java - 如何对 Db 的列求和

Question

抱歉，我是 Java 新手。我想获取所有笔记(KEY_Time)，对它们求和并将总和作为字符串返回。当我想执行如下语句时，我的应用程序崩溃了。

String rawQuery = 
    "SELECT _id, fehlzeiten, 
         SUM(fehlzeiten) as sum 
         FROM notes";
     sqliteCallLogsDB.rawQuery(rawQuery, null);

NotesDbAdapter :

public class NotesDbAdapter {

public static final String KEY_TITLE = "title";
public static final String KEY_BODY = "body";
public static final String KEY_ROWID = "_id";
public static final String KEY_Test = "test";
public static final String KEY_Time = "fehlzeiten";
private static final String TAG = "NotesDbAdapter";
private DatabaseHelper mDbHelper;
private SQLiteDatabase mDb;

/**
 * Database creation sql statement
 */

private static final String DATABASE_CREATE =
    "create table notes (_id integer primary key autoincrement, "
    + "title text not null, body text not null, fehlzeiten text not null, test text not null);";

private static final String DATABASE_NAME = "data";
private static final String DATABASE_TABLE = "notes";
private static final int DATABASE_VERSION = 4;

private final Context mCtx;

private static class DatabaseHelper extends SQLiteOpenHelper {

    DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override

    public void onCreate(SQLiteDatabase db) {

        db.execSQL(DATABASE_CREATE);

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

        Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                + newVersion + ", which will destroy all old data");
        db.execSQL("DROP TABLE IF EXISTS notes");
        onCreate(db);
    }
}

/**
 * Constructor - takes the context to allow the database to be
 * opened/created
 * 
 * @param ctx the Context within which to work
 */
public NotesDbAdapter(Context ctx) {

    this.mCtx = ctx;
}

/**
 * Open the notes database. If it cannot be opened, try to create a new
 * instance of the database. If it cannot be created, throw an exception to
 * signal the failure
 * 
 * @return this (self reference, allowing this to be chained in an
 *         initialization call)
 * @throws SQLException if the database could be neither opened or created
 */
public NotesDbAdapter open() throws SQLException {

    mDbHelper = new DatabaseHelper(mCtx);
    mDb = mDbHelper.getWritableDatabase();
    return this;
}

public void close() {
    mDbHelper.close();
}


/**
 * Create a new note using the title and body provided. If the note is
 * successfully created return the new rowId for that note, otherwise return
 * a -1 to indicate failure.
 * 
 * @param title the title of the note
 * @param body the body of the note
 * @return rowId or -1 if failed
 */
public long createNote(String title, String body, String fehlzeit, String test) {

    ContentValues initialValues = new ContentValues();
    initialValues.put(KEY_TITLE, title);
    initialValues.put(KEY_BODY, body);
    initialValues.put(KEY_Time, fehlzeit);
    initialValues.put(KEY_Test, test);

    return mDb.insert(DATABASE_TABLE, null, initialValues);
}

/**
 * Delete the note with the given rowId
 * 
 * @param rowId id of note to delete
 * @return true if deleted, false otherwise
 */
public boolean deleteNote(long rowId) {

    return mDb.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
}

/**
 * Return a Cursor over the list of all notes in the database
 * 
 * @return Cursor over all notes
 */
public Cursor fetchAllNotes() {

    return mDb.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_TITLE,
            KEY_BODY,KEY_Time,KEY_Test}, null, null, null, null, null);
}




/**
 * Return a Cursor positioned at the note that matches the given rowId
 * 
 * @param rowId id of note to retrieve
 * @return Cursor positioned to matching note, if found
 * @throws SQLException if note could not be found/retrieved
 */
public Cursor fetchNote(long rowId) throws SQLException {

    Cursor mCursor =

        mDb.query(true, DATABASE_TABLE, new String[] {KEY_ROWID,
                KEY_TITLE, KEY_BODY,KEY_Time,KEY_Test}, KEY_ROWID + "=" + rowId, null,
                null, null, null, null);
    if (mCursor != null) {
        mCursor.moveToFirst();
    }
    return mCursor;
}


/**
 * Update the note using the details provided. The note to be updated is
 * specified using the rowId, and it is altered to use the title and body
 * values passed in
 * 
 * @param rowId id of note to update
 * @param title value to set note title to
 * @param body value to set note body to
 * @return true if the note was successfully updated, false otherwise
 */
public boolean updateNote(long rowId, String title, String body, String fehlzeiten, String test) {

    ContentValues args = new ContentValues();
    args.put(KEY_TITLE, title);
    args.put(KEY_BODY, body);
    args.put(KEY_Time, fehlzeiten);
    args.put(KEY_Test, test);

    return mDb.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
}

Answer 1

如果我理解正确，您将 Fehlzeiten 存储为字符串。那不是很方便。将其切换为 INTEGER 并得到如下总和:

public long getFehlzeitenSum() {
    long result = 0;
    Cursor c = mDb.query(DATABASE_TABLE, 
      new String[] { "sum(" + KEY_Time + ")" },
      null, 
      null, 
      null /* you may add a GROUP BY attribute (e.g. KEY_Test) here */,
      null /* and here a HAVING for your group-by-clause */, 
      null);
    if (c.moveToFirst()) {
        result = c.getLong(0);
    }
    c.close();
    return result;
}

正如 tuze 所指出的，您不能将聚合函数与常规值混合使用(GROUP BY 中的除外)。