Android sqlite - 空表上的光标计数不为 0

Question

我在一个更大的项目中有以下代码:

final class DBlifetimeStatisticsHandler{ //implements DBvalueHandler<Cyclist, Double>{

private  final String TAG = getClass().getName();
private static final boolean debug = true;

private final DBminMaxAvgHandler dbMinMaxAvgHandler = new DBminMaxAvgHandler();

// table name
private static final String TABLE_LIFETIME_STATISTICS = "lifetime_statistics";

// column names
private static final String KEY_LIFETIME_STATISTICS_ID = "lifetime_statistics_id";
private static final String KEY_MIN_MAX_AVG = "min_max_avg";

// table create statement
private static final String CREATE_TABLE = "CREATE TABLE "
        + TABLE_LIFETIME_STATISTICS + "(" 
        + KEY_LIFETIME_STATISTICS_ID + " LONG PRIMARY KEY NOT NULL," 
        + KEY_MIN_MAX_AVG   + " LONG"
        + ")";

public void onCreateTable(SQLiteDatabase db) {
    db.execSQL(CREATE_TABLE);
}

public void onUpgrade(SQLiteDatabase db) {
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_LIFETIME_STATISTICS);
    onCreateTable(db);
}

public long addValue(SQLiteDatabase db, Statistics Statistics ) {
    ContentValues values = new ContentValues();

    long ID = getLatestID(db)+1;

    values.put(KEY_STATISTICS_ID, ID);

    ... //not important to the question

}

private long getLatestID(SQLiteDatabase db){
    String selectQuery = "SELECT  MAX(" + KEY_STATISTICS_ID +") FROM " + TABLE_STATISTICS;

    Cursor c = db.rawQuery(selectQuery, null);

    c.moveToFirst();
    int id = 0;
    Log.e("count", String.valueOf(c.getCount()));

    if (c.moveToFirst()){
       ...
    }

    return id;
}
}

更新表后，它又被创建了。因此，当我尝试添加一个新值时，我遇到了问题，因为它总是跳入 if 子句，因为 c.moveToFirst() 总是返回 true。

所以我试着检查 c.getCount() 是否会返回 true 但遗憾的是它总是返回 1。所以问题是:为什么它会在空表上返回 1？ (我确实使用 Questoid SQLite 浏览器并且表格真的是空的)

Answer 1

您使用聚合函数MAX，所以阅读documentation:

There are two types of simple SELECT statement - aggregate and non-aggregate queries. A simple SELECT statement is an aggregate query if it contains either a GROUP BY clause or one or more aggregate functions in the result-set.

An aggregate query without a GROUP BY clause always returns exactly one row of data, even if there are zero rows of input data.