Database incrementing field sqlite error [duplicate]

This question is an exact duplicate of:

• Increment the field by one in sqlite database android 1 answer

I've an simple shop database with a purchase() method. I want from that method to INCREMENT by 1 int called fItemNumberOfPurchases. The problem is that it does not do that and I don't know why. Can you please help me finding a solution to this ? Here is the sourcecode for whole database because I don't know where exactly the problem happens. This is not a duplicate question because the previous one have not correct solution.

public class MonsterTapDatabase extends SQLiteOpenHelper {
static final String dbName = "MonsterTapDb";
static final int version = 1;
static final String tTableName = "Shop";
static final String fItemID = "ItemID";
static final String fItemName = "ItemName";
static final String fItemNumberOfPurchases = "NumberOfPurchases";
static final String fItemIsLocked = "IsItemLocked";


public MonsterTapDatabase(Context context) {
    super(context, dbName, null, version);
}

@Override
public void onCreate(SQLiteDatabase sqLiteDatabase) {
    sqLiteDatabase.execSQL("CREATE TABLE IF NOT EXISTS "+tTableName+" (" +
            fItemID + " INTEGER PRIMARY KEY , " +
            fItemName + " TEXT , " +
            fItemNumberOfPurchases + " INT, " +
            fItemIsLocked + " TEXT" +
            ")");
    ContentValues cv = new ContentValues();
cv.put(fItemID, 1);
cv.put(fItemName, "Lives");
cv.put(fItemNumberOfPurchases, 0);
cv.put(fItemIsLocked, "true");
sqLiteDatabase.insert(tTableName, null, cv);
cv.put(fItemID, 2);
cv.put(fItemName, "Hardmode");
cv.put(fItemNumberOfPurchases, 0);
cv.put(fItemIsLocked, "true");
sqLiteDatabase.insert(tTableName, null, cv);
cv.put(fItemID, 3);
cv.put(fItemName, "Reversed");
cv.put(fItemNumberOfPurchases, 0);
cv.put(fItemIsLocked, "true");
sqLiteDatabase.insert(tTableName, null, cv);
cv.put(fItemID, 4);
cv.put(fItemName, "Reversed Hardmode");
cv.put(fItemNumberOfPurchases, 0);
cv.put(fItemIsLocked, "true");
sqLiteDatabase.insert(tTableName, null, cv);
}

@Override
public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
sqLiteDatabase.execSQL("DROP TABLE IF EXISTS " + tTableName);
onCreate(sqLiteDatabase);
}
public void purchase(int ID){

SQLiteDatabase myDB = this.getWritableDatabase();
myDB.execSQL("UPDATE " + tTableName+
        " SET "+fItemNumberOfPurchases+"=IFNULL("+fItemNumberOfPurchases+",0)+1"+
        " WHERE "+fItemID+"="+ID);
myDB.close();

}
public void lockItem(int ID){
SQLiteDatabase myDB = this.getWritableDatabase();
ContentValues cv = new ContentValues();
cv.put(fItemIsLocked, false);
myDB.update(tTableName, cv, fItemID+"=?", new String []{String.valueOf(ID)});
}
public String isPurchased(int ID){
SQLiteDatabase myDB = this.getReadableDatabase();
String[] mySearch = new String[]{String.valueOf(ID)};
Cursor myCursor = myDB.rawQuery("SELECT "+ fItemIsLocked +" FROM "+ tTableName +" WHERE "+ fItemID +"=?",mySearch);
myCursor.moveToFirst();
int index = myCursor.getColumnIndex(fItemID);
String myAnswer = myCursor.getString(index);
myCursor.close();
return myAnswer;
}
public int numberOfLives(){
int ID = 1;
SQLiteDatabase myDB = this.getReadableDatabase();
String[] mySearch = new String[]{String.valueOf(ID)};
Cursor myCursor = myDB.rawQuery("SELECT "+ fItemNumberOfPurchases +" FROM "+ tTableName +" WHERE "+ fItemID +"=?",mySearch);
myCursor.moveToFirst();
int index = myCursor.getColumnIndex(fItemNumberOfPurchases);
myCursor.close();
return index;
}

and this is how i purchase the item

lives.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            myDB.purchase(1);
            i[0] =String.valueOf(myDB.numberOfLives());
            myDB.close();
            price1.setText(i[0]);
        }
    });