Is there a way to read the value of the last record inserted in an SQLite table without going through the previous records ?
I ask this question for performance reasons.
There is a function named sqlite3_last_insert_rowid() which will return the integer key for the most recent insert operation. http://www.sqlite.org/c3ref/last_insert_rowid.html
This only helps if you know the last insert happened on the table you care about.
If you need the last row on a table, regardless of wehter the last insert was on this table or not, you will have to use a SQL query
SELECT * FROM mytable WHERE ROWID IN ( SELECT max( ROWID ) FROM mytable );
When you sort the records by ID, in reverse order, the last record will be returned first.
(Because of the implicit index on the autoincrementing column, this is efficient.)
If you aren't interested in any other records, use LIMIT:
SELECT *
FROM MyTable
ORDER BY _id DESC
LIMIT 1
Related
I am learning SQLite and constructed a line which I thought would delete dups but it deletes all rows instead.
DELETE from tablename WHERE rowid not in (SELECT distinct(timestamp) from tablename);
I expected this to delete rows with a duplicate (leaving one). I know I can simply create a new table with the distinct rows, but why does what I have done not work? Thanks
If timestamp is a column in the table and this is what you want to compare so to delete duplicates then do this:
delete from tablename
where exists (
select 1 from tablename t
where t.rowid < tablename.rowid and t.timestamp = tablename.timestamp
)
With recent versions of sqlite, the following is an alternative:
DELETE FROM tablename
WHERE rowid IN (SELECT rowid
FROM (SELECT rowid, row_number() OVER (PARTITION BY timestamp) AS rownum
FROM tablename)
WHERE rownum >= 2);
why does what I have done not work?
Consider the WHERE condition:
rowid not in (SELECT distinct(timestamp) from tablename)
The simple answer is that you are not comparing data in the same columns, nor are they columns with the same type of data. rowid is an automatically-incremented integer column and I assume that timestamp column is either a numeric or string column containing time values, or perhaps custom-generated sequential numeric values. Because rowid likely never matches a value in timestamp, then the NOT IN operation will always return true. Thus each row of the table will be deleted.
SQL is rather explicit and so there are no hidden/mysterious column comparisons. It will not automatically compare the rowid's from one query with another. Notice that the various alternative statements do something to distinguish rows with duplicate key values (timestamp in your case), either by direct comparison between main query and subquery, or using windowing functions to uniquely label rows with duplicate values, etc.
Just for kicks, here's another alternative that uses NOT IN like your original code.
DELETE FROM tablename
WHERE rowid NOT IN (
SELECT max(t.rowid) FROM tablename t
GROUP BY t.timestamp )
First notice that this is comparing rowid with max(t.rowid), values which derive from the same column.
Because the subquery groups on t.timestamp, the aggregate function max() will return the greatest/last t.rowid separately for each set of rows with the same t.timestamp value. The resultant list will exclude t.rowid values that are less than the maximum. Thus, the NOT IN operation will not find those lesser values and will return true so they will be deleted.
It also uses basic SQL (no window functions... the OVER keyword). It will likely be more efficient than the alternative that references the outer query from the subquery, because this statement can execute the subquery just once and then use an efficient index to match individual records... it doesn't need to rerun the query for each row. For that matter, it should also be more efficient than the windowing function, because the window partition essentially "groups" on the partitioned columns, but must then execute the windowing function for each row, an extra step not present in the basic aggregate query. Efficiency is not always critical, but something important to consider.
By the way, the distinct keyword is not a function and does not need/accept parenthesis. It is a directive that applies to the entire select statement. The subquery is being interpreted as
SELECT DISTINCT (timestamp) FROM tablename
where DISTINCT is interpreted in isolation and the parenthesis are interpreted as a separate expression.
Update
These two queries will return the same data:
SELECT DISTINCT timestamp FROM tablename;
SELECT timestamp FROM tablename GROUP BY timestamp;
Both results eliminate duplicate rows from the output by showing only unique/distinct values, but neither has a "handle" (other data column) which indicates which rows to keep and which rows to eliminate. In other words, these queries return distinct values, but the results loose all relationship to the source rows and so have no use in specifying which source rows to delete (or keep). To understand better, you should run subqueries separately to inspect what they return so that you can understand and verify what data you're working with.
To make those queries useful, we need to do something to distinguish rows with duplicate key values. The rows need a "handle"--some other key value to select for either deleting or keeping those rows. Try this...
SELECT DISTINCT rowid, timestamp FROM tablename;
But that won't work, because it applies the DISTINCT keyword to ALL returned columns, but since rowid is already unique it will necessarily output each row separately and so there is no use to the query.
SELECT max(rowid), timestamp FROM tablename GROUP BY timestamp;
That query preserves the unique grouping, but provides just one rowid per timestamp as the "handle" to include/exclude for deletion.
try this
DELETE liens from liens where
id in
( SELECT * FROM (SELECT min(id) FROM liens group by lkey having count(*) > 1 ) AS c)
you can do this many times
I have a relatively simple select query which asks for rows by an column value (this is not controlled by me). I pass in a variable argument of id values to be returned. Here's an example:
select * from team where id in (2, 1, 3)
I'm noticing that as the database changes its order over time, my results are changing order as well. Is there a way to make SQLite guarantee results in the same order as the arguments?
If you could have so many IDs that the query becomes unwieldy, use a temporary table to store them:
CREATE TEMPORARY TABLE SearchIDs (
ID,
OrderNr INTEGER PRIMARY KEY
);
(The OrderNr column is autoincrementing so that it automatically gets proper values when you insert values.)
To do the search, you have to fill this table:
INSERT INTO SearchIDs(ID) VALUES (2), (1), (3) ... ;
SELECT Team.*
FROM Team
JOIN SearchIDs USING (ID)
ORDER BY SearchIDs.OrderNr;
DELETE FROM SearchIDs;
Try this!
select * from team order by
case when 2 then 0
when 1 then 1
when 3 then 2
end
I want to delete first 10 records(i.e. records 1 to 10) from my table in SQLite if the record count is 110. Only 100 records I want in my table. If record count is more than 100 then those records should be deleted, as a result if I add new records then also only 100 records should be present. How could it be possible using SQLite query, please provide your suggestions. Thanks.
I gather you want to maintain 100 rows in your table by removing the first 'X' rows every time your table grows beyond 100 rows. Here is what you can do:
Delete from table_name where rowid IN (Select rowid from table_name limit X);
This will keep removing the first 'X' rows as and when your SQLite table grows.
First you have to do a select that returns the last 100 records in your table, then use the delete command to remove all ids not in the result, something like this:
DELETE FROM tbl_name WHERE NOT EXISTS in (SELECT id FROM tbl_name ORDER BY id LIMIT 100 );
Hope this helps!
DELETE FROM {{ table }} LIMIT 0, 10
I'm not entirely sure if that's correct and I don't have the ability to check at this moment.
Maybe i should do this in C# but i have more then one row with linkId X. I would like to remove it but i am unsure how. In code i could just use a foreach from 0 to n and remove any found rows with a greater (or !=) id but thats in code. Is there a less difficult way of doing it using sqlite?
Assuming the table's name is tableName and there is a primary key field named id, the following sql would do it. I think the following SQL query is general enough and should be able to be executed under any database engine.
delete from tableName
where id not in (
select min(id) from tableName
group by linkId
)
I want to get the number of selected rows as well as the selected data. At the present I have to use two sql statements:
one is
select * from XXX where XXX;
the other is
select count(*) from XXX where XXX;
Can it be realised with a single sql string?
I've checked the source code of sqlite3, and I found the function of sqlite3_changes(). But the function is only useful when the database is changed (after insert, delete or update).
Can anyone help me with this problem? Thank you very much!
SQL can't mix single-row (counting) and multi-row results (selecting data from your tables). This is a common problem with returning huge amounts of data. Here are some tips how to handle this:
Read the first N rows and tell the user "more than N rows available". Not very precise but often good enough. If you keep the cursor open, you can fetch more data when the user hits the bottom of the view (Google Reader does this)
Instead of selecting the data directly, first copy it into a temporary table. The INSERT statement will return the number of rows copied. Later, you can use the data in the temporary table to display the data. You can add a "row number" to this temporary table to make paging more simple.
Fetch the data in a background thread. This allows the user to use your application while the data grid or table fills with more data.
try this way
select (select count() from XXX) as count, *
from XXX;
select (select COUNT(0)
from xxx t1
where t1.b <= t2.b
) as 'Row Number', b from xxx t2 ORDER BY b;
just try this.
You could combine them into a single statement:
select count(*), * from XXX where XXX
or
select count(*) as MYCOUNT, * from XXX where XXX
To get the number of unique titles, you need to pass the DISTINCT clause to the COUNT function as the following statement:
SELECT
COUNT(DISTINCT column_name)
FROM
'table_name';
Source: http://www.sqlitetutorial.net/sqlite-count-function/
For those who are still looking for another method, the more elegant one I found to get the total of row was to use a CTE.
this ensure that the count is only calculated once :
WITH cnt(total) as (SELECT COUNT(*) from xxx) select * from xxx,cnt
the only drawback is if a WHERE clause is needed, it should be applied in both main query and CTE query.
In the first comment, Alttag said that there is no issue to run 2 queries. I don't agree with that unless both are part of a unique transaction. If not, the source table can be altered between the 2 queries by any INSERT or DELETE from another thread/process. In such case, the count value might be wrong.
Once you already have the select * from XXX results, you can just find the array length in your program right?
If you use sqlite3_get_table instead of prepare/step/finalize you will get all the results at once in an array ("result table"), including the numbers and names of columns, and the number of rows. Then you should free the result with sqlite3_free_table
int rows_count = 0;
while (sqlite3_step(stmt) == SQLITE_ROW)
{
rows_count++;
}
// The rows_count is available for use
sqlite3_reset(stmt); // reset the stmt for use it again
while (sqlite3_step(stmt) == SQLITE_ROW)
{
// your code in the query result
}