Develop Reference

Is it possible to take column names for sqlite database as input from user in an android app?

I am creating an android app for my college faculties through which they will be able to keep and maintain the attendance of students in their lectures.
I thought of designing the database this way
Date | Student1 | Student2 | . . . . . . |. . .|. . . | Student60
In this structure each INSERT INTO statement will take 61 values, one for the date and rest for presence/absence record of 60 students.
But in this case the column headers have to be named by the user(the column header should be unique identifier for that particular student, like his roll no.). Is it possible? or am I completely on the wrong track?
Please suggest if there is a better database design alternative.
I also need to provide the users the ability to retrieve aggregate attendance % of a student.
Apologies in advance if I've asked something very basic or stupid.

This is on the wrong track. Instead, make the student's ID a primary key column, and use the other columns for storing student metadata, something like this:
Students
ID | first_name | last_name |
1 | Jon | Skeet |
2 | Gordon | Linoff |
...
Attendance
ID | SID | date | status
1 | 1 | 2017-05-24 | absent
2 | 1 | 2017-05-25 | present
3 | 2 | 2017-05-24 | present
4 | 2 | 2017-05-25 | present
Now if you wanted to find out which students were present on a given day you could use the following query:
SELECT
s.first_name,
s.last_name
FROM Students s
INNER JOIN Attendance a
ON s.ID = s.SID
WHERE a.status = 'present' AND
a.date = '2017-05-24'
Note that in practice you might use an integer (0 or 1) to store the attendance.
Towards answering your actual question, if you wanted a summary by student along with his attendance record in percent over the most recent 90 days, you could use this:
SELECT SID, 100*(SUM(CASE WHEN status = 'present' THEN 1 ELSE 0 END) / 90) AS p_attedance
FROM Attendance
WHERE date > date('now', '-90 days');
GROUP BY SID

Does SQLite guarantee the order of results to match a table?

If table Scores looks like this:
_id | score
---------
1 | 1,000
2 | 2,000
3 | 3,000
4 | 4,000
5 | -1
6 | -1
7 | -1
Will the following query always return the rows in _id ascending order?
SELECT * FROM Scores
Also, will the following query always return the first ordered occurrence of _id (that is, 5)?
SELECT _id FROM Scores WHERE Score = -1 LIMIT 0, 1
The key here is ALWAYS. I have tested it and it works as intended but I want to verify this outcome is guaranteed and that an order by clause is not needed in these cases.
Thank you all.

By default, the rows are logically stored in order of increasing rowid.
And if your select stament does not include an order by, or the table has no index, the default sorting order will always be the primary key column, which is the _ID column in this case
Read more here

One of the principles of the databases is that you can not suppose that your registers are ordered in any way, as the internal implementation of SQLite may differ between devices. You should use the operator order by to assure a certain order.

Complex SQL query in Android SQLite

I have a table orders, consisting of 3 columns:
order_id int primary key,
cust_id integer,
order_date integer
with data:
order_id | cust_id | order_date
1 | 10 | 1325376000
2 | 10 | 1325548800
3 | 10 | 1325894400
4 | 11 | 1325462400
5 | 11 | 1325721600
6 | 12 | 1325721600
7 | 12 | 1326326400
I'm trying to write a query to give a Cursor containing the most recent order for a given customer that I can then pass to a SimpleCursorAdapter and bind to a ListView, such that the user sees the following:
10 1325894400 (formatted as human readable date)
11 1325721600
12 1326326400
I've tried joining the table to itself in various ways without any luck:
http://sqlfiddle.com/#!2/77b22d/1/0
If I have to populate an ArrayList and use an ArrayAdapter I will, but I'd like to exhaust this option first. Thanks!
EDIT: Apologize for the differences between here and the SQLFiddle, brain running on two separate threads. The Fiddle is the 'correct' data set.
2nd EDIT: Added a new wrinkle (ignore table above, see the SQL fiddle). Adding a field for free-form text and then running the query returns the first record in the GROUP BY, plus the field for the max_date. I need to pull the whole record containing the date that equals max_date. Adding a WHERE clause breaks the query. Thoughts?

Try this
select
order_number
, cust_number
, order_date
from orders o1
where order_number =
(
select order_number
from orders o2
where o2.cust_number = o1.cust_number
and order_date =
(
select max(order_date)
from orders o3
where o3.cust_number = o2.cust_number
)
)
This will get you the correct records and you can format the date as you like in the main query.
Note: My answer is a bit different form your display since the example here and the Fiddle are different. used the Fiddle one
create table orders (order_number integer primary key,
cust_number integer not null,
order_date integer not null);
insert into orders values (1001,10,1005),
(1,10,1325376000),
(2,10,1325548800),
(3,11,1325894400),
(4,11,1325462400),
(5,11,1325721600),
(6,12,1325721600),
(7,12,1326326400),
(8,12,1326326460);

If you just want the latest record for each customer, I think this will work:
SELECT order_number, cust_number, max(order_date) as max_date FROM orders GROUP BY cust_number

The values you put on the link are different from the ones you posted here but you are looking for:
select o1.cust_number, max(o1.order_date)
from orders o1
group by o1.cust_number
order by o1.cust_number ASC
This will give you for each customer the most recent order.

Simple SQL query not giving proper results

Here's a query i constructed:
SELECT DISTINCT * FROM tagResourceName a INNER JOIN
tagResource b ON a.tagID=b.tagID
However, the result is returning values which are not distinct. e.g.
Tag Name | Tag ID
pink | 13
pink | 13
zoo | 16
Why is that? I'm running this on an Android app btw. Thanks!

SELECT * FROM tagResourceName a
INNER JOIN tagResource b ON a.tagID=b.tagID
GROUP BY a.tagName, a.tagID
ORDER BY a.tagID

Might one of your tags have trailing spaces?

SELECT * FROM tagResourceName a INNER JOIN
tagResource b ON a.tagID=b.tagID
GROUP BY b.tagName, a.tagID
ORDER BY a.tagID

How to run DISTINCT on non-key in SQL

I have a database that can have similar rows, but have different keys and a different boolean column. Here is what the database looks like:
columns: _id, name, img, address, available
Two entries can look like this:
_id | name | img | address | available
-------------------------------------------------------
1 | John | http://img.com/222 | 1 Main St | 1
2 | John | http://img.com/222 | 1 Main St | 0
I want a query that will give me all results that have a distinct key, and if there are duplicate entries(ignoring the fact that _id would be different), it will give back only the first one. Here is the query I have:
SELECT p1.*
FROM (SELECT DISTINCT _id, available FROM people) p
INNER JOIN people p1
ON p1._id=p._id
ORDER BY p1.available DESC;
I know this isn't right, but maybe it explains a little what I am looking for. Would I want to use GROUP BY here?

I want a query that will give me all results that have a distinct key, and if there are duplicate entries(ignoring the fact that _id would be different), it will give back only the first one.....the _id isn't what I want to be distinct, as they [the ids] are already unique. ... . Ideally it will order by 'available' in descending order so that if there are two columns with the same data(aside from _id and available), it will return the row with '1' for the available column
select name, image, address, max(availability) as avail
from T
group by name, image, address
Then you can join the set returned by the query above, as an inline view, to your table:
select * from T
inner join
(
select name, image, address, max(availability) avail
from T
group by name, image, address
) as foo
on T.name = foo.name and T.image = foo.image and T.address = foo.address and T.availability = foo.avail
It would help to have a composite index so: (name, image, address).
Caveat: if there is more than one row where a specific {name, image, address} triad has availablility =1, the query will return multiple rows for the triad:
2 | John | http://img.com/222 | 1 Main St | 1
6 | John | http://img.com/222 | 1 Main St | 1
P.S. It sounds as though you wished the triad (name, image, address) had been created in your table an alternate UNIQUE key.

this sql may solve your problem:
select b.* from （select distinct _id from people) a, people b where a._id = b._id order by b.available

I actually just asked a similar question and received a great answer from an experienced user here:
SQL Populating with distinct data and a sequence
Based on what he told me, perhaps this query would provide you with what you want:
SELECT p1.*
FROM (SELECT DISTINCT _id, name from people) p
INNER JOIN people p1
ON p1._id=p._id
ORDER BY p1.available desc
apologies if that's a fail and doesn't work!
EDIT: It just occurred to me that I have no idea which distinct name+_id combo this will extract.. the available=1 or the available=0 or a random selection..! Let me know what happens anyway..

If you want the first row which has the lowest _id among those that have the highest available value (between 1 and 0), you can "record" the _id inside the aggregated value generated by the grouping.
The value to compare is constructed in a way that orders the record by their available field in descending order and then by their _id field in descending order, and allow to easily retrieve the value of the _id with the modulo operator (assuming available max value is 1 and the ids are never above 100000000).
select people.* from people
inner join (
select name, img, address,
min((1-available)*100000000 + _id) avail_id
from people group by name, img, address
) as foo on people._id = foo.avail_id % 100000000;
I adapted it Tim's query.
You can also do that without subquery:
select people.* from people
left outer join people as other on
other.name = people.name and
other.img = people.img and
people.address=other.address and
(1 - people.available) * 100000000 + people._id >
(1 - other.available) * 100000000 + other._id
where other.available is null;