Increment the field by one in sqlite database android

I have a database with a table which represents Shop for my game. The problem is, that when I use the method to purchase it doesn't increment the value by one but just sets it to zero. So how can I improve the method to increment the value of ItemNumberOfPurchases by one each time it's called for specific ID ? Here is my code for this.

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

here is my code for defining the database:

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);
}