Android SQL查询无法识别第一个条目

Question

Buckle up folks, this is a weird one. I'm currently working on an android app that involves storing and retrieving data in an sqlite database. I was going through the app and testing some of the basic features to make sure everything worked, and lo and behold I found a bug in retrieving data from my database. When a user inputs their very first entry to the app, everything works as expected, the values get processed and stored. However, when I go back and attempt to access that data using SELECT * FROM history; I get a cursor that returns true when I call data.moveToNext() , yet when I loop through it using while(data.moveToNext()) { //get values and add to a List<> } the while loop never gets executed.

I've looked at the contents of the database after moving the file to my computer and opening the database using this db browser and I can see my entry.

Here's the method that I call to get all the points from my database:

List<PointValue> getAllPoints() {
    List<PointValue> points;
    Cursor data = rawQuery("SELECT * FROM history");
    if (data.moveToNext()) {
        points = new ArrayList<>();
        while (data.moveToNext()) {
            System.out.println("Looped");
            long timestamp = data.getLong(data.getColumnIndexOrThrow("timestamp"));
            int level = data.getInt(data.getColumnIndexOrThrow("level"));
            points.add(new PointValue(timestamp, level));
        }
    } else {
        return null;
    }
    data.close();
    if (points.size() == 0) {
        return null;
    }
    return points;
}

The rawQuery method looks like this:

private Cursor rawQuery(String sql) {
    SQLiteDatabase db = this.getReadableDatabase();
    return db.rawQuery(sql, null);
}

When I tried debugging this on my own, the size of points is 0 even though I know that there's at least one point in the database. Thoughts? The class containing all of my sql related stuff extends SQLiteOpenHelper

EDIT: Here's the solution suggested by @Isaac Payne (still doesn't work):

public List<PointValue> getAllPoints() {
    List<PointValue> points = new ArrayList<>();
    Cursor data = rawQuery("SELECT * FROM history");

    while (data.moveToNext()) {
        long timestamp = data.getLong(data.getColumnIndexOrThrow("timestamp"));
        int level = data.getInt(data.getColumnIndexOrThrow("level"));
        points.add(new PointValue(timestamp, level));
    }
    data.close();
    if (points.size() == 0) {
        return null;
    }
    return points;
}

Answer 1

The issue is that when you call data.moveToNext() in the if statement you are moving to the first entry, then you call moveToNext() again in your while loop moving to the second non-existent entry. Try removing the if statement

Answer 2

Add data.moveToFirst() before if loop.

Cursor data = rawQuery("SELECT * FROM history");
//add this line
data.moveToFirst();
if (data.moveToNext()) {