I have a pre-established SQLite Database in my application. It has a table with rows about 20 rows of text. I want to be able to add additional rows to the table without deleting all of the previous information. The only way I have seen which would allow me to do this is to delete all of the previous databases and then recreate it with the new rows. There must be a better way. Thanks for your help!
Are you confusing rows with columns?
If you really do mean rows then as antlersoft points out, using the SQL INSERT INTO statement will simply add a new row to a table without affecting any existing table data. This is one of the most basic and commonly used SQL statements.
If you actually mean you need to add columns then use the SQL ALTER TABLE statement.
See..
SQL INSERT INTO statement
SQL ALTER TABLE statement
The Android framework, as it relates to SQLite (using a SQLiteOpenHelper) provides two distinct methods for handling database lifecycles - onCreate(), used when the database needs to be created from scratch, and onUpgrade(<database>, int oldVersion, int newVersion) for handling updates. You can specify the "new" version number in the constructor for the superclass of your SQLiteOpenHelper, and the framework knows to call onUpgrade() based on this parameter and the internal version # in the actual sqlite database.
So, to modify your database during a version change just override onUpgrade() and run whatever SQLite stuff that you need.
Related
Why sqlite database has the limitation of "not to drop a column from a table"
in a single command?
Are there any chances it would be added in future versions?
ALTER TABLE DROP COLUMN is now supported, as of 3.35.0 which was released 2021-03-12.
SQLite supports a limited subset of ALTER TABLE. The ALTER TABLE
command in SQLite allows the user to rename a table or to add a new
column to an existing table. It is not possible to rename a column,
remove a column, or add or remove constraints from a table.
https://sqlite.org/lang_altertable.html
No, you can't.
Are there any chances it would be added in future versions?
Well, we can't tell you that. And even if they will add it, there are still a lot of Android devices out there which have an older version of SQLite without that feature.
However, you can delete a column like this from this answer:
https://stackoverflow.com/a/5987838/5457878
The ALTER TABLE statement supports exactly those operations that can be implemented easily without having to rewrite the entire table. (When adding a new column, that value is missing in all rows; it gets replaced with the column's default value when read.)
The ability to drop columns (or other things, like reordering columns) would add lots of complex code to the SQLite library.
However, SQLite is designed as an embedded database, so it must take care to conserve resources. It is very unlikely that such a large amount of code will ever be added.
http://www.sqlite.org/faq.html#q11
SQLite has limited ALTER TABLE support that you can use to add a column to the end of a table or to change the name of a table. If you want to make more complex changes in the structure of a table, you will have to recreate the table. You can save existing data to a temporary table, drop the old table, create the new table, then copy the data back in from the temporary table.
So you can't just delete the column
I am working on a project on android where thousands of tables exist.Unfortunately I do not have fresh data base so what can I do to delete all record from all tables. i am using sqlite database .If it is possible please tell me.
If you want to remove the entire rows/data from table, then it's better to drop the table and recreate it. This is the usual convention and for this reason SqLiteOpenHelper has onCreate() and onUpgrade() method which creates table or upgrades it, when table is dropped, or database version is changed
For deleting/dropping table, the code fragment is
db.execSQL("DROP TABLE IF EXISTS " + YOUR_TABLE);
Check the first answer for reference What happens if an android app, which creates database, is then uninstalled?
I'm trying to write an android app that contains a database that would dynamically change its schema based on user input.
For example, suppose that you initially have a table in which the only column is a column for different breeds of puppies. This would be the primary key. The user can then dynamically add new attributes which would correspond to new columns in this table (e.g. color, has spots, size, etc.)
I was wondering whether the ALTER TABLE query must be executed in onUpgrade or whether I can do it in another method within the SQLiteOpenHelper subclass. I don't really know if it is really necessary to increment the database version every time the user wants to add a new attribute. Thanks!
I was wondering whether the ALTER TABLE query must be executed in onUpgrade or whether I can do it in another method within the SQLiteOpenHelper subclass.
You are welcome to execute ALTER TABLE statements whenever you want, though (as with all database I/O) on a background thread, please.
In your case, I do not know why you are using SQLiteOpenHelper, though. The point behind SQLiteOpenHelper is to help developers building apps with fixed (per version) schemas. That is not the route that you are taking, in which case SQLiteOpenHelper may not really be helping much.
onUpgrade will only be called when the database file already exists but the stored user version is lower than requested in constructor. I have used this function to carry out the changes I made to the database schema when I release a new version of the app. But if you want to add columns dynamically, yes you can use the ALTER TABLE command.
I have an app published in the play store.
The app uses a database which holds a table which has a column of type int.
I'm doing a new change where I need to change the column type to long.
How do I go about handling it in the DatabaseHandler I'v created.
I want to preserve the data stored in the older apps database, so what should ideally be the code in the onUpgrade() function???
You don't need to change the database column type. An INTEGER column will happily contain all the bits needed to represent a Java long.
In fact, there's no long column type in sqlite.
I think using SQLite, the best way is to create a temporary table, copy all your table content, drop the old table and recreate the table with the right type on your column, then you can just copy the content from the temporary table and drop it...
I know this don't fell like the best approach, but I don't think SQLite have some alter table function.
As far I know you can t do this . But You can drop your table if it exists and create it again . Maybe you can find out some useful information here SQLite Modify Column or here Modify a Column's Type in sqlite3
The Android app that I am currently working on dynamically adds columns to an SQLite database. The problem I have is that I cannot figure out a way to remove these columns from the database.
If I add column A, B, C, D, and E to the database, is it possible to later remove column C?
I have done a lot of looking around and the closest thing I could find was a solution that requires building a backup table and moving all the columns (except the one to be deleted) into that backup table.
I can't figure out how I would do this, though. I add all the columns dynamically so their names are not defined as variables in my Java code. There doesn't seem to be a way to retrieve a column name by using Android's SQLiteDatabase.
SQLite has limited ALTER TABLE support that you can use to add a column to the end of a table or to change the name of a table.
If you want to make more complex changes in the structure of a table, you will have to recreate the table. You can save existing data to a temporary table, drop the old table, create the new table, then copy the data back in from the temporary table.
For example, suppose you have a table named "t1" with columns names "a", "b", and "c" and that you want to delete column "c" from this table. The following steps illustrate how this could be done:
BEGIN TRANSACTION;
CREATE TEMPORARY TABLE t1_backup(a,b);
INSERT INTO t1_backup SELECT a,b FROM t1;
DROP TABLE t1;
CREATE TABLE t1(a,b);
INSERT INTO t1 SELECT a,b FROM t1_backup;
DROP TABLE t1_backup;
COMMIT;
SQLite doesn't support a way to drop a column in its SQL syntax, so its unlikely to show up in a wrapper API. SQLite doesn't often support all features that traditional databases support.
The solutions you've identified make sense and are ways to do it. Ugly, but valid ways to do it.
You can also 'deprecate' the columns and not use them by convention in newer versions of your app. That way older versions of your app that depend on column C won't break.
Oh... just noticed this comment:
The app is (basically) an attendance tracking spreadsheet. You can add
a new "event" and then indicate the people that attended or didn't.
The columns are the "events".
Based on that comment you should just create another table for your events and link to it from your other table(s). You should never have to add columns to support new domain objects like that. Each logical domain object should be represented by its own table. E.g. user, location, event...
Was writing this initially. Will keep it if you're interested:
Instead of dynamically adding and removing columns you should consider using an EAV data model for that part of your database that needs to be dynamic.
EAV data models store values as name/value pairs and the db structure never needs to change.
Based on your comment below about adding a column for each event, I'd strongly suggest creating a second table in which each row will represent an event, and then tracking attendance by storing the user row id and the id of the event row in the attendance table. Continually piling columns onto the attendance table is a definite anti-pattern.
With regards to how to find out about the table schema, you can query the sqlite_master table as described in this other SO question - Is there an SQLite equivalent to MySQL's DESCRIBE [table]?
As per SQLite FAQ, there is only limited support to the ALTER TABLE SQL command. So, the only way you can do is that ou can save existing data to a temporary table, drop the old table, create the new table, then copy the data back in from the temporary table.
Also you can get the column name from the database using a query. Any query say "SELECT * FROM " gives you a cursor object. You can use the method
String getColumnName(int columnIndex);
or
String[] getColumnNames();
to retrieve the names of the columns.