i have read tons of articles regarding last n records in Oracle SQL by using rownum functionality, but on my case it does not give me the correct rows.
I have 3 columns in my table: 1) message (varchar), mes_date (date) and mes_time (varchar2).
Inside lets say there is 3 records:
Hello world | 20-OCT-14 | 23:50
World Hello | 21-OCT-14 | 02:32
Hello Hello | 20-OCT-14 | 23:52
I want to get the last 2 records ordered by its date and time (first row the oldest, and second the newest date/time)
i am using this query:
SELECT *
FROM (SELECT message
FROM messages
ORDER
BY MES_DATE, MES_TIME DESC
)
WHERE ROWNUM <= 2 ORDER BY ROWNUM DESC;
Instead of getting row #3 as first and as second row #2 i get row #1 and then row #3
What should i do to get the older dates/times on top follow by the newest?
Maybe that helps:
SELECT *
FROM (SELECT message,
mes_date,
mes_time,
ROW_NUMBER() OVER (ORDER BY TO_DATE(TO_CHAR(mes_date, 'YYYY-MM-DD') || mes_time, 'YYYY-MM-DD HH24:MI') DESC) rank
FROM messages
)
WHERE rank <= 2
ORDER
BY rank
I am really sorry to disappoint - but in Oracle there's no such thing as "the last two records".
The table structure does not allocate data at the end, and does not keep a visible property of time (the only time being held is for the sole purpose of "flashback queries" - supplying results as of point in time, such as the time the query started...).
The last inserted record is not something you can query using the database.
What can you do? You can create a trigger that orders the inserted records using a sequence, and select based on it (so SELECT * from (SELECT * FROM table ORDER BY seq DESC) where rownum < 3) - that will assure order only if the sequence CACHE value is 1.
Notice that if the column that contains the message date does not have many events in a second, you can use that column, as the other solution suggested - e.g. if you have more than 2 events that arrive in a second, the query above will give you random two records, and not the actual last two.
AGAIN - Oracle will not be queryable for the last two rows inserted since its data structure do not managed orders of inserts, and the ordering you see when running "SELECT *" is independent of the actual inserts in some specific cases.
If you have any questions regarding any part of this answer - post it down here, and I'll focus on explaining it in more depth.
select * from table
minus
select * from table
where rownum<=(select count(*) from table)-n
Related
I have been given a task which I should look on items table and grab first item of 2019 and last item for 2019 and set the active flags on them as active , the query I wrote only I can grab one by one depends on the store, and it takes days to finish if I have no other choice, here is my query in SQL Server:
SELECT *
FROM NODES
WHERE NODE ID = 5562
AND DATE BETWEEN '2019/01/01' AND '2019/12/30'
Basically I need the first and the last item for the year, but the problem is every Node is a specific store which has many record and I have run the query for million of records in many Nodes, is it possible if I for example say OK SQL from the given nodes take first and last item for 2019 and display to me and then update their active flag = 'Y'
Is it possible with a CTE, do I need a CTE at all?
Thank you
If I understood correctly, you could try using a CTE with a windowed function to fetch only the first row from each store after ordering by date in ascending order and the first row from each store after ordering by date in descending order.
For instance :
CREATE TABLE NODES (NodeId int,NodeDate DATETIME2,status NVARCHAR(128))
INSERT INTO NODES(NodeId,NodeDate,Status) VALUES
(1,'2019/01/01','inactive'),
(1,'2019/03/01','inactive'),
(1,'2019/06/01','inactive'),
(1,'2019/09/01','inactive'),
(1,'2019/12/01','inactive'),
(2,'2019/01/01','inactive'),
(2,'2019/03/01','inactive'),
(2,'2019/06/01','inactive'),
(2,'2019/09/01','inactive'),
(2,'2019/12/01','inactive'),
(3,'2019/01/01','inactive'),
(3,'2019/03/01','inactive'),
(3,'2019/06/01','inactive'),
(3,'2019/09/01','inactive'),
(3,'2019/12/01','inactive')
;WITH cte AS
(
SELECT status,
ROW_NUMBER() OVER (PARTITION BY NodeId ORDER BY NodeDate ASC) AS FirstDate,
ROW_NUMBER() OVER (PARTITION BY NodeId ORDER BY NodeDate DESC) AS LastDate
FROM NODES
WHERE NodeDate >= '2019/01/01' AND NodeDate < '2020/01/01'
)
UPDATE CTE SET status = 'active'
WHERE FirstDate = 1 OR LastDate = 1
SELECT * FROM NODES
Try it online
Please do note however that this operation can be non deterministic if multiple rows have the same date.
See also :
Get top 1 row of each group
Background: I am running TeslaMate/Grafana for monitoring my car status, one of the gauges plots the battery level fetched from database. My server is located remotely and running in a Dock from an old NAS, so both query performance and network overhead matters.
I found the koisk page frequently hangs and by investigation, it might caused by the query -- two of the plots returns 10~100k rows of results from database. I want to limit the number of rows returned by SQL queries, as the plots certainly don't have that much precision for drawing such detailed intervals.
I tried to follow this answer and use row_number() to pop only 100-th rows of results, but more complicated issues turned up, that is, the time intervals among rows are not consistent.
The car has 4 status, driving / online / asleep / offline.
If the car is at driving status, the time interval could be less than 200ms as the car pushes the status whenever it has new data.
If the car is at online status, the time interval could be several minutes as the system actively fetches the status from the car.
Even worse, if the system thinks the car is going to sleep and need to stop fetching status (to avoid preventing the car to sleep), the interval could be 40 minutes maximum depend on settings.
If the car is in asleep/offline status, no data is recorded at all.
This obviously makes skipping every n-th rows a bad idea, as for case 2-4 above, lots of data points might missing so that Grafana cannot plot correct graph representing the battery level at satisfactory precision.
I wonder if there's any possible to skip the rows by time interval from a datetime field rather than row_number() without much overhead from the query? i.e., fetch every row with minimal 1000ms from the previous row.
E.g., I have following data in the table, I want the rows returned are row 1, 4 and 5.
row date
[1] 1610000001000
[2] 1610000001100
[3] 1610000001200
[4] 1610000002000
[5] 1610000005000
The current (problematic) method I am using is as follows:
SELECT $__time(t.date), t.battery_level AS "SOC [%]"
FROM (
SELECT date, battery_level, row_number() OVER(ORDER BY date ASC) AS row
FROM (
SELECT battery_level, date
FROM positions
WHERE car_id = $car_id AND $__timeFilter(date)
UNION ALL
SELECT battery_level, date
FROM charges c
JOIN charging_processes p ON p.id = c.charging_process_id
WHERE $__timeFilter(date) AND p.car_id = $car_id) AS data
ORDER BY date ASC) as t
WHERE t.row % 100 = 0;
This method clearly gives problem that only returns alternate rows instead of what I wanted (given the last row reads t.row % 2 = 0)
PS: please ignore the table structures and UNION from the sample code, I haven't dig deep enough to the tables which could be other tweaks but irrelevant to this question anyway.
Thanks in advance!
You can use a recursive CTE:
WITH RECURSIVE rec(cur_row, cur_date) AS (
(
SELECT row, date
FROM t
ORDER BY date
LIMIT 1
)
UNION ALL
(
SELECT row, date
FROM t
JOIN rec
ON t.date >= cur_date + 1000
ORDER BY t.date
LIMIT 1
)
)
SELECT *
FROM rec;
cur_row
cur_date
1
1610000001000
4
1610000002000
5
1610000005000
View on DB Fiddle
Using a function instead would probably be faster:
CREATE OR REPLACE FUNCTION f() RETURNS SETOF t AS
$$
DECLARE
row t%ROWTYPE;
cur_date BIGINT;
BEGIN
FOR row IN
SELECT *
FROM t
ORDER BY date
LOOP
IF row.date >= cur_date + 1000 OR cur_date IS NULL
THEN
cur_date := row.date;
RETURN NEXT row;
END IF;
END LOOP;
END;
$$ LANGUAGE plpgsql;
SELECT *
FROM f();
row
date
1
1610000001000
4
1610000002000
5
1610000005000
I have been trying to write a query to perfect this instance but cant seem to do the trick because I am still receiving duplicated. Hoping I can get help how to fix this issue.
SELECT DISTINCT
1.Client
1.ID
1.Thing
1.Status
MIN(1.StatusDate) as 'statdate'
FROM
SAMPLE 1
WHERE
[]
GROUP BY
1.Client
1.ID
1.Thing
1.status
My output is as follows
Client Id Thing Status Statdate
CompanyA 123 Thing1 Approved 12/9/2019
CompanyA 123 Thing1 Denied 12/6/2019
So although the query is doing what I asked and showing the mininmum status date per status, I want only the first status date. I have about 30k rows to filter through so whatever does not run overload the query and have it not run. Any help would be appreciated
Use window functions:
SELECT s.*
FROM (SELECT s.*,
ROW_NUMBER() OVER (PARTITION BY id ORDER BY statdate) as seqnum
FROM SAMPLE s
WHERE []
) s
WHERE seqnum = 1;
This returns the first row for each id.
Use whichever of these you feel more comfortable with/understand:
SELECT
*
FROM
(
SELECT *, ROW_NUMBER() OVER (PARTITION BY id ORDER BY statusdate) as rn
FROM sample
WHERE ...
) x
WHERE rn = 1
The way that one works is to number all rows sequentially in order of StatusDate, restarting the numbering from 1 every time ID changes. If you thus collect all the number 1's togetyher you have your set of "first records"
Or can coordinate a MIN:
SELECT
*
FROM
sample s
INNER JOIN
(SELECT ID, MIN(statusDate) as minDate FROM sample WHERE ... GROUP BY ID) mins
ON s.ID = mins.ID and s.StatusDate = mins.MinDate
WHERE
...
This one prepares a list of all the ID and the min date, then joins it back to the main table. You thus get all the data back that was lost during the grouping operation; you cannot simultaneously "keep data" and "throw away data" during a group; if you group by more than just ID, you get more groups (as you have found). If you only group by ID you lose the other columns. There isn't any way to say "GROUP BY id, AND take the MIN date, AND also take all the other data from the same row as the min date" without doing a "group by id, take min date, then join this data set back to the main dataset to get the other data for that min date". If you try and do it all in a single grouping you'll fail because you either have to group by more columns, or use aggregating functions for the other data in the SELECT, which mixes your data up; when groups are done, the concept of "other data from the same row" is gone
Be aware that this can return duplicate rows if two records have identical min dates. The ROW_NUMBER form doesn't return duplicated records but if two records have the same minimum StatusDate then which one you'll get is random. To force a specific one, ORDER BY more stuff so you can be sure which will end up with 1
I have the following table, let's call it Names:
Name Id Date
Dirk 1 27-01-2015
Jan 2 31-01-2015
Thomas 3 21-02-2015
Next I have the another table called Consumption:
Id Date Consumption
1 26-01-2015 30
1 01-01-2015 20
2 01-01-2015 10
2 05-05-2015 20
Now the problem is, that I think that doing this using SQL is the fastest, since the table contains about 1.5 million rows.
So the problem is as follows, I would like to match each Id from the Names table with the Consumption table provided that the difference between the dates are the lowest, so we have: Dirk consumes on 27-01-2015 about 30. In case there are two dates that have the same "difference", I would like to calculate the average consumption on those two dates.
While I know how to join, I do not know how to code the difference part.
Thanks.
DBMS is Microsoft SQL Server 2012.
I believe that my question differs from the one mentioned in the comments, because it is much more complicated since it involves comparison of dates between two tables rather than having one date and comparing it with the rest of the dates in the table.
This is how you could it in SQL Server:
SELECT Id, Name, AVG(Consumption)
FROM (
SELECT n.Id, Name, Consumption,
RANK() OVER (PARTITION BY n.Id
ORDER BY ABS(DATEDIFF(d, n.[Date], c.[Date]))) AS rnk
FROM Names AS n
INNER JOIN Consumption AS c ON n.Id = c.Id ) t
WHERE t.rnk = 1
GROUP BY Id, Name
Using RANK with PARTITION BY n.Id and ORDER BY ABS(DATEDIFF(d, n.[Date], c.[Date])) you can locate all matching records per Id: all records with the smallest difference in days are going to have rnk = 1.
Then, using AVG in the outer query, you are calculating the average value of Consumption between all matching records.
SQL Fiddle Demo
I have a SQL statement that I am currently using to return a number of rows from a database:
SELECT
as1.AssetTagID, as1.TagID, as1.CategoryID,
as1.Description, as1.HomeLocationID, as1.ParentAssetTagID
FROM Assets AS as1
INNER JOIN AssetsReads AS ar ON as1.AssetTagID = ar.AssetTagID
WHERE
(ar.ReadPointLocationID='Readpoint1' OR ar.ReadPointLocationID='Readpoint2')
AND (ar.DateScanned between 'LastScan' AND 'Now')
AND as1.TagID!='000000000000000000000000'
I am wanting to do a query that will get the row with the oldest DateScanned from this query and also get another row from the database if there was one that was within a certain period of time from this row (say 5 seconds for an example). The oldest record would be relatively simple by selecting the first record in a descending sort, but how would I also get the second record if it was within a certain time period of the first?
I know I could do this process with multiple queries, but is there any way to combine this process into one query?
The database that I am using is SQL Server 2008 R2.
Also please note that the DateScanned times are just placeholders and I am taking care of that in the application that will be using this query.
Here is a fairly general way to approach it. Get the oldest scan date using min() as a window function, then use date arithmetic to get any rows you want:
select t.* -- or whatever fields you want
from (SELECT as1.AssetTagID, as1.TagID, as1.CategoryID,
as1.Description, as1.HomeLocationID, as1.ParentAssetTagID,
min(DateScanned) over () as minDateScanned, DateScanned
FROM Assets AS as1
INNER JOIN AssetsReads AS ar ON as1.AssetTagID = ar.AssetTagID
WHERE (ar.ReadPointLocationID='Readpoint1' OR ar.ReadPointLocationID='Readpoint2')
AND (ar.DateScanned between 'LastScan' AND 'Now')
AND as1.TagID!='000000000000000000000000'
) t
where datediff(second, minDateScanned, DateScanned) <= 5;
I am not really sure of sql server syntax, but you can do something like this
SELECT * FROM (
SELECT
TOP 2
as1.AssetTagID,
as1.TagID,
as1.CategoryID,
as1.Description,
as1.HomeLocationID,
as1.ParentAssetTagID ,
ar.DateScanned,
LAG(ar.DateScanned) OVER (order by ar.DateScanned desc) AS lagging
FROM
Assets AS as1
INNER JOIN AssetsReads AS ar
ON as1.AssetTagID = ar.AssetTagID
WHERE (ar.ReadPointLocationID='Readpoint1' OR ar.ReadPointLocationID='Readpoint2')
AND (ar.DateScanned between 'LastScan' AND 'Now')
AND as1.TagID!='000000000000000000000000'
ORDER BY
ar.DateScanned DESC
)
WHERE
lagging IS NULL or DateScanned - lagging < '5 SECONDS'
I have tried to sort the results by DateScanned desc and then just the top most 2 rows. I have then used the lag() function on DateScanned field, to get the DateScanned value for the previous row. For the topmost row the DateScanned shall be null as its the first record, but for the second one it shall be value of the first row. You can then compare both of these values to determine whether you wish to display the second row or not
more info on the lagging function: http://blog.sqlauthority.com/2011/11/15/sql-server-introduction-to-lead-and-lag-analytic-functions-introduced-in-sql-server-2012/