile SQL sentence , Repeated operation use SQLiteStatement
2、 With a transaction , Improve performance when doing data update operations
3、 Shut down in time Cursor
4、 Time consuming asynchronization

Basic use

// Create database : Inherit the of the system SQLiteOpenHelper, stay onCreate and onUpgrade Realize the creation and update of database 
public class MyDbHelper extends SQLiteOpenHelper {
    

    private static final String TAG = "MyDbHelper";
    private static final String DB_NAME = "my.db";
    private static final int DB_VERSION = 1;

    public MyDbHelper(@Nullable Context context) {
    
        super(context, DB_NAME, null, DB_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
    
        //  Create database 
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    
        //  Update the database 
    }
}

//2. Database management : Use singleton to manage the operation of database , Used to get a readable and writable SQLiteDatabase
public class MyDbManage {
    

    private static MyDbManage myDbManage;
    private final MyDbHelper myDbHelper;

    private MyDbManage(Context context) {
    
        myDbHelper = new MyDbHelper(context);
    }

    public static MyDbManage getInstance(Context context) {
    
        if (myDbManage == null) {
    
            synchronized (MyDbManage.class) {
    
                if (myDbManage == null) {
    
                    myDbManage = new MyDbManage(context);
                }
            }
        }
        return myDbManage;
    }

    /** *  Get a writable database  * * @return SQLiteDatabase */
    public SQLiteDatabase getWritableDatabase() {
    
        return myDbHelper.getWritableDatabase();
    }
}

//3. Database operation : For each entity , It is recommended to create a separate DAO, Easier maintenance 
public class MyDaoSample {
    
    public static final String TABLE_NAME = "user";//  Table name 
    public static final String USER_NAME = "username";
    public static final String AGE = "age";
    public static final String SEX = "sex";

    private static MyDaoSample myDaoSample;
    private final SQLiteDatabase mDb;

    private MyDaoSample() {
    
        MyDbManage dbManage = MyDbManage.getInstance(CoreApplication.mApp);
        mDb = dbManage.getWritableDatabase();
    }

    public static MyDaoSample getInstance() {
    
        if (myDaoSample == null) {
    
            synchronized (MyDaoSample.class) {
    
                if (myDaoSample == null) {
    
                    myDaoSample = new MyDaoSample();
                }
            }
        }
        return myDaoSample;
    }

    public void insert() {
    
        ContentValues values = new ContentValues();
        values.put(USER_NAME, "susu");
        values.put(AGE, 18);
        mDb.beginTransaction();
        mDb.insert(TABLE_NAME, null, values);
        mDb.setTransactionSuccessful();
        mDb.endTransaction();
    }
}

Android Upgrade and update the native database OnUpgrade

Be careful ：
1. Modify the database version number
2. rewrite OnUpgrade Method

//DBOpenHelper The database class 
public class DBOpenHelper extends SQLiteOpenHelper

// Database version number 
private static final int DB_VERSION = 2;

// initialization 
private DBOpenHelper(Context context) {
    
        super(context, DB_NAME, null, DB_VERSION);
    }

// Upgrade database 
@Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    
        if (oldVersion == 1) {
    
            db.beginTransaction();
            try {
    
                // upgrade note table column
                db.execSQL("ALTER TABLE " + NOTE_TABLE_NAME + " ADD COLUMN " + NOTE_IS_TOP_ID + " int;");
                db.execSQL("ALTER TABLE " + NOTE_TABLE_NAME + " ADD COLUMN " + NOTE_TOP_DATE_ID + " long;");
                db.execSQL("ALTER TABLE " + NOTE_TABLE_NAME + " ADD COLUMN " + NOTE_IS_CONTAIN_VOICE_ID + " int;");
                db.execSQL("ALTER TABLE " + NOTE_TABLE_NAME + " ADD COLUMN " + NOTE_PIC_PATH_ID + " text;");

                // upgrade folder table column
                db.execSQL("ALTER TABLE " + FOLDER_TABLE_NAME + " ADD COLUMN " + FOLDER_IS_TODO + " int;");
               
                //insert default values 
                String travel = mContext.getString(R.string.folder_travel);
        String life = mContext.getString(R.string.folder_life);
        String work = mContext.getString(R.string.folder_work);
        db.execSQL("INSERT INTO "
                + FOLDER_TABLE_NAME
                + " VALUES ('1','"+travel+"', '0', '0','0');");
        db.execSQL("INSERT INTO "
                + FOLDER_TABLE_NAME
                + " VALUES ('2','"+life+"', '0', '0','0');");
        db.execSQL("INSERT INTO "
                + FOLDER_TABLE_NAME
                + " VALUES ('3','"+work+"', '0', '0','0');");

                // create todo table
                db.execSQL(" CREATE TABLE IF NOT EXISTS " + TODO_TABLE_NAME + " ( "
                        + ID + " integer primary key autoincrement , "
                        + TODO_CONTENT + " text , "
                        + TODO_IS_IMPORTANT + " int , "
                        + TODO_TIME + " long , "
                        + TODO_REPEAT + " int , "
                        + TODO_IS_COMPLETE + " int , "
                        + TODO_IS_DELAY + " int , "
                        + TODO_IS_DELETED + " int , "
                        + TODO_PARENT_FOLDER_ID + " int , "
                        + TODO_DATE + " long);");

                db.setVersion(DB_VERSION);
                db.setTransactionSuccessful();
            } finally {
    
                db.endTransaction();
            }
        }
    }

// If the database file does not exist , Only onCreate() Called （ This method is called once when the database is created ）
public abstract void onCreate(SQLiteDatabase db);
// If the database file exists , Would call onUpgrade() Method to upgrade the database , And update the version number .
public abstract void onUpgrade(SQLiteDatabase db,int oldVersion,int newVersion);

OnCreate : If the database file does not exist ,SQLiteOpenHelper Creating a database file , Open the database , call onCreate() Method , In this method, you usually need to create a table 、 View and other components . The database is usually empty before creation , Therefore, there is no need to delete the relevant components in the database first .

OnUpgrade : When the system is constructing SQLiteOpenHelper Class , If the version number is different , Will automatically call onUpgrade function , Let you upgrade the database here .

Both the new version number and the old version number will be used as onUpgrade The arguments to the function come in , It is convenient for developers to know which version of the database should be upgraded from . When the upgrade is complete , The database will automatically store the latest version number as the current database version number .

Need to be right SQLite Upgrade the structure of the database :
SQLite Provides ALTER TABLE command , Allow users to rename or add new fields to existing tables , But you can't delete fields from the table .
And you can only add fields at the end of the table , such as , by Student Add two fields ：
1 ALTER TABLE Student ADD COLUMN UserPhone VARCHAR;
2 ALTER TABLE Student ADD COLUMN UserNickName VARCHAR;

If you encounter complex modification operations , For example, while modifying , Data transfer is required , Then you can execute the following statement in a transaction to meet the requirement of modifying the table .

1. Change the table name to a temporary table
   ALTER TABLE Student RENAME TO __temp__Student;

2. Create new table
   CREATE TABLE Student (UserId VARCHAR(32) PRIMARY KEY ,UserName VARCHAR(32) NOT NULL ,UserAddress VARCHAR(16) NOT NULL);

3. Import data 　　
   INSERT INTO Student SELECT UserId, “”, UserAddress FROM __temp__Student;
   perhaps 　　
   INSERT INTO Student() SELECT UserId, “”, UserAddress FROM __temp__Student;

• Be careful Double quotes ”” It is used to supplement the data that did not exist before

4. Delete temporary table 　　
   DROP TABLE __temp__Student;

Through the above four steps , You can complete the migration from the old database structure to the new database structure , And it can also ensure that the data will not be lost for upgrading .
　　 Of course , If the field is reduced , It can also be achieved by creating temporary tables .

Relevant reference
https://blog.csdn.net/linglingchenchen/article/details/123632277