Develop Reference

Search in sqlite database by criterias selected with checkbox

I have in my app a database with two tables : country and rights. Long story short, the db tells me whether a right (there is 10 rights in total) is legal or not in a specific country.
Now, I want the user to be able to search in my db by criterias. I have a layout with checkbox. If the user check a box, it mean he want to see every country in where the right is legal. For exemple, if he check the box "criteria1" and "criteria6", the user want the list of every country where criteria1 and criteria6 are legal, but we don't care wether the other rights are legal or not.
I asigned values to the checkboxs (1 if legal, 0 if illegal, just like in my db) and passes all of them to the activity who display the result of the search.
My problem is, I can't figure out how to search in my database. I need to only get the country where where the selected criters are equal to 1, but I don't know how to formulate my sql request (since I never know which criterias are going to be checked or not). My request need to only be about the criterias who has the value 1.
I had the idea of sending all my values to a function (witch returns a cursor) where I excecute a select statement if the value is equal to one, but I don't know how I could join all the result of my selects in a cursor. I also thought about using "CASE WHEN..." but it doesn't seem to work.
Does anyone have a clue on how I could deal with my search ?
If you need precisions on my problem, please ask.

This guy here (https://www.youtube.com/watch?v=NGRV2qY9ZiU&list=PL200JxfhYIgCrrpH4rCz-uNfBTb5sng1e) has the right idea.
The clip may be a bit slow but it does exactly what you want.
He creates a custom string based on if checkbox is checked and removes it from the string if unchecked.
To get what you want, you need to do a couple of things.
First, create a table with countries as rows, and rights as columns. Add 1 for right is present in country and 0 if not. Get this into an sqlite database (eg import via csv in DB browser for SQLite, free software; don't forget to create the android_metadata table in the sqlite database - search online for this). Import the database in the app (there is plenty of documentation for this online).
Second, change the text inputed in the if/else checkbox part of the script (he writes fruit names, you write for ex. "right1 = 1", or the exact query the checkbox should do on the column right1).
You also need to pay attention to the selection.add and selection.remove (know that selection is an array list which will store all your criteria for search by column).
Third, you need to change the content of his finalSelection (View view).
Delete all he has written and just create two strings:
String final1 = android.text.TextUtils.join(" or ", selection);
String final2 = "select country from table where " + final1;
The string final2 is your key for a cursor with a rawQuery. Just create a cursor in the database and pass the key to it. This can be done easily.
PS the method android.text.TextUtils.join() is amazing :)
You can place the operator of choice there.
If you need more than one operator (and, or etc), you can create different ArrayLists which you fill in the if/else checkbox is filled and join later in the finalSelection.

Oh, btw, if you have too many checkboxes, you will get a warning in the XML file (layout has more than 80 views is bad for performance).
In order to get around that, you need to get to know grid views a bit better. After reading a few tutorials on the basic use of GridViews, a good start for checkboxes inside them is here.
It may seem like a lot, but you need to learn to use holders to get information out of the getView of the modified BaseAdapter.
If you want to understand it better, follow the arrPath.
It is a String[] filled with all the paths of images found inside the cursor (string values from the dataColumnIndex, which contains paths of images).
Within the onClick() listener of the Button, from the arrPath he extracts only the rows of the cursor that were selected by checkbox click (thumbnailsselection[i] is a boolean - with a value TRUE/FALSE for each row in the cursor).
The selected paths are placed in the selectImages String, separated by OR.

MySQL relation and entities

I have two tables: Place and Discount with an N<->N relation. I also have another table called place_discount which has two foreign keys (idPlace and idDiscount).
When I add a discount to a place, e.g. discount for a bar, I add it in place_discount table.
So my issue is: I have an android app which queries on that database. when I want to list all the places that I have loaded, I want to show a star on those who have a discount. To list that I do a select * from place. Is there a way to do that query?
Tip: I can not use triggers because I do not have super privileges.

Why not use a join?
SELECT
Place.*,
IF(IFNULL(MIN(place_discount.idDiscount),0)>0,1,0) AS hasDiscount
FROM place
LEFT JOIN place_discount ON Place.id=place_discount.idPlace
GROUP BY Place.id
will give you a 1 in hasDiscount if a discount is available, a 0 if not.
On a side note: You do not need super privilege for a trigger on a table you own.

SQLite in Android: is there a way to use a limited number of rows in my application, or should I remove them by hand?

I am currently building a database recording events on the phone, but as I don't want to make this a huge database, 100 events are more than enough.
This will keep my database light en efficient.
Unfortunately, I don't see a way to limit the number of rows other than
String sql = "DELETE FROM myTable WHERE _id <= "+limitId;
and I could run this code when the user launch/leaver the app, but I am expecting a better way to achieve this
Is there a more convenient way to achieve this?

If you are using a ContentProvider, you can implement your DELETE in onInsert, deleting a single row on every insert of a single row:
String sql = "DELETE FROM myTable WHERE _id IN (SELECT min(_id) FROM myTable)";

I guess you mean limit to the 100 newest events ?
if so, there is no better way to do it as you did: checking for entries on every insert and delete old entries if necessary.
It's just a matter of taste how or where you do your check, as flx mentioned you could do it in the ContentProvider or as you probably did in the BroadcastReceiver or Service where you actually add the new row. You could also set up a Trigger on your table, but the main idea remains the same. Here a link if you're interested in triggers:
http://www.tutorialspoint.com/sqlite/sqlite_triggers.htm

how to read a specific column from existing SQLiteBatabase in Android?

I have a database having 28 rows just for saving app data. My goal is to read One specific column (not one value) from it.
However, I don't want to read all columns at once, because it's slow as I have blob's in db too. Instead, I'd like to read just one column at a time, separately. Can you plese help me to solve this problem.

If you want to read one ROW do something along those lines:
SELECT * FROM tablename WHERE id='5' LIMIT 1;
where 'id' is supposer do be in the table 'tablename'
or else try something like this:
SELECT datafield FROM tablename WHERE 1;
which will give you all the 'datafield' data from the table (ie. a Column).
If this doesn't help you at all then post the code you are already using.

Android's SimpleCursorAdapter with queries using DISTINCT

Here's an interesting question that I'm shocked hasn't been asked more often on the internet. Android's CursorAdapters are extremely useful once you get a ContentProvider up and running and learn how to use it, but they are limited due to their requirement on having the _id field as part of their query (an error is thrown without it). Here's why:
My specific problem is that I have two spinners: One spinner should contain unique "category" names from the database, and the other should populate with all the database entries from the selected "category" (category being the column name, here). This seems like a pretty simple setup that many programs might use, no? Trying to implement that first spinner is where I've run into problems.
Here's the query that I would like for that first spinner:
SELECT DISTINCT category FROM table;
Making this query throws an error on CursorAdapter because the _id column is required as part of the query. Adding the _id column to the projection naturally returns every row of the table, since you're now asking for distinct id's as well, and every id is distinct (by definition). Obviously I would rather see only one entry per category name.
I've already implemented a work around, which is to simply make the query above and then copy the results into an ArrayAdapter. My reason for this post is to see if there was a more elegant solution to this odd little problem and start a discussion on what I could be doing better. Alternate implementation suggestions, such as using different kinds of controls or adapters, are very welcome.

Here's the query I ended up with:
SELECT _id, category FROM table_name GROUP BY category;
I used the rawQuery() function on an SQLiteDatabase object to carry this out. The "GROUP BY" piece was the key towards getting the right results, so thanks to user Sagar for pointing me in the right direction.
Do consider user Al Sutton's answer to this question as well, as it may be a more elegant solution to this problem.
Thanks everyone!

I'd suggest having a separate table with just _id & category in it which contains one row per unique category. Your data rows can then replace their category field with the _id from the category table.
This has the added advantage you can change the category in the categories table and it will show up in all entries in that category.

SELECT DISTINCT category,_id FROM table GROUP BY category;
I think this should give you what you are looking for. The results from this will be the category, and the first _id for that category. You can ignore the second column (_id).

You can specify an _id field alias in your select statement that is just a constant value, for example:
SELECT DISTINCT 0 _id, category FROM table;

Better yet, I solved this problem by using:
SELECT DISTINCT category AS _id FROM table
Now, you have a column with the name _id which has what you want in it