Develop Reference

Sqlite Dropping Column from table

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

Update database schema while preserving data (and relationships)

I have a simple database schema containing 5 tables and a few foreign key relationships. Now I want to update the schema without losing the relationships.
SQLite does not support the operation I want to perform. I want to mark a column AUTOINCREMENT. So I searched and found that I need to recreate the tables.
In all the relationships, foreign key is the auto-generated row ID. This will change when I re-insert the data (right?). And the best case scenario would be having mismatched relationships. The worst case (I think) would be having foreign keys that correspond to nothing. How do I avoid this?
One way that I can think of doing this is through dumping all data into a special model designed for this very exercise. This special model will encapsulate all the relationships. Then I can start creating new tables in order of ascending dependence, from least to most. Table that has no foreign keys (but other tables use its ID) goes in first. And on inserting rows, I update the model instance.
Is there a better way of doing this? Thank you for reading.

Values in an autoincrementing column can be set to an explicit value:
INSERT INTO MyTable(MyAutoincID, Name) VALUES(42, 'hello')
Just copy the ID together with the other column values.
Furthermore, as long as you do not enable foreign key checking with PRAGMA foreign_keys or setForeignKeyConstraintsEnabled(), your DB is allowed to have inconsistent data temporarily.

Changing a Column type in my Android App's Database

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

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

Can you delete columns in an SQLite database?

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.