Android SQLite grop_concat 多列

Question

我正在尝试在 android 中运行以下查询。当我尝试检索“d”列的值时，它返回 null。我可以得到 a.* 值。 :(

查询：

select a.*,group_concat(b._id || ', ' || b.product_id || ', ' || b.no_of_units || ', ' || b.unit_price || ', ' || b.discount || ', ' || b.discount, '; ') as d
    from transactions a  join transactions b on a._id=b.local_ref_txn_id 
group by a.id

代码：

mCursor = new MyDbHelper(mContext).getReadableDatabase().rawQuery("select group_concat(b._id || ', ' || b.product_id || ', ' || b.no_of_units || ', ' || b.unit_price || ', ' || b.discount || ', ' || b.discount, '; ') as d,a.* \n" +
                "from transactions a left join transactions b on a._id=b.local_ref_txn_id " +
                "group by a._id", null); 

String details  = cursor.getString(cursor.getColumnIndex("d"));

Answer 1

您上面的代码将无法工作/无法按预期工作，原因是：-

首先，当您生成的游标名称为 mCursor 时，您尝试访问名称为 cursor 的游标。

您似乎正在访问以前创建/提取的游标，假设该游标中存在列 d，则该游标很可能具有列 d 的错误数据。

因此，代码应该是（仅用于纠正上述问题）：-

String details  = mCursor.getString(mCursor.getColumnIndex("d"));

然而，上述操作会失败，并出现

android.database.CursorIndexOutOfBoundsException: Index -1 requested, with a size of 0

这是因为没有尝试移动到光标内的一行，因此光标位于“第一行之前”的位置（即位置 -1）。

要更正上述问题，您需要移动到有效行（如果有的话）。假设您将提取任意数量的行，那么您可以使用以下内容处理每一行：-

private void SO51603383() {
    Cursor mCursor;
    SO51603383DBHelper MyDbHelper = new SO51603383DBHelper(this);
    mCursor = MyDbHelper.getReadableDatabase().rawQuery("select group_concat(b._id || ', ' || b.product_id || ', ' || b.no_of_units || ', ' || b.unit_price || ', ' || b.discount || ', ' || b.discount, '; ') as d,a.* \n" +
            "from transactions a left join transactions b on a._id=b.local_ref_txn_id " +
            "group by a._id", null);

    while (mCursor.moveToNext()) {
        String details = mCursor.getString(mCursor.getColumnIndex("d"));
    }
}

当/如果您在使用 LEFT JOIN 时运行，您可能会得到空值，如下所示：-

如果连接操作符是“LEFT JOIN”或“LEFT OUTER JOIN”，那么在 ON 或 USING 过滤子句已被应用，额外的一行是 添加到原始左侧输入中每一行的输出 与复合数据集中完全不对应的数据集 （如果有的话）。添加的行在列中包含 NULL 值 通常包含从右侧输入数据集中复制的值。

SQL As Understood By SQLite - SELECT

所以你可能想改用 JOIN。

示例案例

使用以下作为数据库助手：-

public class SO51603383DBHelper extends SQLiteOpenHelper {

    public static final String DBNAME = "SO51603383.db";
    public static final int DBVERSION = 1;

    public static final String TB_TRANSACTIONS = "transactions";
    public static final String COL_TRANSACTIONS_ID = BaseColumns._ID;
    public static final String COl_TRANSACTIONS_NAME = "name";
    public static final String COL_TRANSACTIONS_PRODUCTID = "product_id";
    public static final String COL_TRANSACTIONS_NO_OF_UNITS = "no_of_units";
    public static final String COL_TRANSACTIONS_UNIT_PRICE = "unit_price";
    public static final String COL_TRANSACTIONS_DISCOUNT = "discount";
    public static final String COl_TRANSACTIONS_LOCAL_REF_TXN = "local_ref_txn_id";

    SQLiteDatabase mDB;


    public SO51603383DBHelper(Context context) {
        super(context, DBNAME, null, DBVERSION);
        mDB = this.getWritableDatabase();
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String crtsql = "CREATE TABLE IF NOT EXISTS " + TB_TRANSACTIONS + "(" +
                COL_TRANSACTIONS_ID + " INTEGER PRIMARY KEY," +
                COl_TRANSACTIONS_NAME + " TEXT," +
                COL_TRANSACTIONS_PRODUCTID + " INTEGER, " +
                COL_TRANSACTIONS_NO_OF_UNITS + " INTEGER, " +
                COL_TRANSACTIONS_UNIT_PRICE + " INTEGER, " +
                COL_TRANSACTIONS_DISCOUNT + " INTEGER, " +
                COl_TRANSACTIONS_LOCAL_REF_TXN + " INTEGER" +
                ")";
        db.execSQL(crtsql);
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {

    }

    public long add(String name, long productid, int units, int price, int discount, long localref) {
        ContentValues cv = new ContentValues();
        cv.put(COl_TRANSACTIONS_NAME,name);
        cv.put(COL_TRANSACTIONS_PRODUCTID,productid);
        cv.put(COL_TRANSACTIONS_NO_OF_UNITS,units);
        cv.put(COL_TRANSACTIONS_UNIT_PRICE,price);
        cv.put(COL_TRANSACTIONS_DISCOUNT,discount);
        cv.put(COl_TRANSACTIONS_LOCAL_REF_TXN,localref);
        return mDB.insert(TB_TRANSACTIONS,null,cv);
    }
}

使用加载 2 行（参见上面的添加方法）：-

    MyDbHelper.add("Test001",1,10,22,3,0);
    MyDbHelper.add("Test002",1,20,24,5,1);

然后运行：-

    mCursor = MyDbHelper.getReadableDatabase().rawQuery(
            "select group_concat(b._id || ', ' || b.product_id || ', ' || b.no_of_units || ', ' || b.unit_price || ', ' || b.discount || ', ' || b.discount, '; ') " +
                    "as d,a.* \n" +
            "from transactions a left join transactions b on a._id=b.local_ref_txn_id " +
            "group by a._id", null);

    while (mCursor.moveToNext()) {
        String details = mCursor.getString(mCursor.getColumnIndex("d"));
        Log.d("DETAILS","Column D from query is :- " + details);
    }

导致以下输出到日志（1 行数据，1 行空值）：-

07-31 02:26:21.255 1580-1580/soanswers.soanswers D/DETAILS: Column D from query is :- 2, 1, 20, 24, 5, 5
    Column D from query is :- null

改用：-

    mCursor = MyDbHelper.getReadableDatabase().rawQuery(
            "select group_concat(b._id || ', ' || b.product_id || ', ' || b.no_of_units || ', ' || b.unit_price || ', ' || b.discount || ', ' || b.discount, '; ') " +
                    "as d,a.* \n" +
                    //"from transactions a left join transactions b on a._id=b.local_ref_txn_id " +
                    "from transactions a join transactions b on a._id=b.local_ref_txn_id " +
            "group by a._id", null);

    while (mCursor.moveToNext()) {
        String details = mCursor.getString(mCursor.getColumnIndex("d"));
        Log.d("DETAILS","Column D from query is :- " + details);
    }

导致（清除数据库后）：-

07-31 05:56:38.872 2380-2380/soanswers.soanswers D/DETAILS: Column D from query is :- 2, 1, 20, 24, 5, 5

使用第一个代码 (LEFT JOIN) 并第二次运行，从而添加另外两行（除了 _id 值之外的重复），结果是：-

07-31 05:59:04.095 2440-2440/? D/DETAILS: Column D from query is :- 2, 1, 20, 24, 5, 5; 4, 1, 20, 24, 5, 5
    Column D from query is :- null
    Column D from query is :- null
    Column D from query is :- null

使用第二个（只是加入）代码并运行两次，结果是：-

07-31 06:05:00.610 2565-2565/soanswers.soanswers D/DETAILS: Column D from query is :- 2, 1, 20, 24, 5, 5; 4, 1, 20, 24, 5, 5