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?
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 ...;
Related
I'm trying to a have a table with an auto incremented primary key. The SQL query for table creation is included.
Problem is the auto-increment does not work. Meaning when I insert a row with NULL as the value of conversation_id it just inserts null. I have this problem on multiple tables.
-- Table: conversations
CREATE TABLE conversations (
conversation_id INTEGER (64) PRIMARY KEY
UNIQUE,
target_id BIGINT (64),
sender_id BIGINT (64),
status STRING (16) NOT NULL
DEFAULT unseen,
is_group INTEGER (1) NOT NULL
DEFAULT (0),
last_update INTEGER (32) DEFAULT (0),
target_name STRING (64),
target_photo STRING (256),
unread_count INTEGER (10) DEFAULT (0),
last_message STRING (256)
);
The following is the method I use to insert into table:
public Conversation addConversation(Conversation conversation) {
SQLiteDatabase db = getWritableDatabase();
ContentValues row = new ContentValues();
row.put("target_id", conversation.getTargetID());
row.put("sender_id", conversation.getSenderID());
row.put("target_name", conversation.getTargetName());
row.put("target_photo", conversation.getTargetPhoto());
row.put("status", conversation.getStatus());
row.put("unread_count", conversation.getUnreadCount());
row.put("last_message", conversation.getLastMessage());
conversation.setConversationID(db.insert(TBL_CONVERSATIONS, null, row));
Log.d(TAG, "conversation added: "+conversation.getConversationID());
db.close();
return conversation;
}
The curious thing here is when I retrieve the insert id from insert method it returns the correct value, but the actual database field is null.
If I understand correctly A column declared INTEGER PRIMARY KEY will autoincrement. [Cite]
From documentation:
A table created using CREATE TABLE AS has no PRIMARY KEY and no
constraints of any kind. The default value of each column is NULL.
You don't have to add UNIQUE constraint on a COLUMN that has PRIMARY KEY constraint.
Explanation:
A UNIQUE constraint is similar to a PRIMARY KEY constraint, except
that a single table may have any number of UNIQUE constraints.
Instead add NOT NULL.
This is why:
According to the SQL standard, PRIMARY KEY should always imply NOT
NULL. Unfortunately, due to a bug in some early versions, this is not
the case in SQLite. Unless the column is an INTEGER PRIMARY KEY or the
table is a WITHOUT ROWID table or the column is declared NOT NULL,
SQLite allows NULL values in a PRIMARY KEY column. SQLite could be
fixed to conform to the standard, but doing so might break legacy
applications. Hence, it has been decided to merely document the fact
that SQLite allowing NULLs in most PRIMARY KEY columns.
I recommend using this Column definition:
CREATE TABLE conversations (
conversation_id INTEGER PRIMARY KEY NOT NULL AUTOINCREMENT,
...
}
Most likely the return value you are seeing is the row's ROWID. A ROWID is a hidden column available in every table, unless explicitly removed. According to the official documentation, when you define an INTEGER PRIMARY KEY, it should automatically become an alias for the ROWID. That's also why AUTOINCREMENT is not needed when you define your column in this way.
With one exception noted below, if a rowid table has a primary key
that consists of a single column and the declared type of that column
is "INTEGER" in any mixture of upper and lower case, then the column
becomes an alias for the rowid. Such a column is usually referred to
as an "integer primary key". A PRIMARY KEY column only becomes an
integer primary key if the declared type name is exactly "INTEGER".
Other integer type names like "INT" or "BIGINT" or "SHORT INTEGER" or
"UNSIGNED INTEGER" causes the primary key column to behave as an
ordinary table column with integer affinity and a unique index, not as
an alias for the rowid.
See: CREATE TABLE documentation
Either your column is not an INTEGER, or it is not a PRIMARY KEY. Taking a closer look at your create-statement I can see one or two possible culprits.
UNIQUE vs. PRIMARY KEY
A primary key is unique by default. According to the syntax definition (which you can find on the same documentation page as the citation above) you should choose either PRIMARY KEY or UNIQUE, not both.
COLUMN length restrictions
ROWID is already 64-bit by default. You have specified length 64, but lengths are not specified in bits. You may have specified a 64-byte integer here, which I'm sure was not intended. This should actually not be a problem however, since SQLite ignores length-constraints. So it is not meaningful to specify them.
TLDR
Replace this code:
conversation_id INTEGER (64) PRIMARY KEY UNIQUE
With this:
conversation_id INTEGER PRIMARY KEY AUTOINCREMENT
I just put autoincrement in the query and it works fine .
like this
id integer primary key autoincrement
According to some blogs like http://reigndesign.com/blog/using-your-own-sqlite-database-in-android-applications/ and even in some of the aswers here.
One of the first steps before including the datababe into the project is to rename the primary id field of your tables to "_id" so Android will know where to bind the id field of your tables.
What should be done with a table that have a combined primary key
Assume that i'm creating the relation between the product id and the store id to assign it's price.
CREATE TABLE `Products-Stores` (
`product` INTEGER NOT NULL,
`store` INTEGER NOT NULL,
`price` INTEGER NOT NULL,
PRIMARY KEY(product,store)
);
There is no need to rename any column in your database. SQL allows column aliases like this:
SELECT integer_primary_key AS _id
...
The only time this is necessary is when you are using a ListAdapter to display the contents of a cursor queried from your DB. You must have an integer primary key column, named "_id" in the cursor, to do that
Better yet, every SQLite database table has an implicit column named "rowid". You don't even have to have your own integer primary key column. Just use rowid, like so:
SELECT rowid AS _id
...
EDITED TO INCLUDE #CL's EXPLANATION OF WORKING JOINS
Obviously, this trick won't work, for many kinds of joins. As long as the rowids are unique over all the rows in the join, though, it works fine.
I am using SQLite to work on an android application. What I have learned so far is that a table made in SQLite requires a column called _id. So, for instance, to create a table, I use the following SQL Statement:
CREATE TABLE IF NOT EXISTS Semesters(
_id INTEGER PRIMARY KEY AUTOINCREMENT,
semester_name TEXT,
is_current TEXT
GPA REAL);
However, now if I want to create a seperate table in the same database, what would I name the primary key column? For instance, this is the table I want to create:
CREATE TABLE IF NOT EXISTS Classes(
_id INTEGER PRIMARY KEY AUTOINCREMENT,
course_name TEXT);
Now, both the semesters table and the courses table have a column called _id, and if I want to make a foreign key reference to the _id column in the semesters table, I will have to call it by the table name. Is there any way to make this simpler by using different names for ids?
Thanks.
For example, you make a third table and want to use both primary keys from your tables as foreign keys:
CREATE TABLE ClassSemester(
_id INTEGER PRIMARY KEY AUTOINCREMENT,
semester_id INTEGER,
class_id INTEGER,
FOREIGN KEY(semester_id) REFERENCES Semesters(_id),
FOREIGN KEY(class_id) REFERENCES Classes(_id)
);
More details in here:
http://www.sqlite.org/foreignkeys.html
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
I have a table with a composite primary key and I am having trouble inserting. The code used to create the table is:
CREATE TABLE ClassEvent (
EventName varchar(10) NOT NULL,
CourseId varchar(10) NOT NULL,
EventType varchar(20),
EventWeight number(3),
DueDate DATE NOT NULL,
FOREIGN KEY (CourseId) REFERENCES Courses(CourseId),
PRIMARY KEY (CourseId, EventName));
The problem I am having is when I want to insert records that have values that may not be unique for the columns CourseId or EventName, but are a unique combination of the 2.
for example, if I try to run the following 2 inserts:
INSERT INTO ClassEvent VALUES('Assignment 1','60-415','Assignment',10,'12/10/2010');
INSERT INTO ClassEvent VALUES('Project 1','60-415','Project',15,'5/12/2010');
I get the following error:
Error: columns CourseId, EventName are not unique.
and the second insert does not make it into the DB. Why does this error out? I thought that a composite primary key requires that the combination of both values are unique. In my above inserts, the values for the EventName column are different even though the values for CourseId are the same. Shouldn't this be seen as 2 unique combinations and thus 2 different primary keys?
My table needs to be able to hold several different events for each CourseId, but each Event must be unique for each Course. I need to be able to insert values into the table like:
EventName CourseId
Assignment 1 60-415
Project 1 60-415
Assignment2 60-415
Project 2 60-415
Assignment 1 60-367
Project 1 60-367
and so on. Can anyone tell me how I can get this to work? Why are these composite PK's not being seen as unique entries? Any help would be much appreciated.
Here is the java function I am using for the insert:
public void addNewClassEvent(ContentValues values) {
SQLiteDatabase db = openConnection();
db.insert("ClassEvent", null, values);
db.close();
}
Could this be causing the problem?
You can have a composite primary key in SQLite, but you
have to create the key when you create the table:
CREATE TABLE example1(
field1 FLOAT,
field2 TEXT,
PRIMARY KEY(field1, field2)
);
You cannot create the primary key after the fact using ALTER TABLE.
On the other hand, you can create a UNIQUE INDEX after the fact
which has essentially the same effect as a PRIMARY KEY:
CREATE UNIQUE INDEX pk_index ON "table1"("field1","field2");
I am not sure how you have created, the tables, and if you have added the primary index later, but grab the database to your desktop, and check out how works in a desktop environment.
You can't make combinations like that, but you don't need them. What is stopping you from just having a truly id column ?