Develop Reference

How to uniquely identify a contact on ContactsContract.Contacts table

I have an app that gets the ContactsContract.Contacts.LOOKUP_KEY of a contact on the device and saves it on the app Db.
After reading this page I thought I could use the LOOKUP_KEY to uniquely identify a contact, even when a contact is edited (for example after editing the name of the contact).
Actually I saw that after editing a contact, its LOOKUP_KEY changes, so I cannot use anymore the LOOKUP_KEY I saved on my app DB.
My question is: is there a way to uniquely identify a contact on ContactsContract.Contacts from when it is created for the first time on the device until it is deleted from the device?
Thank you

A LOOKUP_KEY is not meant to be used as a key on its own, instead it should be used together with a contact's _ID to form a full lookupUri.
The lookupUri can then be used to find a contact in CONTENT_LOOKUP_URI tables.
The CONTENT_LOOKUP_URI basically first looks for the contact by _ID, if it fails to find it, or the _ID seems like the wrong contact, it uses hints from the LOOKUP_KEY part to try and track down the correct contact for you.
From CONTENT_LOOKUP_URI
A content:// style URI for this table that should be used to create
shortcuts or otherwise create long-term links to contacts. This URI
should always be followed by a "/" and the contact's LOOKUP_KEY. It
can optionally also have a "/" and last known contact ID appended
after that. This "complete" format is an important optimization and is
highly recommended.
As long as the contact's row ID remains the same, this URI is
equivalent to CONTENT_URI. If the contact's row ID changes as a result
of a sync or aggregation, this URI will look up the contact using
indirect information (sync IDs or constituent raw contacts).
Lookup key should be appended unencoded - it is stored in the encoded
form, ready for use in a URI.
From getLookupUri(long contactId, String lookupKey)
Build a CONTENT_LOOKUP_URI lookup Uri using the given _ID and
LOOKUP_KEY.
From LOOKUP_KEY
An opaque value that contains hints on how to find the contact if its
row id changed as a result of a sync or aggregation.

The row id (primary key) for each contact called _ID.

extracting most recent row contatining longitude and latitude added into table in database

We have created an android app where multiple users of this app sends its coordinates on server in a table named 'data' after a regular intervals of time. which has 3 columns "bus_id", "longitude", "latitude".Each user has a particular id. The table structure is:
data
bus_id longitude latitude
--------------------------------
Along with coordinates they also send their id to server. There is no primary key in that table. Now we want to extract the most recent entry for a particular user. so is there a way to do so or we have to define a primary key??
or is there any other way to do so?

To extract a 'most recent' entry (by specific user or otherwise), you need some way of keeping track of the individual records, either by an autoincrement ID or by a date field.
With just the three columns that you show, there's no basis on which to do a query for 'most recent' - you can only search, sort and filter by the attributes present in the table.
Using an autoincrement ID field:
SELECT * FROM data
WHERE bus_id = 'specific_user'
ORDER BY id DESC LIMIT 1
will select records for a specific user (specified by bus_id), order them in descending order by the autoincrement ID, and return the top record, which will be the record with the highest ID number and hence the most recent record (for that user).

in Android 2.0 or later, is it possible to identify all the raw contacts from which a single aggregate contact is formed?

as far as you might know, there are contacts (aggregate contacts) which are formed by aggregation of two or more raw contacts in Android V2.x
is it possible to identify all the raw contacts from which a single aggregate contacts is formed through a query on the ContactsContract.Contacts or is there a way to identify these
contacts at all?
i could not find any flag or database field that tells me that this aggregate contacts is linked with these raw contacts.
any suggestions?

You can check AggregationExceptions.CONTENT_URI Table where relationship type are AggregationExceptions.TYPE_KEEP_TOGETHER, AggregationExceptions.TYPE_KEEP_SEPARATE, etc.
and you can find Raw_contact_id1 and raw_contact_id2.
Example of data into database. Lets say 1,2,3,4 are in relation so you can find following pairs.
Raw_contact_id1 raw_contact_id2 Relationship type
1-> 2, 1->3, 1->4, 2->3, 2->4, 3->4

A Contact cannot be created explicitly. When a raw contact is inserted, the provider will first try to find a Contact representing the same person. If one is found, the raw contact's CONTACT_ID column gets the _ID of the aggregate Contact. If no match is found, the provider automatically inserts a new Contact and puts its _ID into the CONTACT_ID column of the newly inserted raw contact.
So, while reading all the contacts one by one we can take its _ID value and can retrieve all the contacts from raw_contacts where _ID matches with raw_contacts.CONTACT_ID.
If the count is greater than 1 then we can conclude that it is linked with those numbers of contacts else it is not linked with any other contact.

Help with MySQL Friends List

I am making an Android app that will use a friends list. As far as I know, you can't store an array in a MySQL table, so it seems that I will have to make a separate table from my "Users" table and call it, say, "Friends". This table will just have Friend1 and Friend2.
My question is the preferred way to store this table. I can use the UserName field (string) from my "Users" table to store them, or I could use the UserID field (integer) from my "Users" table. Using the ID would make the table smaller because the small integers take up more space than the string, but at the same time, I access this data mainly using the UserName field (so I have to query the Users table to get the UserID from the Users table).
Which method is preferred for a MySQL table? Using the users name directly so I do not have to find the UserID from the Users table, or saving the table as two integers, and querying to find the ID from the UserName? Thanks.

Store two userID keys from the users table.
Let's say that you change a name of a contact from "Guy from bar" to "Mr. McMoneypants". If this contact was a friend, it will still show up as "Guy from bar" even after the change.
Try to keep data from living in multiple places.

The preferred method is to use the UserID from the Users table in the Friends table as a way to reference that user. That way, as Phillip says, the User can change their name and you only have to change it in one place. Plus, as you say, your Friends table will take up less space with a numeric column as compared to a string column.
And in regards to "(so I have to query the Users table to get the UserID from the Users table)", the following query is not too cumbersome:
SELECT FriendName
FROM Users Natural Left Join Friends
WHERE UserName = 'Ralph';
As far as your query is concerned, you never had to deal with the UserID column.
That's not that much harder than your method:
SELECT FriendName
FROM Friends
WHERE UserName = 'Ralph';

Using foreign keys and ContactsContract

I'm building an app where I need the user to select some "favorite" contacts and a phone number. I'm able to select a contact using
startActivityForResult(new Intent(Intent.ACTION_PICK, Contacts.CONTENT_URI), PICK_CONTACT_REQUEST);
and I'm able to extract all the information I need.
I then proceed to save the _id of the contact into my own database. My plan is to later list all the "favorited" contacts and display the name and phonenumber in a listview.
I want to save the contact id instead of the name and number so my listview will reflect any changes the user makes to his or her contacts.
But now I'm stuck. I don't know how to transform my table with contacts ids into a table with contact names.
I would like to something like this
my_table LEFT OUTER JOIN contacts_table ON (my_table.contact_id = contacts_table._id)

What you will want to do here is to store those IDs then when you want to pull the names from the user's address book, you'll have to cross reference the IDs with the contacts intent you have store in order to filter them out.
Basically you cant join a non-existent table (contacts_table).

You could probably just save the contect IDs however you wish, and then loop through the contact IDs and look up the contacts one-by-one extracting the data you need. If that's quick enough you wouldn't need any SQL joins ("optimize by need").
Perhaps its even quicker to use some Google API and use HTTP/XML to fetch the contacts (perhaps cache them for a while as well).