How would you implement a query that selects every nth row, with NHibernate QueryOver, HQL or Criteria?
Currently I use the following T-SQL query:
SELECT * FROM (
SELECT *, ROW_NUMBER() OVER (ORDER BY Id) AS [Row]
FROM [TABLE_NAME]
) x WHERE (x.[Row] % 100) = 0
(Thanks to Marc Gravell)
Have you considered the solution of using an indexing table in a cross join? What I mean is that you have a table with as many rows as you think you will need with an indexed column of integers going from 1-n in each row. This can be in a master database perhaps with a date column beside it - its amazing how useful this method is. The query would then look like
SELECT * FROM (
SELECT *, ROW_NUMBER() OVER (ORDER BY Id) AS [Row]
FROM [TABLE_NAME]
) x INNER JOIN [Index_Table] i ON i.Id*100=x.[Row]
Same as L2S - there's no easy way to do this without SQL. And the syntax would be DBMS-specific anyway.
Related
I can't use a Union because it's not the result I want, and I can't use join because I haven't any common column. I have tried many different SQL query structures and nothing works as I want.
I need help to achieve what I believe is a really simple SQL query. What I am doing now is
select a, b
from (select top 4 a from element_type order by c) as Y,
(SELECT * FROM (VALUES (NULL), (1), (2), (3)) AS X(b)) as Z
The first is a part of a table and the second is a hand created select that gives results like this:
select a; --Give--> a,b,c,d (1 column)
select b; --Give--> 1,2,3,4 (1 column)
I need a query based on the two first that give me (2 column) :
a,1
b,2
c,3
d,4
How can i do this? UNION, JOIN or anything else? Or maybe I can't.
All I can get for now is this:
a,1
a,2
a,3
a,4
b,1
b,2
...
If you want to join two tables together purely on the order the rows appear, then I hope your database support analytic (window) functions:
SELECT * FROM
(SELECT t.*, ROW_NUMBER() OVER(ORDER BY x) as rown FROM table1 t) t1
INNER JOIN
(SELECT t.*, ROW_NUMBER() OVER(ORDER BY x) as rown FROM table2 t) t2
ON t1.rown = t2.rown
Essentially we invent something to join them on by numbering the rows. If one of your tables already contains incrementing integers from 1, you dont need to ROW_NUMBER() OVER() on that table, because it already has suitable data to join to; you just invent a fake column of incrementing nubmers in the other table and then join together
Actually, even if it doesn't support analytics, there are ugly ways of doing row numbering, such as joining the table back to itself using id < id and COUNT(*) .. GROUP BY id to number the rows. I hate doing it, but if your DB doesnt support ROW_NUMBER i'll post an example.. :/
Bear in mind, of course, that RDBMS have R in the name for a reason - related data is.. well.. related. They don't do so well when data is unrelated, so if your hope is to join the "chalks" table to the "cheese" table even though the two are completely unrelated, you're finding out now why it's hard work! :)
Try using row_number. I've created something that might help you. See below:
declare #tableChar table(letter varchar)
insert into #tableChar(letter)
select 'a';
insert into #tableChar(letter)
select 'b';
insert into #tableChar(letter)
select 'c';
insert into #tableChar(letter)
select 'd';
select letter,ROW_NUMBER() over(order by letter ) from #tableChar
You can user row_number() to achieve this,
select a,row_number() over(order by a) as b from element_type;
As you are not taking second part from other table, so you do not need to use join. But if you are doing this on different tables the you can use row_number() to create key for both the tables and bases on those keys, you can join.
Hope it will help.
I'm trying to implement pagination in a query that is built using information from a view, and I need to use the row_number() function over a column when I don't know which table it is from.
SELECT * FROM (
SELECT class.ID as ID, user.ID as USERID, row_number() over (ORDER BY
ID desc) as row_number FROM class, user
) out_q WHERE row_number > #startrow ORDER BY row_number
The problem is that I only have the result column name (ID or USERID) that came from a previous query. If I execute this query, it will raise the error 'Ambiguous column name "ID"'. Is there a way to specify that I'm referencing the column ID that is being selected and not from a different table?
Is it possible to specify an alias to the query result itself?
I have already tried the following,
SELECT TOP 30 * FROM (
SELECT *, row_number() over (ORDER BY ID desc) as row_number FROM(
SELECT class.ID as ID, user.ID as USERID FROM class, user
) in_q
) out_q WHERE row_number > #startrow ORDER BY row_number
It works, but the SGBD gets confused on which query plan it has to use, because of the small row goal present in the outer query and the big set of results returned by the inner query, when #startrow is a small number, the query executes in less than one second, when it is a big number the query takes minutes to execute.
Your problem is the id in the row_number itself. If you want a stable sort, then include both ids:
SELECT *
FROM (SELECT class.ID as ID, user.ID as USERID,
row_number() over (ORDER BY class.ID desc, user.id) as row_number
FROM class CROSS JOIN user
) out_q
WHERE row_number > #startrow
ORDER BY row_number;
I assume the cartesian product is intentional. Sometimes, this indicates an error in the query. In general, I would advise you to avoid using commas in the from clause. If you do want a cartesian product, then be explicit by using CROSS JOIN.
You could try using the option you already tried, then use the OPTIMIZE FOR hint.
OPTION ( OPTIMIZE FOR (#startrow = 100000) );
See a description of the hint in MSDN docs here: https://msdn.microsoft.com/en-us/library/ms181714.aspx.
I have a Toplist table and I want to get a user's rank. How can I get the row's index?
Unfortunately, I am getting all rows and checking in a for loop the user's ID, which has a significant impact on the performance of my application.
How could this performance impact be avoided?
You can use ROW.NUMBER
This is a example syntax for MySQL
SELECT t1.toplistId,
#RankRow := #RankRow+ 1 AS Rank
FROM toplist t1
JOIN (SELECT #RankRow := 0) r;
This is a example syntax for MsSQL
SELECT ROW_NUMBER() OVER(ORDER BY YourColumn) AS Rank,TopListId
FROM TopList
You may also do something like this:
SELECT ROW_NUMBER() OVER(ORDER BY (SELECT 1)) AS MyIndex
FROM TopList
ROW_NUMBER() MS SQL
Use in your code analytic functions
I was wondering is there a way to retrieve, for example, 2nd and 5th row from SQL table that contains 100 rows?
I saw some solutions with WHERE clause but they all assume that the column on which WHERE clause is applied is linear, starting at 1.
Is there other way to query a SQL Server table for a specific rows in case table doesn't have a column whose values start at 1?
P.S. - I know for a solution with temporary tables, where you copy your select statement output and add a linear column to the table. I am using T-SQL
Try this,
SELECT * FROM (
SELECT
ROW_NUMBER() OVER (ORDER BY ColumnName ASC) AS rownumber
FROM TableName
) as temptablename
WHERE rownumber IN (2,5)
With SQL Server:
; WITH Base AS (
SELECT *, ROW_NUMBER() OVER (ORDER BY id) RN FROM YourTable
)
SELECT *
FROM Base WHERE RN IN (2, 5)
The id that you'll have to replace with your primary key or your ordering, YourTable that is your table.
It's a CTE (Common Table Expression) so it isn't a temporary table. It's something that will be expanded together with your query.
There is no 2nd or 5th row in the table.
There is only the 2nd or 5th result in a resultset that you return, as determined by the order you specify in that query.
If you are on SQL Server 2005 or above, you could use Row_Number() function. Ex:
;With CTE as (
select col1, ..., row_number() over (order by yourOrderingCol) rn
from yourTable
)
select col1,...
from cte
where rn in (2,5)
Please note that yourOrderingCol will decide the value of row number (i.e. rn).
I'm using SQL Azure with asp script, and for the life of me, have had no luck trying to get this to work. The table I'm running a query on has many columns, but I want to query for distinct values on 2 columns (name and email), from there I want it to return the entire row's values.
What my query looks like now:
SELECT DISTINCT quote_contact, quote_con_email
FROM quote_headers
WHERE quote_contact LIKE '"&query&"%'
But I need it to return the whole row so I can retrieve other data points. Had I been smart a year ago, I would have created a separate table just for the contacts, but that's a year ago.
And before I implemented LiveSearch features.
One approach would be to use a CTE (Common Table Expression). With this CTE, you can partition your data by some criteria - i.e. your quote_contact and quote_con_email - and have SQL Server number all your rows starting at 1 for each of those partitions, ordered by some other criteria - i.e. probably SomeDateTimeStamp.
So try something like this:
;WITH DistinctContacts AS
(
SELECT
quote_contact, quote_con_email, (other columns here),
RN = ROW_NUMBER() OVER(PARTITION BY quote_contact, quote_con_email ORDER BY SomeDateTimeStamp DESC)
FROM
dbo.quote_headers
WHERE
quote_contact LIKE '"&query&"%'
)
SELECT
quote_contact, quote_con_email, (other columns here)
FROM
DistinctContacts
WHERE
RowNum = 1
Here, I am selecting only the last entry for each "partition" (i.e. for each pair of name/email) - ordered in a descending fashion by the time stamp.
Does that approach what you're looking for??
You need to provide more details.
This is what I could come up with Without them:
WITH dist as (
SELECT DISTINCT quote_contact, quote_con_email, RANK() OVER(ORDER BY quote_contact, quote_con_email) rankID
FROM quote_headers
WHERE quote_contact LIKE '"&query&"%'
),
data as (
SELECT *, RANK() OVER(ORDER BY quote_contact, quote_con_email) rankID FROM quote_headers
)
SELECT * FROM dist d INNER JOIN data src ON d.rankID = src.rankID