I am wondering how can I insert an element at the beginning of the data base? I want to do this because:
a) I want to display the elements (ListView) as 'last-inserted on top and first-inserted on the bottom' (like a stack)
b) I want to limit the amount of elements in my db. When a new element is added (over the limit) I can put the new one (at the beginning) and delete the last one. (Don't know yet how to delete the last element).
I was searching for a solution but I am starting to wonder I have any control of how the element are inserted. If not I was thinking about displaying the database from end to bottom. But don't actually know how to do it since the cursor is always set at the beginning of the db. If could achieve this I can solve b) by deleting the first element (again don't know how to achieve this yet).
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"
To get last inserted at top and first inserted at bottom....
SELECT * from mytable order by ROWID desc;
Read more about ROWID here
And, to delete the oldest:
DELETE from mytable WHERE ROWID = (SELECT MIN(ROWID) FROM mytable);
If you use the ROWID, you do not have to modify your existing table. And, the value of ROWID is guaranteed.
You can store a timestamp (as a new field) when you insert a record into your database. Then you delete the oldest record you can find if it is over the limit (e.g. using MAX SQL keyword). You can display the records in reverse-chronological order by sorting DESC with your timestamp field.
Related
I am using this query
"select * from SomeTable group by SomeColumn"
It is returns list with accenting order, but i need to same order like in database.
For example the order in database is:
p
a
s
But result is:
a
i
p
Sample
The result need to be like distinct by CityEN but with all columns and order like 1.Paris 2.Amsterdam 3.Istanbul
In Sqlite, each row of a table has a unique rowid, which you can use for sorting.
select * from SomeTable group by SomeColumn order by rowid;
In your statement, add this line to sort the results:
order by min(rowid)
Your query does not enforce any order with ORDER BY clause so no assumption about row order should be made. If you want specific order add i.e. ORDER BY SomeColumn. See docs about all available order options: https://www.sqlite.org/lang_select.html#orderby
By the rules of SQL, you can't count on getting records back in any specific order without specifying an ORDER BY clause in your SQL query.
In practice servers sometimes return values in the order in which they're inserted, in the order of the first index created, or in the order of the primary key--but you can't count on this behavior, and in fact I've seen the behavior change between database maintenance windows or after the database version is upgraded. You definitely wouldn't want to count on a DB engine to give you back records in any particular order if you write a SELECT statement without an ORDER BY clause.
The only real way to get your records back in the order you inserted them is to create a timestamp column and then sort on it during the SELECT. If you don't want to worry about populating that column on INSERT, have that column auto-populate itself with a timestamp (depending on your DB engine).
I was just wondering if it's possible to just input/update a single value into an SQLiteDatabase (as opposed to an entire row).
The SQLiteDatabase API on the Android developer site makes it sound like you can only insert one row at a time (which erases previous information).
An alternative I've been thinking of is to copy the contents of the entire row into an array, alter a value in the array, and then insert the values of the array as an update row in the SQL Database. Does anyone have a more efficient and less roundabout solution?
The SQLiteDatabase API on the Android developer site makes it sound like you can only insert one row at a time (which erases previous information).
A standard SQL INSERT statement does not erase "previous information". It adds a new row to a table.
I was just wondering if it's possible to just input/update a single value into an SQLiteDatabase (as opposed to an entire row).
By definition, it is not possible to insert a single value into a table. You insert rows into a table. Any columns that you do not provide values for in your INSERT statement need to either allow NULL values or have default values defined on the table.
A SQL UPDATE statement, however, can update a single column in a row. This is true whether you use update() or execSQL() on SQLiteDatabase to update that row.
then insert the values of the array as an update row in the SQL Database
A standard SQL INSERT statement does not update a row. It adds a new row to a table.
I've got two SQLite databases, each with a table that I need to keep synchronized by merging rows that have the same key. The tables are laid out like this:
CREATE TABLE titles ( name TEXT PRIMARY KEY,
chapter TEXT ,
page INTEGER DEFAULT 1 ,
updated INTEGER DEFAULT 0 );
I want to be able to run the same commands on each of the two tables, with the result that for pairs of rows with the same name, whichever row has the greater value in updated will overwrite the other row completely, and rows which do not have a match are copied across, so both tables are identical when finished.
This is for an Android app, so I could feasibly do the comparisons in Java, but I'd prefer an SQLite solution if possible. I'm not very experienced with SQL, so the more explanation you can give, the more it'll help.
EDIT
To clarify: I need something I can execute at an arbitrary time, to be invoked by other code. One of the two databases is not always present, and may not be completely intact when operations on the other occur, so I don't think a trigger will work.
Assuming that you have attached the other database to your main database:
ATTACH '/some/where/.../the/other/db-file' AS other;
you can first delete all records that are to be overwritten because their updated field is smaller than the corresponding updated field in the other table:
DELETE FROM main.titles
WHERE updated < (SELECT updated
FROM other.titles
WHERE other.titles.name = main.titles.name);
and then copy all newer and missing records:
INSERT INTO main.titles
SELECT * FROM other.titles
WHERE name NOT IN (SELECT name
FROM main.titles);
To update in the other direction, exchange the main/other database names.
For this, you can use a trigger.
i.e.
CREATE TRIGGER sync_trigger
AFTER INSERT OR UPDATE OF updated ON titles
REFERENCING NEW AS n
FOR EACH ROW
DECLARE updated_match;
DECLARE prime_name;
DECLARE max_updated;
BEGIN
SET prime_name = n.name;
ATTACH database2name AS db2;
SELECT updated
INTO updated_match
FROM db2.titles t
WHERE t.name=prime_name)
IF updated_match is not null THEN
IF n.updated > updated_match THEN
SET max_updated=n.updated;
ELSE
SET max_updated=updated_match;
END IF;
UPDATE titles
SET updated=max_updated
WHERE name=prime_name;
UPDATE db2.titles
SET updated=max_updated
WHERE name=prime_name;
END IF;
END sync_trigger;
The syntax may be a little off. I don't use triggers all that often and this is a fairly complex one, but it should give you an idea of where to start at least. You will need to assign this to one database, exchanging "database2name" for the other database's name and then assign it again to the other database, swapping the "database2name" out for the other database.
Hope this helps.
Say my SQLite Databate has 2 columns, the first being an auto-incrementing ID and the 2nd being some string. Say right now it's
1 random
2 jellybean
3 ImTired
if I were to delete entry 2, it would then be
1 random
3 ImTired
What I want is a way to make it so when you delete entry 2, it turns it into
1 random
2 ImTired
I thought about updating the entries to shift them all down one and delete the last one, but even if it worked(in my case, it deleted all of my entries, but whatever...), and even if I did get it to
1 random
2 ImTired
the next time I create a new entry, it'll be entry 4. I don't think this necessary to my app, but it seriously bugs me.
The ID column on your DB is working as a Primary Key, which is a column or group of columns used to uniquely identify a row. Once you set a Primary Key on a row you shouldn't change it, else you risk losing the consistency of the DB. For instance, suppose you later create another table that references the rows in your first table. That reference will be made using the Primary Key, and if you later change it your data won't make sense anymore.
If you wanted the ID column to keep changing just to reflect the number of rows in your table you can solve that problem with other methods. For instance. SQL offers a COUNT operator that will return the number of rows in your table:
SELECT COUNT(*) FROM Table_name;
I want to get the number of NOT NULL records from my SQLite database. Since I'm using autoincrement, the last inserted row won't give me the real number of records in the table as even if I delete any middle record, it'll insert at the position higher than the last inserted record.
The insert statement returns me the number of last inserted row, but I want this value on the fly.
Doing a count before on the table should work. Simply query for the id column with the where check of NOT NULL and on the returned cursor just call the getCount()
Just to be sure: You should never ever, really never ever, manipulate the auto increment in a productive database. If you delete a record, than the "gap" should stay there. It has no impact on performance or anything else. If you insert a new record in the gap, you can create a lot of trouble...
So you just want to find the number of rows? (There no such thing as a "null record" as far as I'm aware.)
Can you not just do
select count(1) from YourTableName
or
select count(*) from YourTableName
? (Some databases are faster using one form or other... I don't know about sqlite.)