I have several tables that use an ID number plus a column called xsequence that are both primary keys. Currently, I have a bunch of data that looks like this:
ID_NUMBER,XSEQUENCE
001,2
001,5
001,8
002,1
002,6
What I need to end up with is:
ID_NUMBER,XSEQUENCE
001,1
001,2
001,3
002,1
002,2
What is the best way of going about starting this? Every time I try, I just end up spinning my wheels.
Try something like this:
select id_number,
row_number() over (partition by id_number order by xsequence) new_xsequence
from yourtable
That's an analytic function really handy for this sort of thing. Using the Partition keyword - "resets" the counter at each id_number. (so 1,2,3 .. then starts again 1,2,3 ... etc.).
(The Partition keyword in analytic functions behaves very similar to the GROUP by keyword)
[edit]
To UPDATE the original table, I actually prefer the MERGE statement - it's a bit simpler syntax wise, and seems a bit more intuitive ;) )
MERGE INTO yourtable base
USING (
select rowid rid,
id_number,
row_number() over (partition by id_number order by xsequence) new_xsequence,
xsequence old_xsequence
from yourtable
) new
ON ( base.rowid = new.rid )
WHEN MATCHED THEN UPDATE
SET base.xsequence = new.new_xsequence
[edit]
Related
I have a table with the following Columns...
Node, Date_Time, Market, Price
I would like to delete all but 1 record for each Node, Date time.
SELECT Node, Date_Time, MAX(Price)
FROM Hourly_Data
Group BY Node, Date_Time
That gets the results I would like to see but cant figure out how to remove the other records.
Note - There is no ID for this table
Here are steps that are rather workaround than a simple one-command which will work in any relational database:
Create new table that looks just like the one you already have
Insert the data computed by your group-by query to newly created table
Drop the old table
Rename new table to the name the old one used to have
Just remember that locking takes place and you need to have some maintenance time to perform this action.
There are simpler ways to achieve this, but they are DBMS specific.
here is an easy sql-server method that creates a Row Number within a cte and deletes from it. I believe this method also works for most RDBMS that support window functions and Common Table Expressions.
;WITH cte AS (
SELECT
*
,RowNum = ROW_NUMBER() OVER (PARTITION BY Node, Date_Time ORDER BY Price DESC)
FROM
Hourly_Data
)
DELETE
FROM
cte
WHERE
RowNum > 1
I have a table with many duplicate items – Many rows with the same id, perhaps with the only difference being a requested_at column.
I'd like to do a select * from the table, but only return one row with the same id – the most recently requested.
I've looked into group by id but then I need to do an aggregate for each column. This is easy with requested_at – max(requested_at) as requested_at – but the others are tough.
How do I make sure I get the value for title, etc that corresponds to that most recently updated row?
I suggest a similar form that avoids a sort in the window function:
SELECT *
FROM (
SELECT
*,
MAX(<timestamp_column>)
OVER (PARTITION BY <id_column>)
AS max_timestamp,
FROM <table>
)
WHERE <timestamp_column> = max_timestamp
Try something like this:
SELECT *
FROM (
SELECT
*,
ROW_NUMBER()
OVER (
PARTITION BY <id_column>
ORDER BY <timestamp column> DESC)
row_number,
FROM <table>
)
WHERE row_number = 1
Note it will add a row_number column, which you might not want. To fix this, you can select individual columns by name in the outer select statement.
In your case, it sounds like the requested_at column is the one you want to use in the ORDER BY.
And, you will also want to use allow_large_results, set a destination table, and specify no flattening of results (if you have a schema with repeated fields).
I want to number the rows in my result set. I want some way I can have a result set of 3 records use some SQL keyword to generate a column that would read 1,2,3 for watch of the records...
I know I can make a temp table with an auto increment column but i wanted to know if there was a way I can get this back from a SQL query?
SELECT row_count,
project_name,
project_id
FROM Project
anything available like "row_count" that i am dreaming of?
Ordering by SELECT 0 will give you an incrementing column and saves a potentially unnecessary sort.
SELECT ROW_NUMBER() over (order by (select 0)) as row_count,
project_name,
project_id
FROM Project
Sounds like you want ROW_NUMBER, an analytic/ranking function. But it's unclear what you want to base the numbering on - this assumes the project_id, starting at the smallest value:
SELECT ROW_NUMBER() OVER(ORDER BY p.project_id) AS row_count,
p.project_name,
p.project_id
FROM PROJECT p
I have an Oracle table with two columns ID and START_DATE, I want to run a query to get the ID of the record with the most recent date, initially i wrote this:
select id from (select * from mytable order by start_date desc) where rownum = 1
Is there a more cleaner and efficient way of doing this? I often run into this pattern in SQL and end up creating a nested query.
SELECT id FROM mytable WHERE start_date = (SELECT MAX(start_date) FROM mytable)
Still a nested query, but more straightforward and also, in my experience, more standard.
This looks to be a pretty clean and efficient solution to me - I don't think you can get any better than that, of course assuming that you've an index on start_date. If you want all ids for the latest start date then froadie's solution is better.
I have a SQL query, that returns a set of rows:
SELECT id, name FROM users where group = 2
I need to also include a column that has an incrementing integer value, so the first row needs to have a 1 in the counter column, the second a 2, the third a 3 etc
The query shown here is just a simplified example, in reality the query could be arbitrarily complex, with several joins and nested queries.
I know this could be achieved using a temporary table with an autonumber field, but is there a way of doing it within the query itself ?
For starters, something along the lines of:
SELECT my_first_column, my_second_column,
ROW_NUMBER() OVER (ORDER BY my_order_column) AS Row_Counter
FROM my_table
However, it's important to note that the ROW_NUMBER() OVER (ORDER BY ...) construct only determines the values of Row_Counter, it doesn't guarantee the ordering of the results.
Unless the SELECT itself has an explicit ORDER BY clause, the results could be returned in any order, dependent on how SQL Server decides to optimise the query. (See this article for more info.)
The only way to guarantee that the results will always be returned in Row_Counter order is to apply exactly the same ordering to both the SELECT and the ROW_NUMBER():
SELECT my_first_column, my_second_column,
ROW_NUMBER() OVER (ORDER BY my_order_column) AS Row_Counter
FROM my_table
ORDER BY my_order_column -- exact copy of the ordering used for Row_Counter
The above pattern will always return results in the correct order and works well for simple queries, but what about an "arbitrarily complex" query with perhaps dozens of expressions in the ORDER BY clause? In those situations I prefer something like this instead:
SELECT t.*
FROM
(
SELECT my_first_column, my_second_column,
ROW_NUMBER() OVER (ORDER BY ...) AS Row_Counter -- complex ordering
FROM my_table
) AS t
ORDER BY t.Row_Counter
Using a nested query means that there's no need to duplicate the complicated ORDER BY clause, which means less clutter and easier maintenance. The outer ORDER BY t.Row_Counter also makes the intent of the query much clearer to your fellow developers.
In SQL Server 2005 and up, you can use the ROW_NUMBER() function, which has options for the sort order and the groups over which the counts are done (and reset).
The simplest way is to use a variable row counter. However it would be two actual SQL commands. One to set the variable, and then the query as follows:
SET #n=0;
SELECT #n:=#n+1, a.* FROM tablename a
Your query can be as complex as you like with joins etc. I usually make this a stored procedure. You can have all kinds of fun with the variable, even use it to calculate against field values. The key is the :=
Heres a different approach.
If you have several tables of data that are not joinable, or you for some reason dont want to count all the rows at the same time but you still want them to be part off the same rowcount, you can create a table that does the job for you.
Example:
create table #test (
rowcounter int identity,
invoicenumber varchar(30)
)
insert into #test(invoicenumber) select [column] from [Table1]
insert into #test(invoicenumber) select [column] from [Table2]
insert into #test(invoicenumber) select [column] from [Table3]
select * from #test
drop table #test