[database] SQLite version upgrade and downgrade

Reference resources ：https://www.jianshu.com/p/65923fa3e3dc

1 Normal all use process

1.1 Define global variables

public static SQLiteHelper dbHelper;
public static String folder = "android.xxx.xxx"; //  Database storage address 
public static String file = "database.db"; //  Database name 
public static int DB_VERSION = 1; //  Version number of the database 
public static SQLiteDatabase db; //  database 

1.2 Initialize database （ stay MainActivity Medium or Services in ）

The resulting database is stored in a public folder , Even if you uninstall APP, The database will not be lost .

static void init_db(Context context) {
    
    String pathname = GetDir.getDir(Constant.folder);
    File file = new File(pathname, Constant.file);
    try {
    
        if(!file.exists()){
    
            file.createNewFile();
            Uri uri = Uri.fromFile(file);
            Intent intent = new Intent(Intent.ACTION_MEDIA_SCANNER_SCAN_FILE, uri);
            context.sendBroadcast(intent);
        }
    } catch (IOException e) {
    
        Log.v("main", "failed1");
        e.printStackTrace();
    }
    try {
    
        Constant.dbHelper = new SQLiteHelper(context, pathname+"/"+Constant.file, null, Constant.DB_VERSION);
        Constant.db = Constant.dbHelper.getWritableDatabase();    //  call SQLiteHelper.OnCreate()
        Log.v("main", "new db");
    } catch (IllegalArgumentException e) {
    
        Log.v("main", "failed2");
        e.printStackTrace();
        Constant.dbHelper.onUpgrade(Constant.db, Constant.DB_VERSION - 1, Constant.DB_VERSION);
    }
}

among GetDir.getDir as follows ：

public class GetDir {
    
    public static String getDir(String pathname) {
    
        String sdcardPath = Environment.getExternalStorageDirectory().toString();
        File dir = new File(sdcardPath + File.separator + pathname /*+ File.separator + "Files"*/);
        if (dir.exists()) {
    
            return dir.toString();
        } else {
    
            dir.mkdirs();
            return dir.toString();
        }
    }
}

1.3 stay SQLiteHelper Define the steps of database initialization in , Database upgrade and downgrade

public class SQLiteHelper extends SQLiteOpenHelper {
    
	static  final String TAG = "SQLiteHelper";

	public SQLiteHelper(Context context, String name, CursorFactory factory, int version) {
    
		super(context, name, factory, version);
	}
	
	/** *  Create new table  */
	@Override
	public void onCreate(SQLiteDatabase db) {
    
		Log.v(TAG,"onCreate");
		db.execSQL("CREATE TABLE IF NOT EXISTS mytable (name varchar(15), age varchar(5))");
	}

	@Override
	public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    
		Log.v(TAG,"onDowngrade");
		if(oldVersion == 3 && newVersion == 2){
     //  from 3 drop to 2
			Log.i(TAG, "onDowngrade:  from 3 drop to 2");
			db.execSQL("create table tmp_mytable as select name, age from mytable"); //  Create a new table from the old table , It is equivalent to deleting score Column 
			db.execSQL("drop table mytable");
			db.execSQL("alter table tmp_mytable rename to mytable");
		}
	}

	/** *  When the detection is different from the database version created last time , Delete the table before creating a new table  */
	@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    
		Log.v(TAG,"onUpgrade");
		switch (newVersion){
    
			case 2: {
     //  Add... To the table score Field , Initial value 0
				Log.i(TAG, "onUpgrade: newVersion = 2");

				//  The way 1
				db.execSQL("alter table mytable add score varchar(5)");
				ContentValues values = new ContentValues();
				values.put("score", "0");
				db.update("mytable ", values, "name!=?", new String[]{
    ""});


				//  The way 2
				//  First create a temporary table that meets the requirements  
				db.execSQL("CREATE TABLE IF NOT EXISTS mytable (name varchar(15), age varchar(5), score varchar(5))");
				
				//  Copy data from old table to temporary table 
				ContentValues values = new ContentValues();
				values.put("score", "0");
				db.execSQL("INSERT INTO tmp_mytable (name, age) SELECT name, age from mytable");
				db.update("tmp_mytable", values, "name!=?", new String[]{
    ""}); //  as long as name Not empty , All modified score by 0

				//  Delete old table 
				db.execSQL("DROP TABLE IF EXISTS mytable");

				//  Rename the temporary table to the old table name 
				db.execSQL("ALTER TABLE tmp_mytable RENAME TO mytable");
				break;
			}

			case 3: {
     //  Delete... From both tables region Field 
				Log.i(TAG, "onUpgrade: newVersion = 3");
				db.execSQL("create table tmp_mytable as select name, age from mytable"); //  If you write  where 1 = 2  Only the table structure will be copied , Don't copy content 
				db.execSQL("drop table mytable");
				db.execSQL("alter table tmp_mytable rename to mytable");
				break;
			}
			
			default:
				break;
		}
	}
}

2. sqlite Version update

2.1 Method 1 Hard upgrade Not recommended

The first is a hard upgrade method , Delete all old tables , Then recreate all the tables in the database . This method is not elegant , Deleting data is at risk of being unrecoverable , This approach is not recommended .

@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    
		//  Hard upgrade 
		db.execSQL("DROP TABLE IF EXISTS numbers");
		db.execSQL("DROP TABLE IF EXISTS recordInfo");
		onCreate(db);

2.2 Recommended approach 2 The species are as follows

/** *  When the detection is different from the database version created last time , Delete the table before creating a new table  */
	@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    
		Log.v(TAG,"onUpgrade");
		switch (newVersion){
    
			case 2: {
     //  Add... To the table score Field , Initial value 0
				Log.i(TAG, "onUpgrade: newVersion = 2");

				//  The way 1
				//  First, check whether there is score Field , without , To upgrade it 
				ContentValues values = new ContentValues();
				values.put("score", "0");
				if(!checkColumnExist(db, "mytable", "score")){
    
					db.execSQL("alter table mytable add score varchar(5)");
					db.update("mytable", values, "name!=?", new String[]{
    ""});
				}


				//  The way 2
				//  First create a temporary table that meets the requirements  
				db.execSQL("CREATE TABLE IF NOT EXISTS mytable (name varchar(15), age varchar(5), score varchar(5))");
				
				//  Copy data from old table to temporary table 
				ContentValues values = new ContentValues();
				values.put("score", "0");
				db.execSQL("INSERT INTO tmp_mytable (name, age) SELECT name, age from mytable");
				db.update("tmp_mytable", values, "name!=?", new String[]{
    ""}); //  as long as name Not empty , All modified score by 0

				//  Delete old table 
				db.execSQL("DROP TABLE IF EXISTS mytable");

				//  Rename the temporary table to the old table name 
				db.execSQL("ALTER TABLE tmp_mytable RENAME TO mytable");
				break;
			}

			case 3: {
     //  Delete... From both tables region Field 
				Log.i(TAG, "onUpgrade: newVersion = 3");
				db.execSQL("create table tmp_mytable as select name, age from mytable"); //  If you write  where 1 = 2  Only the table structure will be copied , Don't copy content 
				db.execSQL("drop table mytable");
				db.execSQL("alter table tmp_mytable rename to mytable");
				break;
			}
			
			default:
				break;
		}
	}
}

Check whether a table column exists

/** *  Check whether a table column exists  * @param db * @param tableName  Table name  * @param columnName  Name  * @return */
private boolean checkColumnExist(SQLiteDatabase db, String tableName, String columnName) {
    
	boolean result = false ;
	Cursor cursor = null ;
	try{
    
		// Look up a line 
		cursor = db.rawQuery( "SELECT * FROM " + tableName + " LIMIT 0", null );
		result = cursor != null && cursor.getColumnIndex(columnName) != -1 ;
	}catch (Exception e){
    
		Log.e(TAG,"checkColumnExists1..." + e.getMessage()) ;
	}finally{
    
		if(null != cursor && !cursor.isClosed()){
    
			cursor.close() ;
		}
	}

	return result ;
}

2. Downgrade

@Override
public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    
	if(oldVersion == 3 && newVersion == 2){
     //  from 3 drop to 2
		Log.i(TAG, "onDowngrade:  from 3 drop to 2");

		db.execSQL("create table tmp_mytable as select name, age from mytable");
		db.execSQL("drop table mytable");
		db.execSQL("alter table tmp_tb1 rename to mytable");
	}
}