Develop Reference

New to Android - Is _id a must for databases?

I have an already functioning app running on iOS whose database uses a composite primary key. For discussions sake, lets say "CID" and "RID" make up that composite pk, resulting in something that looks like:
CID-RID
F6uuDTEU1c-1
F6uuDTEU1c-2
F6uuDTEU1c-3
However, there are conditions under which the CID column is altered, resetting the RID column. For example:
CID-RID
...
F6uuDTEU1c-4
F6uuDTEU1c-5
WQq6JnyrDI-1
WQq6JnyrDI-2
WQq6JnyrDI-3
...etc
These databases are to be shared cross-platform (ios - android) and going back and editing the current ios structure is not an option. What issues am I going to run into not having an _id column as my pk running on Android?
I found this here on SO - which seems to state that the db itself does not have to have the _id column, only that ...
"The result set for the cursor must contain _id, not the cursor itself."
... but I could be reading this all wrong. Any input/help is much appreciated.
PS: I already looked at a few (what I thought were) similar questions here, here, and here.

You are free to have any database schema you want. Android doesn't impose any additional restrictrions there.
Only if you use a CursorAdapter, then the Cursor needs an _id column. Any app can be written without using CursorAdapter, it's just there to provide some convenience. sqlite tables always have a ROWID column that aliases to the INTEGER PRIMARY KEY column if the table has one. You can always select it as the _id, e.g. SELECT rowid AS _id ... if needed.

SQLite insert record with _id value for two or more fields

I have a case that I would like to insert record in SQLite with database.insert(table, null, values).
TABLE t2 (_id, field1, field2)
..
val.setVal1(null);
val.setVal2(val2);
..
if(val.getVal1==null){
values.put(field1, _id);
}else{
values.put(field1, var.val1);
}
values.put(field2, var.val2);
database.insert("t2", null, values);
Is possible to do sth like this "values.put(field1, _id);"?
_id is generated at database.insert().
Note: I am looking for solution for one insert call. Insert and update row with (field1=_id) is easy.

i think i see now. you're asking if you can enter a value into a specific SQLite row _id field if it's available in your val object. Else, you want the database to automatically create a unique id for that column while inserting, like normally done. Is this correct?
To that end, i would seriously reconsider this purpose. You should never be specifying values for the _id column because it needs to be unique or else you'll get exceptions thrown. Moreover, it's only purpose is to be a unique identifier for the system, so you personally knowing this value should be of no use to you.
If you still need this functionality, i'd suggest making another field in your table (much like the _id column but not it), which you can fill with randomly generated numbers or val.getVal1 values.

SQLite CREATE table with an alias for ROWID

Looking at this answer I was trying to work out how to actually create a table with a column that is just an alias for ROWID. I would like to do this as some android classes require a unique column named '_id' but i dont want that col to be a PRIMARY KEY. I cant use AUTO_INCREMENT on the col that not a PK either, so I would just like _id to be an alias. I know I could do that in the query but thats not practical for my situation. Any pointers? Is this even possible?
Thanks :)
EDIT: really i just want to be able to have my own PK but also have an id field for CursorAdapters to work correctly etc
EDIT: Looking at Do i have to use _ID as a SQlite primary key? and does it have to be an INT? (Android Dev) it suggests to do it in the query if my PK is a number type also, but what if its a TEXT col that im using for my PK? (sort of thinking aloud here) - I guess i could copy CursorAdaptor and just getString instead of a long for the _id col (plus, pass the name of the col to use as the PK in to cursor adapter, get rid of _id!) OR just add a alias for the ROWID as _id in the SELECT, feels a bit hacky though...

You may have a primary key that is completely independent of SQLite's built-in rowid.
Every row of every SQLite table has a 64-bit signed integer key that uniquely identifies the row within its table. This integer is usually called the "rowid". The rowid value can be accessed using one of the special case-independent names "rowid", "oid", or "rowid" in place of a column name.
You may formulate your query so that the built-in rowid is called _id for the benefit of the CursorAdaptor by prepending the hidden rowid column to your results:
select _rowid_ as _id, ...
The rowid is unique, but need not be the primary key. You can make the primary key anything you like in the CREATE TABLE statement.

TBH, I don't think that this is really feasible. You are required to have that kind of PK column (even though with a lot of hacking you could create some kind of workaround).
I would rather suggest to have the _id column as a technical PK and have an additional column as logical PK i.e. just a freely defined column with a unique constraint. Your program logic should then be able to simply use the logical PK for all operations. Of course this would require you to use custom queries for find operations etc. but that is usually more fitting anyways.
hth..

Need a good method to change (SQLite) column data type

I am having a table with 'int' column. During software upgrade, I want to change it to 'long' data type. It seems SQLite does not provide an option (in alter statement) to change/modify column properties. Since I want to do an upgrade, the table might contain data which the user should not lose. So please suggest me a good method to change the column data type property without data loss.
One way suggested in the search links is to create a temporary table, copy the records from the existing table, delete the existing table, and rename the temporary table. I doubt that is efficient.
Your help appreciated!
Regards
Vivek Ragunathan

I used the follow statements to change the type of the column.
CREATE TABLE IF NOT EXISTS **TEMP_TABLE** (id integer primary key autoincrement, **col2change integer not null**, ...)
INSERT INTO TEMP_TABLE SELECT * FROM EXISTING_TABLE
DROP TABLE EXISTING_TABLE
ALTER TABLE TEMP_TABLE RENAME TO EXISTING_TABLE
I changed the int type column in the existing table to integer type. For a few hundred rows, it was reasonably fast.

SQLite3 columns do not have data types, only affinities -- there is no benefit in changing the column type from int to long.
If a wrapper program is truncating values before giving them to SQLite3, there is a way to fix this by editing the schema. It is a dangerous operation, so do it only after backing up your database. The schema is stored in the table sqlite_master; normally it is read-only, but you can modify it if you enable it with the writable_schema pragma. Be careful to only make changes that do not invalidate your data; you may change int to long int since they both have INTEGER affinity.

From SQLite documentation
It is not possible to rename a column, remove a column, or add or
remove constraints from a table.
Check this link
Please do remember that column data types are not rigid in SQLite. Check this link
Edit:
Following your comments on another answer, I guess the option you mentioned - working through the temp table - is the only one, which is not efficient off course.

you could add a new colum, copy the values form the old to the new column, delete the old column and then rename the new column to the old name

AFAIK there is no way in Android to change column data types once a table is created and used. The practiced way is to make a new table and copy the data which you read about

SQLite rename fts3 rowid column

I'm using a tutorial here to provide an AutoCompleteTextView with a SimpleCursorAdapter. It works perfectly as is, but I've changed the database to use fts3 because I've heard it's faster (hence, the name).
It seems something in the code is hard-wired to use the column _id because after changing to an fts3 table, I get this error:
01-28 21:31:53.018: E/AndroidRuntime(16284): java.lang.IllegalArgumentException: column '_id' does not exist
01-28 21:31:53.018: E/AndroidRuntime(16284): at android.database.AbstractCursor.getColumnIndexOrThrow(AbstractCursor.java:314)
Even though I haven't declared the autoincrement key anywhere (as it's superceded by rowid in fts3). The error occurs in AbstractCursor, so there's not much I can do about it.
I'm thinking there might be a way to force the code to recognize rowid as _id by using SELECT rowid,* FROM mytable and then changing the column name somehow.. I'm pretty new to sql so any help is appreciated!

Renaming a column in SQLite can be done as described here. Note that it is highly advisable to do all these operations in transaction. One detail when you are doing this in Android - I don't know how you execute sql scripts in your solution, but keep in mind this if you use execSQL calls.
By the way if you prefer not to rename the column you can try the technique proposed here.