Android sqlite - 如何计算sqlite中2个REAL条目之间的差异？

Question

In Android/Java, I am trying to compute and store the difference between 2 values in sqlite when one value is entered. My table looks like this:

When weights are added/stored in the table in the column 'Weight', the column 'Diff_Weight' shall receive "Weight(N) - Weight(N-1)". Example: the last cell of Diff_Weight (row 6) = 88.0 - 55.2 = 32.8. // Row 5 shall get '-0.7' etc. Their type is REAL (col Weight & col Diff_Weight).

This 32.8 should be calculated and added at the same time when 88.0 is added to the table.

So far, I have read lots of tutorials and can't figure how to proceed. (My code to create and insert in the DB is fine, but reading is somehow more complex).

My code to read the entry is very bad because I don't see how to set it up:

public Bouble getData() {
        String selectQuery= "SELECT * FROM " + TABLE_NAME2 + " ORDER BY COL_4 DESC LIMIT 1";
        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(TABLE_NAME2, null);
           result2 = Double.valueOf(cursor.getString(cursor.getColumnIndex("Weight")));
           result1 = Double.valueOf(cursor.getString(cursor.getColumnIndex("Weight")-1));
           insertdata(result2-result1);  //insert in row 6 of Diff_Weight
        return
    }

Can anybody help there?

If that is unclear, I was needing some help for the sqlite command AND the java to get the difference result.

Answer 1

Simplistically you can get the data by joining to the same table

SELECT a.id, a.weight, b.weight, (b.weight - a.weight)  FROM TABLE_NAME2 a
join TABLE_NAME2 b on (b.id = a.id + 1);

Answer 2

One way is to use the lag() window function to get the value of the previous row (As ordered by id ; using timestamps would be better but between splitting up the date and time into different columns and not using a date format that can be meaningfully sorted, this is easier.):

SELECT id, weight,
       round(coalesce(weight - lag(weight, 1) OVER (ORDER BY id), weight), 1) AS diff_weight
FROM example
ORDER BY id

which gives

id          weight      diff_weight
----------  ----------  -----------
1           22.0        22.0
2           22.2        0.2
3           55.0        32.8
4           55.9        0.9
5           55.2        -0.7
6           88.0        32.8

You can make a view of this query use that like a normal table if you like. Generating the differences dynamically like this has the advantage that if an existing weight value changes, everything that depends on it doesn't have to be updated.

Answer 3

ok, after a long search, here is a possible result (sqlite + java):

• first, you need to query the last row of the table...
• ...and handle the case if there is no row in your table (blank or new table)
• then you must query the 'Weight' value from the known column 'Weight' (Y) and the row with ID you already have (X)
• and when you have your value (last_weight), you need to write the difference (weight-last_weight) in the column 'Diff_Weight'.

Here is the code:

SQLiteDatabase db = this.getWritableDatabase();
//query the last row of the table
Cursor cursor = db.rawQuery("SELECT ID FROM TABLE_NAME2 ORDER BY ID DESC LIMIT 1", null);
cursor.moveToLast();
int lastID;
//handle the case if there is no row in your table
try {
    lastID = cursor.getInt(0);
} catch (Exception e) {
    lastID = 0;
}
Double lastWeight = 0.0;
//query the 'Weight' value
if (lastID >= 1) {
   Cursor cursor2 = db.rawQuery("SELECT Weight FROM TABLE_NAME2 WHERE ID=" + lastID, null);
   if (cursor2.moveToFirst()) { //this is boundary otherwise 'lastWeight' doesn't get the value
        lastWeight= cursor2.getDouble(0);
   }
} else {
    lastWeight = 0.0;
}
//write the difference in the Diff_Weight column (=COL_5)
ContentValues cValues2 = new ContentValues();
//add your data in COL_1 to COL_4 here...
cValues2.put(COL_5, weight - lastWeight);
long id2 = db.insert(TABLE_NAME2, null, cValues2);

... And so you get the red figures in the column Diff_Weight from the table photo in the question.