I am trying to wrap my head around a problem I hit exporting data from one system to another.
Let's say I have a table like:
id | item_num
1 1
2 1
3 2
4 3
5 3
6 3
I need to add a column to the table and update it to contain an incrementing product_num field based on item. This would be the end result given the above table.
id | item_num | product_num
1 1 1
2 1 2
3 2 1
4 3 1
5 3 2
6 3 3
Any ideas on going about this?
Edit: This is being done in Access 2010 from one system to another (sql server source, custom/unknown ODBC driven destination)
Perhaps you could create a view in your SQL Server database and then select from that in Access to insert into your destination.
Possible solutions in SQL Server:
-- Use row_number() to get product_num in SQL Server 2005+:
select id
, item_num
, row_number() over (partition by item_num order by id) as product_num
from MyTable;
-- Use a correlated subquery to get product_num in many databases:
select t.id
, t.item_num
, (select count(*) from MyTable where item_num = t.item_num and id <= t.id) as product_num
from MyTable t;
Same result:
id item_num product_num
----------- ----------- --------------------
1 1 1
2 1 2
3 2 1
4 3 1
5 3 2
6 3 3
Related
I wonder how to update order in this table for many-to-many relationship using SQL based on PostsId.
So my table now looks like:
I'm using SQL Server
BlogsId
PostsId
Order
1
1
1
0
2
2
1
0
3
3
2
0
3
4
2
0
3
5
3
0
3
6
3
0
but I want to update Order using SQL to this:
BlogsId
PostsId
Order
1
1
1
1
2
2
1
2
3
3
2
1
3
4
2
2
3
5
3
1
3
6
3
2
So for example: Blog with Id 3 is the first blog in Post with Id 2, Blog with Id 4 is the second Blog in Post with Id 2 and etc...
I've tried this:
DECLARE #myVar int
SET #myVar = 0
UPDATE [dbo].[BlogPost]
SET #myVar = [Order] = #myVar + 1
but then I got this:
BlogsId
PostsId
Order
1
1
1
1
2
2
1
2
3
3
2
3
3
4
2
4
3
5
3
5
3
6
3
6
So, I think I should do something in WHERE part (with Distinct maybe) but I don't know exactly what. I could write something in C# to do what I want but I don't know how to write something like this in SQL.
Physically maintaining an order or sequence of rows is rarely a good idea and can lead to data inconsistencies and other unforseen issues.
You would be better off creating a view that provides the additional Order column which you can do using row_number()
Create view BlogPosts as
select *,
Row_Number() over(partition by PostsId order by BlogsId) as [Order]
from blogpost;
If you really want to update an actual column in the table you could use a CTE
with b as (
select *,
Row_Number() over(partition by PostsId order by BlogsId) as seq
from blogpost
)
update b
set [Order] = seq;
You can update from a calculated row_number.
update t
set [Order] = rn
from (
select BlogsId, PostsId, [Order]
, rn = row_number() over (partition by PostsId order by BlogsId asc)
from BlogPost
) t
where ([Order] is null or [Order]!=rn);
select *
from BlogPost
order by BlogsId, PostsId
BlogsId
PostsId
Order
1
1
1
2
1
2
3
2
1
4
2
2
5
3
1
6
3
2
Demo on db<>fiddle here
I am working on an API-based simple chat module. I am trying to get chat conversations for a particular user but due to 2 columns having the same value swapped between each other is causing my data to be duplicated.
I want to merge rows having the same values swapped between 2 columns and the merged row should be based on the latest entry inserted in the database.
The data looks like this :
Id To From Message ConversationTime
1 1 2 hello 11:00AM
2 3 1 hi 12:00PM
3 1 3 how are you? 12:15PM
4 3 1 I am fine. 12:30PM
5 4 5 Hi! 04:30PM
6 5 4 Hello 04:35PM
7 1 5 Hola! 06:30PM
So for example if user with Id 1 My result needs to look like this:
Id To From Message ConversationTime
1 1 2 hello 11:00AM
4 3 1 I am fine. 12:30PM
7 1 5 Hola! 06:30PM
If Id is 5 then result would be like this:
Id To From Message ConversationTime
6 5 4 Hello 04:35PM
7 1 5 Hola! 06:30PM
My result set looks like this:
Id To From Message ConversationTime
1 1 2 hello 11:00AM
3 1 3 how are you? 12:15PM
4 3 1 I am fine. 12:30PM
7 1 5 Hola! 06:30PM
Any help would be grateful. Thanks in advance!
The idea is the same as the linked duplicate Get top 1 row of each group ; just use a CASE expression to get the ID of the other user:
DECLARE #ID int = 1;
WITH RNs AS(
SELECT ID,
[To], --TO is a reserved keyword and should not be used for object names
[From], --FROM is a reserved keyword and should not be used for object names
Message,
ConversationTime, --I assume this is a time
ROW_NUMBER() OVER (PARTITION BY CASE [To] WHEN #ID THEN [From] ELSE [To] END ORDER BY ConversationTime DESC) AS RN --TO and FROM are reserved keywords and should not be used for object names
FROM dbo.YourTable
WHERE #ID IN ([To],[From])) --TO and FROM are reserved keywords and should not be used for object names
SELECT ID,
[To], --TO is a reserved keyword and should not be used for object names
[From], --FROM is a reserved keyword and should not be used for object names
Message,
ConversationTime --I assume this is a time
FROM RN
WHERE RN = 1;
SQL Server allows you to do this without a case expressions by unpivoting the data and then using window functions:
select t.*
from (select t.*,
row_number() over (partition by v.user_other order by t.conversationTime desc) as seqnum
from t cross apply
(values (t.to, t.from), (t.from, to.to)
) v(user, user_other)
where v.user = 1
) t
where seqnum = 1;
I have the following table structure.
ITEM TOTAL
----------- -----------------
ID | TITLE ID |ITEMID|VALUE
1 A 1 2 6
2 B 2 1 4
3 C 3 3 3
4 D 4 3 8
5 E 5 1 2
6 F 6 5 4
7 4 5
8 2 8
9 2 7
10 1 3
11 2 2
12 3 6
I am using Apache Derby DB. I need to perform the average calculation in SQL. I need to show the list of item IDs and their average total of the last 3 records.
That is, for ITEM.ID 1, I will go to TOTAL table and select the last 3 records of the rows which are associated with the ITEMID 1. And take average of them. In Derby database, I am able to do this for a given item ID but I cannot make it without giving a specific ID. Let me show you what I've done it.
SELECT ITEM.ID, AVG(VALUE) FROM ITEM, TOTAL WHERE TOTAL.ITEMID = ITEM.ID GROUP BY ITEM.ID
This SQL gives the average of all items in a list. But this calculates for all values of the total tables. I need last 3 records only. So I changed the SQL to this:
SELECT AVG(VALUE) FROM (SELECT ROW_NUMBER() OVER() AS ROWNUM, TOTAL.* FROM TOTAL WHERE ITEMID = 1) AS TR WHERE ROWNUM > (SELECT COUNT(ID) FROM TOTAL WHERE ITEMID = 1) - 3
This works if I supply the item ID 1 or 2 etc. But I cannot do this for all items without giving an item ID.
I tried to do the same thing in ORACLE using partition and it worked. But derby does not support partitioning. There is WINDOW but I could not make use of it.
Oracle one
SELECT ITEMID, AVG(VALUE) FROM(SELECT ITEMID, VALUE, COUNT(*) OVER (PARTITION BY ITEMID) QTY, ROW_NUMBER() OVER (PARTITION BY ITEMID ORDER BY ID) IDX FROM TOTAL ORDER BY ITEMID, ID) WHERE IDX > QTY -3 GROUP BY ITEMID ORDER BY ITEMID
I need to use derby DB for its portability.
The desired output is this
RESULT
-----------------
ITEMID | AVERAGE
1 (9/3)
2 (17/3)
3 (17/3)
4 (5/1)
5 (4/1)
6 NULL
As you have noticed, Derby's support for the SQL 2003 "OLAP Operations" support is incomplete.
There was some initial work (see https://wiki.apache.org/db-derby/OLAPOperations), but that work was only partially completed.
I don't believe anyone is currently working on adding more functionality to Derby in this area.
So yes, Derby has a row_number function, but no, Derby does not (currently) have partition by.
I have the table below in an SQL database.
user rating
1 10
1 7
1 6
1 2
2 8
2 3
2 2
2 2
I would like to keep only the best two ratings by user to get:
user rating
1 10
1 7
2 8
2 3
What would be the SQL query to do that? I am not sure how to do it.
It will work
;with cte as
(select user,rating, row_number() over (partition by user order by rating desc) maxval
from yourtable)
select user,rating
from cte
where maxval in (1,2)
I have table like this-
id object_id product_id
1 1 1
2 1 1
4 2 2
6 3 2
7 3 2
8 1 2
9 1 1
I want to delete all rows except these-
1 1 1
4 2 2
6 3 2
9 1 2
Basically there are duplicates and I want to remove them but keep one copy intact.
what would be the most efficient way for this?
If this is a one-off then you can simply identify the records you want to keep like so:
SELECT MIN(id) AS id
FROM yourtable
GROUP BY object_id, product_id;
You want to check that this works before you do the next thing and actually throw records out. To actually delete those duplicate records you do:
DELETE FROM yourtable WHERE id NOT IN (
SELECT MIN(id) AS id
FROM yourtable
GROUP BY object_id, product_id
);
The MIN(id) obviously always returns the record with the lowest id for a set of (object_id, product_id). Change as desired.