Why do CursorAdapter subclasses requires the primary key to be necessarily _id ?
Isn't there a method to override, or something like that, to change this behaviour ?
I have read this trick many times, and I am aware of that ... I just want to understand better !
Thanks
Why does CursorAdapter subclasses requires the primary key to be necessarily _id ?
It turns around and provides that value in various places, such as the long id value in getView().
Isn't there a method to override, or something like that, to change this behaviour ?
No, sorry. If you do not have a suitable column, just add ROWID AS _ID to your list of columns to return in rawQuery():
SELECT ROWID AS _id, foo, bar FROM really_important_table ORDER BY foo;
ROWID is automatically added to all SQLite tables by default, and is a unique integer, which fits the _id requirements nicely.
Related
I'm using SQLite DB in my Android app.
Now I need to implement data extraction in the same order in which it was inserted.
Is it enough to do SELECT ... ORDER BY _id ... if the _id column is INTEGER PRIMARY KEY AUTOINCREMENT?
Or should I add a column to store the date and time a row has been created?
You can use ORDER BY _id. Make sure you are using a List and not a Set....
No need, an id generated by AUTOINCREMENT is enough.
Yes, if you just need the order, you may use such statement or just simple "SELECT ... FROM sometable". Results will be equals.
This is the query that I use to create a table
create table site_table(
_id integer primary key autoincrement,
name_site text,
url text,
login text,
pass text
);
I called Cursor.getColumnNames() and noticed that columns order are id, login, pass, name, url.
So, if I want a value I have to get it by the index Cursor.getString(index). Until I debugged I was messing up calling the wrong index, but now I wonder, why SQLite saves that way? Why it does not follow that way I created id, name_site, url, login and pass?
Thanks
So, if I want a value I have to get it by the index
Cursor.getString(index)
So for example for this reason you should always use
c.getString(c.getColumnIndex("ColName")); // or better getColumnIndex(CONSTANT)
This method saves all of us and ensure that you never get wrong results. Generally this method is recommended and also storing COLUMN_NAMES as CONSTANTS in separated class is very, very useful and efficient practise.
Note: Order depends on projection i.e. select name, lastname from table
That data is ordered by the order your requested it in your query, not the order you created the table with. So you probably changed the order in your query that generated said cursor.
Columns order in your cursor depends on projection. To be sure you use correct column index use c.getString(c.getColumnIndexOrThrow("COLUMN_NAME")) where c is your cursor.
I just made the experience first hand:
The indices of the columns of the cursor as a result of a
SELECT * FROM mytable WHERE ...
query have sometimes (not always) a different order that what SQLITE Database Browser shows as column order in the Database Structure tab. So referencing the columns via getColumnIndex seems to be the only safe way.
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..
Can anyone tell me how to declare a composite primary key in Android 1.6 which includes an autoincrement _id column? I'm not sure of the syntax. I've ended up just enforcing it in Java when I try to add values (where registrationNumber + date has to be unique in the table):
Cursor fuelUpsCursor = getFuelUps(registrationNumber, date);
if(!fuelUpsCursor.moveToNext())
{
//add registrationNumber and date
}
I don't really need the _id column but it can make life tricky if tables don't have one.
Cheers,
Barry
Your question does not make much sense. Your subject line asks for a "composite foreign key", your first sentence asks for a "composite primary key" with an AUTOINCREMENT that your sample code then ignores.
I am going to interpret your question this way: You want an _ID INTEGER PRIMARY KEY AUTOINCREMENT column in your table to be able to use Android's CursorAdapter, but you want to also make sure that the combination of two other columns is unique.
In that case, I think that you want to use a UNIQUE constraint:
Multiple Unique Columns in SQLite
SQLite table constraint - unique on multiple columns
http://sqlite.org/lang_createtable.html
Is the field "_id" necessary in Android SQLite?
_id is useful when you are using the enhanced Adapters which make use of a Cursor (e.g. ResourceCursorAdapter). It's used by these adapters to provide an ID which can be used to refer to the specific row in the table which relates the the item in whatever the adapter is being used for (e.g. a row in a ListView).
It's not necessary if you're not going to be using classes which need an _id column in a cursor, and you can also use "as _id" to make another column appear as though it's called _id in your cursor.
Why not make use of _ROWID_?
SQLite provides this anyway for every row, so you can just alias it to _id in your select statement.
Technically no the field _id is not required, however if you are making use of the CursorAdapter class (which you probably are, especially if you are working with the Notepad example) then yes
"The Cursor must include a column named "_id" or this class will not
work"
as explained in the documentation here. Unfortunately the code examples do not make this very clear.
It's quite convenient in many cases to have an id field. I prefer mine to be auto-incrementing (as shown below). I'm always finding new uses for the id field :)
When it comes time to attach the data to an adapter, I like to use a table name alias to query the id field as _id. Example: SELECT id _id, msg from message order by id. That way the adapter sees a field called _id and everybody's happy.
Here's a sample of how I define my tables:
CREATE TABLE message (_id INTEGER PRIMARY KEY AUTOINCREMENT, timestamp INTEGER, tripID TEXT, msg TEXT);
From the official docs...
The Cursor must include a column named "_id" or this class will not work. Additionally, using MergeCursor with this class will not work if the merged Cursors have overlapping values in their "_id" columns.
And the Cursor is:
This interface provides random read-write access to the result set returned by a database query.
In other words, you need _id for Android SQLite ( which usually uses Cursor )
If you define your _id column as an autoincrementing integer it is actually an alias for the ROWID column that SQLite provides by default (https://www.sqlite.org/lang_createtable.html#rowid).
Your create statement needs take the form...
CREATE TABLE t(_id INTEGER PRIMARY KEY ASC, y, z);
To prove this works...
UPDATE t SET _id=22 WHERE _id=11;
then
SELECT ROWID, _id FROM t;
and you'll find both _id and ROWID have the same value.
Note, that if you use DESC in the CREATE a new column is created and ROWID is not aliased.
Surely not.
Its a convenience field that some widgets like ListView uses to populate data. See this good article:
http://www.casarini.org/blog/2009/android-contentprovider-on-sqlite-tables-without-the-_id-column/
Of course if you are creating your own UI widget and your own adapter, you don't have to name your primary key as "_id". It can be any name you want. But you would be responsible for managing your collections of UI widgets and binding them to the right row in your database. "_id" is only useful for ListView as Brad has pointed out.
The _id field is indeed necessary in sqlite, it will help you to select a particular data from sqlite.
SELECT name from table_name where _id = ?
And if your are creating a recyclerview/ listview and you want a detailed activity for that list item you indeed need an id for this to fetch data of that item.
if you are creating a class for constants there is a BaseColumn interface in android,
which provide _ID field to that constant class.
//from android documentation..
public static class FeedEntry implements BaseColumns {
public static final String TABLE_NAME = "entry";
public static final String COLUMN_NAME_TITLE = "title";
public static final String COLUMN_NAME_SUBTITLE = "subtitle";
}