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How to retrieve data from more than one table in parse.com

I put my data in 3 tables(Links, Images and PDF)
each table has columns(university, faculty, grade, and description,...)
I want to retrieve description column in the 3 tables.
where university, faculty, and grade equal to certain values.
and sort them with creation date.
how can I perform that query in parse?

I'm not familiar with Android, but I'm pretty sure Parse does not support "Join" in the way a SQL database does. You could nest the queries, performing the next one in the previous one's completion block.
However, if you regularly want to get data from those 3 tables, I'd suggest you make it 1 table instead, with a column "Content" instead of Link/Img/PDF. Images and PDFs would probably be stored as PFFiles anyway, and you can put link as either its own string column or putting it in a file. You could also add a column "type" if you want to be able to query a specific type, or just keep track of which columns contains which data.
Then you could query the "Content" class, on the keys you want.

I think this link might help you
https://parse.com/docs/js/guide#relations and it is quite simple and nicely explained . You can't do it directly in the database, though.

Magical join, or refresh multiple objects at once?

I have some Items with a foreign key to a parent Category. Is there an efficient way to query some Items and get fully populated Category objects?
The only approach I'm aware of is to use foreignAutoRefresh, or to refresh the Categories manually after the Item query, but this would result in an extra db hit for EACH Item object.
This can be done with a single JOIN, but if I do that is there any support for automatically building out the Category objects? Part (maybe all) of the problem is I don't fully understand the QueryBuilder's join functionality, but based on this answer it sounds like it doesn't do this:
Notice, however, that you can only get entities from the query builder
using this mechanism. If you want to get your two description fields
from different objects then you would have to still use a raw-query.
Alternatively, is there a way to refresh a collection of Categories in place with a single query, so that I can take the bare id-only Categories from the Item query and refresh them all?
In case it's of interest, the goal is to display these hierarchically in an ExpandableListView. Please let me know if I can provide any more info. I am comfortable throwing some SQL at it and populating Java objects myself if need be, but I'd rather stay within the framework if possible.

Which adapter choose?

I have two methods which read the same data from database, the first returns Cursor and the second returns List of objects.Now I show my items in activity using SimpleCursorAdapter and the first method, byt I can also use the second method and appropriate adapter.
Which of these two ways is beter to use and in the second way which adapter I should use?
P.S sorry for poor english

Definitely go with SimpleCursorAdapter. If possible, always use Cursor if your data comes from database, you save memory by not creating List of objects. Creating objects in Java is expensive with regards to time and memory consumption and you have to bear in mind you are on mobile platform with limited resources. If you are using List of objects for your ListView than use custom adapter extending from ArrayAdapter.
It's not always straightforward to use Cursor although your data comes from database. Let's say you store places in the database defined by its name and location and you want to display them in a ListView sorted by distance from current location. It makes it difficult to execute a query which returns sorted results unless you don't store relative distance in additional column. But you can get Cursor convert it to List of objects and sort this collection before sending it to your ListView.

How to organize sqlite database

this is more of a question of theory than anything else. I am writing an android app that uses a pre-packaged database. The purpose of the app is solely to search through this database and return values. Ill provide some abstract examples to illustrate my implementation and quandary. The user can search by: "Thing Name," and what I want returned to the user is values a, b, and c. I initially designed the database to have it all contained on a single sheet, and have column 1 be key_index, column 2 be name, column 3 be a, etc etc. When the user searches, the cursor will return the key_index, and then use that to pull values a b and c.
However, in my database "Thing alpha" can have a value a = 4 or a = 6. I do not want to repeat data in the database, i.e. have multiple rows with the same thing alpha, only separate "a" values. So what is the best way to organize the data given this situation? Do I keep all the "Thing Names" in a single sheet, and all the data separately. This is really a question of proper database design, which is definitely something foreign to me. Thanks for your help!

There's a thing called database normalization http://en.wikipedia.org/wiki/Database_normalization. You usually want to avoid redundancy and dependency in the DB entities using a corresponding design with surrogate keys and foreign keys and so on. Your "thing aplpha" looks like you want to have a many-to-many table like e.g. one or many songs belong/s to the same or different genres. You may want to create dictionary tables to hold your id,name pairs and have foreign keys referencing these tables. In your case it will be mostly a read-only DB so you might want to consider creating indexes with high FILLFACTOR percentage don't think sqlite allows it to do though. There're many ways to design the database. Everything depends on the purpose of DB. You can start with a design of your hardware like raids/file systems/db block sizes to match the F-System's block sizes in order to keep the I/O optimal and where to put your tablespaces/filegroups/indexes to balance the i/o load. The whole DB design theory/task is really a deep subject which is not to be underestimated nor is a matter of few sentences in the answer of stackoverflow. :)

without understanding your data better here is my guess at what you are looking for.
table: product
- _id
- name
table: attribute
- product_id
- a

Need help with a really simple database read and write

I'm using a SQL database to store a single float value, and I'm really having trouble getting my brain around everything that needs to be done to make it work. I've been reading the NotePad tutorial Google provides for a few days now and Googling around but it just isn't clicking in my head.
Could anyone explain to me (no code needed) just what exactly I need to have for a simple database, and how to read the value from it into my float variable and how to write the value of the variable back to the table?
Much thanks, I think my brain is starting to seep out my ears.

A SQL database is not a very good way to store a single float value. It's overkill. Instead, I recommend just using Android's SharedPreference class, which provides a simple key-value store.
If you're still going to create a database, what you need is:
The database file.
A SQL schemea for that database file. This describes the tables in the database, as well as the columns. If you're just storing a single float, then you could just create a single table ("data"), with two columns ("key", "value") -- like SharedPreferences, we're just implementing a key-value store of our own (another reason not to do this -- you're reinventing the wheel).
Once that's created, you can just insert a record with some arbitrary key ("myFloat") for lookups later and your chosen value.
So, your initial SQL statement would look something like this:
INSERT INTO data (key, value) VALUES ("myFloat", 3.14);
Later, you'd retrieve it with a SELECT statement:
SELECT key, value FROM data WHERE key="myFloat";
And you can update the value with an UPDATE statement:
UPDATE data SET value=3.14 WHERE key="myFloat";

It is as simple as shown above or just try reading more on SQL queries