Android SQLite：在删除行之前如何知道数据是否存在

Question

Trying to deleting a row inside a table using query by id. Suppose that id not exists, then how to know that id or row info not exists?

code below:

public void deletePerson(String personId){
        SQLiteDatabase sqldb = getWritableDatabase();
        sqldb.execSQL("DELETE FROM " + TABLE_NAME + " WHERE " + COLUMN_NAME + " = '" + personId+"'");
        Toast.makeText(context,"Record removedId", Toast.LENGTH_SHORT).show();
    }

Answer 1

You don't necessarily need to know, DELETE won't fail if there is no record that meets the where criteria.

However, if you need to know if the row(s) where deleted, then if you use the delete convenience method it returns the number of affected rows. If 0 then no rows were deleted.

As such the code could be :-

public void deletePerson(String personId){
    SQLiteDatabase sqldb = getWritableDatabase();
    if (sqldb.delete(TABLE_NAME,COLUMN_NAME + "=?",new String[]{personId}) > 0) {
        Toast.makeText(context,"Record removedId", Toast.LENGTH_SHORT).show();
    }
}

• The delete convenience methods generates from the minimal set of parameters passed, the encoding including enclosing the value in single quotes, so it reduces the chance of syntax errors.
• Additionally, it offers protection against SQL injection attacks.
• The 3rd parameter, being an array can have multiple parameters, each matches a ? (which is replaced by the value) according to the sequence.
  • ie the first ? is replaced by the value from the first element, the second ? is replaced by the value from 2nd element and so on.
  • ?'s and number of elements must match.

Answer 2

public boolean getPerson(String personId){
    SQLiteDatabase sqldb = getWritableDatabase();
    Cursor c=     sqldb.execSQL("SELECT * FROM " + TABLE_NAME + " WHERE " + COLUMN_NAME + " = '" + personId+"'");
    if(c!=null){
        return true;
    }
    return false;
}

Answer 3

You can create a function which returns boolean value based on existence of data in sqlite.

public boolean isExists(String personId)
{
    SQLiteDatabase sqldb = getWritableDatabase();
    String selectString = "SELECT * FROM " + _TABLE + " WHERE " + _ID + " =?";
    Cursor cursor = sqldb.rawQuery(selectString, new String[] {personId}); 

    boolean isExist = false;

    if(cursor.moveToFirst()){
       isExist = true;
    }

    sqldb.close();              
    return isExist;
}

Referenced from Android SQLite Database CRUD .

Answer 4

Okay, after getting all of your answers, I solved my problem and what I did is:

public void deletePerson(String personId){
        SQLiteDatabase sqldb = getWritableDatabase();
        Cursor cursor = sqldb.rawQuery("SELECT " + COLUMN_NAME + " FROM " + TABLE_NAME + " WHERE " + COLUMN_NAME + " = '" + personId + "'", null);
        if(cursor.getCount()!=0){
            sqldb.execSQL("DELETE FROM " + TABLE_NAME + " WHERE " + COLUMN_NAME + " = '" + personId + "'");
            Toast.makeText(context,"Successfully deleted", Toast.LENGTH_SHORT).show();
        }else{
            Toast.makeText(context,personId+" not found", Toast.LENGTH_LONG).show();
        }
    }

Thanks !