CODEWITHSUNDEEP
androidsqlite
Willy
Willy

Reputation: 10650

What is the use of BaseColumns's _ID primary key in Android

It is a bit confusing.... If I implement BaseColumns for each of my tables it automatically create an autoincremented primary key for me called _ID but then Do I need to create my own primary keys for each of my tables as well or is it redundant or even not necessary?

In case I need my own primary key, let's say, _MyID, I guess the primary key will be formed by _ID and my own (_MyID), right? so in this case, it would be possible to insert the more than one register with the same _MyID.... as _ID is autoincremented automatically, that is:

_ID      _MyID  Other Fields.....
1         1000  ....
2         1000  ....
3         1000  ....

... and so on

so in this case, how to control that only one register can have the value 1000 for _MyID?

Also, I guess I can use _ID column to act as a foreign key with other tables, right?

Upvotes: 1

Views: 2609

Answers (1)

Karakuri
Karakuri

Reputation: 38605

The main use for BaseColumns._ID is that Android's CursorAdapter will look for that column name in the cursor you give it. There may be other classes that do the same, but I can't think of any off the top of my head. If you aren't using CursorAdapter, then there's really nothing binding you to using _id as a column name in your table and you can name the column however you like.

it automatically create an autoincremented primary key for me called _ID

There is nothing automatic about it based on what you've shown so far. You will only have such a column if you executed SQL like this:

CREATE TABLE tableName (_id INTEGER PRIMARY KEY AUTOINCREMENT, ...);

You can just as easily leave it out or give a different name to the primary key. Furthermore, there is nothing that requires your primary key to be auto-incremented; as long as the values are unique, it satisfies the primary key requirement. In other words, this is fine too:

CREATE TABLE tableName (_id INTEGER PRIMARY KEY, ...);

In case I need my own primary key, let's say, _MyID, I guess the primary key will be formed by _ID and my own (_MyID), right?

Not quite. You would have to do something like this:

CREATE TABLE tableName (_id INTEGER, _myId INTEGER, ..., PRIMARY KEY(_id, myId));

This creates a composite key, but note that neither of the two columns are themselves declared as primary key. Honestly though, if you don't need such an arrangement, then stick to one primary key.

One last thing:

If you are planning to use CursorAdapter, you might want to name the column _id for convenience, but even then you don't have to. All that matters is the cursor has a column by that name. The actual column in the table can have a different name, you just have to alias it at query time so that it has the proper name in the cursor:

SELECT _myId as _id, ... FROM ...;

Upvotes: 5

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