I am trying to select rows 3 - 5 of:
SELECT *
FROM Finance_User
ORDER BY email DESC
I originally had just:
SELECT
ROW_NUMBER() OVER (ORDER BY email DESC) AS RowNum, *
FROM
Finance_User
WHERE
RowNum BETWEEN 3 AND 5
But this did not work as RowNum was an invalid column.
Instead I did the below:
WITH OrderedUsers AS
(
SELECT
ROW_NUMBER() OVER (ORDER BY email DESC) AS RowNum, *
FROM
Finance_User
)
SELECT *
FROM OrderedUsers
WHERE RowNum BETWEEN 3 AND 5
This works perfectly fine. However, I am concerned that there might be performance issues with this as it seems to be selecting from the table twice?
ROW_NUMBER() with a CTE (or a subquery) won't scan the table twice. However using the window function might incur additional processing for the RDBMS.
You could achieve the same results with ORDER BY ... OFFSET ... FETCH ..., available starting SQL-Server 2012, that are provided specifically for the purpose of paging a resultset:
SELECT *
FROM Finance_User
ORDER BY email DESC
OFFSET 2 ROWS FETCH NEXT 3 ROWS ONLY
From the documentation:
We recommend that you use the OFFSET and FETCH clauses instead of the TOP clause to implement a query paging solution and limit the number of rows sent to a client application.
Your query is fine.
WITH AS clause is a Common Table Expression which means that this query may be reused later and should be cached if needed and possible. So there should not be any problem with "selecting from the table twice".
The same result you can get with this query:
SELECT * from
(SELECT ROW_NUMBER() OVER (ORDER BY email DESC) AS RowNum, * FROM Finance_User)
where RowNum between 3 and 5
And finally, you can always check execution plan and make sure of it as well.
Related
I am writing a very simple query for an Oracle DB (version 9).
Somehow I can get first 5 rows:
select * from cities where rownum <= 5
But skipping 5 rows returns an empty result:
select * from cities where rownum >= 5
Using:
Oracle SQL Developer
Oracle DB version 9
Why is the second query returning an empty result?
In Oracle Database 12c (release 1) and above, you can do this very simple, for skip 5 rows:
SELECT * FROM T OFFSET 5 ROWS
and for skip 5 rows and take 15 rows:
SELECT * FROM T OFFSET 5 ROWS FETCH NEXT 15 ROWS ONLY
You can use the following query to skip the first not n of rows.
select * from (
select rslts.*, rownum as rec_no from (
<<Query with proper order by (If you don't have proper order by you will see weird results)>>
) rslts
) where rec_no > <<startRowNum - n>>
The above query is similar to pagination query below.
select * from (
select rslts.*, rownum as rec_no from (
<<Query with proper order by (If you don't have proper order by you will see weird results)>>
) rslts where rownum <= <<endRowNum>>
) where rec_no > <<startRowNum>>
Your cities query:
select * from (
select rslts.*, rownum as rec_no from (
select * from cities order by 1
) rslts
) where rec_no > 5 <<startRowNum>>
Note: Assume first column in cities table is unique key
Oracle increments rownum each time it adds a row to the result set. So saying rownum < 5 is fine; as it adds each of the first 5 rows it increments rownum, but then once ruwnum = 5 the WHERE clause stops matching, no more rows are added to the result, and though you don't notice this rownum stops incrementing.
But if you say WHERE rownum > 5 then right off the bat, the WHERE clause doesn't match; and since, say, the first row isn't added to the result set, rownum isn't incremented... so rownum can never reach a value greater than 5 and the WHERE clause can never match.
To get the result you want, you can use row_number() over() in a subquery, like
select *
from (select row_number() over() rn, -- other values
from table
where -- ...)
where rn > 5
Update - As noted by others, this kind of query only makes sense if you can
control the order of the row numbering, so you should really use row_number() over(order bysomething) where something is a useful ordering key in deciding which records are "the first 5 records".
rownum is being increased only when a row is being output, so this type of condition won't work.
In any case, you are not ordering your rows, so what's the point?
Used row_number() over (order by id):
select * from
(select row_number() over (order by id) rn, c.* from countries c)
where rn > 5
Used ROWNUM:
select * from
(select rownum rn, c.* from countries c)
where rn > 5
Important note:
Using alias as countries c instead of countries is required! Without, it gives an error "missing expression"
Even better would be:
select * from mytab sample(5) fetch next 1 rows only;
Sample clause indicates the probability of each row getting picked up in the sampling process. FETCH NEXT clause indicates the number of rows you want to select.
With this code, you can query your table with skip and take.
select * from (
select a.*, rownum rnum from (
select * from cities
) a
) WHERE rnum >= :skip + 1 AND rnum <= :skip + :take
This code works with Oracle 11g. With Oracle 12, there is already a better way to perform this queries with offset and fetch
I am struggling to fetch the data based on rownum. When I execute the below query to get the results based rownum 1 to 4 then it is working fine.
SELECT ROWNUM TOTAL,MI.* FROM (SELECT USER_ID,CUSTOMER_NAME FROM ELEC_AUTO_MERC
ORDER BY CREATION_DATE DESC ) MI WHERE ROWNUM BETWEEN 1 AND 4;
But when I am executing same query to get result from rownum 2 to 4 then it is not working, it doesn't return anything.
SELECT ROWNUM TOTAL,MI.* FROM (SELECT USER_ID,CUSTOMER_NAME FROM ELEC_AUTO_MERC
ORDER BY CREATION_DATE DESC ) MI WHERE ROWNUM BETWEEN 2 AND 4;
As a workaround, when I use one more SELECT statement then it is working fine, but I don't think it is good approach to use SELECT multiple times only for rownum.
SELECT * FROM (SELECT ROWNUM TOTAL,MI.* FROM (SELECT USER_ID,CUSTOMER_NAME FROM ELEC_AUTO_MERC
ORDER BY CREATION_DATE DESC ) MI) WHERE TOTAL BETWEEN 2 AND 4;
Can you please help me out to create optimize query?
ROWNUM is weird in that it can be evaluated as part of a condition in the query - but if the row then fails to pass that filter, the ROWNUM value that it was assigned becomes available to be used again for the next row.
One important effect of this is that if you use any condition that excludes a ROWNUM value of 1, you will never get a match. The first row to be tested against this condition will be row 1; but then it will fail the test, so the next row will then be considered row 1; and so on.
So your condition ROWNUM BETWEEN 2 AND 4 can never be true.
The workaround you have found is the traditional one. Another would be to use an analytic function to rank the rows, then filter on the rank, e.g.:
SELECT MI.* FROM (
SELECT USER_ID,CUSTOMER_NAME, RANK() OVER (ORDER BY CREATION_DATE DESC) AS the_rank
FROM ELEC_AUTO_MERC
) MI
WHERE the_rank BETWEEN 2 AND 4;
Several analytic functions - RANK, DENSE_RANK, and ROW_NUMBER - can be used for this purpose, and will produce slightly different results, especially if there are ties. Check out the docs.
I'm looking to some expresion like this (using SQL Server 2008)
SELECT TOP 10 columName FROM tableName
But instead of that I need the values between 10 and 20. And I wonder if there is a way of doing it using only one SELECT statement.
For example this is useless:
SELECT columName FROM
(SELECT ROW_NUMBER() OVER(ORDER BY someId) AS RowNum, * FROM tableName) AS alias
WHERE RowNum BETWEEN 10 AND 20
Because the select inside brackets is already returning all the results, and I'm looking to avoid that, due to performance.
Use SQL Server 2012 to fetch/skip!
SELECT SalesOrderID, SalesOrderDetailID, ProductID, OrderQty, UnitPrice, LineTotal
FROM AdventureWorks2012.Sales.SalesOrderDetail
OFFSET 10 ROWS FETCH NEXT 10 ROWS ONLY;
There's nothing better than you're describing for older versions of sql server. Maybe use CTE, but unlikely to make a difference.
WITH NumberedMyTable AS
(
SELECT
Id,
Value,
ROW_NUMBER() OVER (ORDER BY Id) AS RowNumber
FROM
MyTable
)
SELECT
Id,
Value
FROM
NumberedMyTable
WHERE
RowNumber BETWEEN #From AND #To
or, you can remove top 10 rows and then get next 10 rows, but I double anyone would want to do that.
There is a trick with row_number that does not involve sorting all the rows.
Try this:
SELECT columName
FROM (SELECT ROW_NUMBER() OVER(ORDER BY (select NULL as noorder)) AS RowNum, *
FROM tableName
) as alias
WHERE RowNum BETWEEN 10 AND 20
You cannot use a constant in the order by. However, you can use an expression that evaluates to a constant. SQL Server recognizes this and just returns the rows as encountered, properly enumerated.
Why do you think SQL Server would evaluate the entire inner query? Assuming your sort column is indexed, it'll just read the first 20 values. If you're really nervous you could do this:
Select
Id
From (
Select Top 20 -- note top 20
Row_Number() Over(Order By Id) As RowNum,
Id
From
dbo.Test
Order By
Id
) As alias
Where
RowNum Between 10 And 20
Order By
Id
but I'm pretty sure the query plan is the same either way.
(Really) Fixed as per Aaron's comment.
http://sqlfiddle.com/#!3/db162/6
One more option
SELECT TOP(11) columName
FROM dbo.tableName
ORDER BY
CASE WHEN ROW_NUMBER() OVER (ORDER BY someId) BETWEEN 10 AND 20
THEN ROW_NUMBER() OVER (ORDER BY someId) ELSE NULL END DESC
You could create a temp table that is ordered the way you want like:
SELECT ROW_NUMBER() OVER(ORDER BY someId) AS RowNum, * FROM tableName
into ##tempTable
...
That way you have an ordered list of rows.
and can just query by row number the subsequent times instead of doing the inner query multiple times.
In my SP I have the following:
with Paging(RowNo, ID, Name, TotalOccurrences) as
(
ROW_NUMBER() over (order by TotalOccurrences desc) as RowNo, V.ID, V.Name, R.TotalOccurrences FROM dbo.Videos V INNER JOIN ....
)
SELECT * FROM Paging WHERE RowNo BETWEEN 1 and 50
SELECT COUNT(*) FROM Paging
The result is that I get the error: invalid object name 'Paging'.
Can I query again the Paging table? I don't want to include the count for all results as a new column ... I would prefer to return as another data set. Is that possible?
Thanks, Radu
After more research I fond another way of doing this:
with Paging(RowNo, ID, Name, TotalOccurrences) AS
(
ROW_NUMBER() over (order by TotalOccurrences desc) as RowNo, V.ID, V.Name, R.TotalOccurrences FROM dbo.Videos V INNER JOIN ....
)
select RowNo, ID, Name, TotalOccurrences, (select COUNT(*) from Paging) as TotalResults from Paging where RowNo between (#PageNumber - 1 )* #PageSize + 1 and #PageNumber * #PageSize;
I think that this has better performance than calling two times the query.
You can't do that because the CTE you are defining will only be available to the FIRST query that appears after it's been defined. So when you run the COUNT(*) query, the CTE is no longer available to reference. That's just a limitation of CTEs.
So to do the COUNT as a separate step, you'd need to not use the CTE and instead use the full query to COUNT on.
Or, you could wrap the CTE up in an inline table valued function and use that instead, to save repeating the main query, something like this:
CREATE FUNCTION dbo.ufnExample()
RETURNS TABLE
AS
RETURN
(
with Paging(RowNo, ID, Name, TotalOccurrences) as
(
ROW_NUMBER() over (order by TotalOccurrences desc) as RowNo, V.ID, V.Name, R.TotalOccurrences FROM dbo.Videos V INNER JOIN ....
)
SELECT * FROM Paging
)
SELECT * FROM dbo.ufnExample() x WHERE RowNo BETWEEN 1 AND 50
SELECT COUNT(*) FROM dbo.ufnExample() x
Please be aware that Radu D's solution's query plan shows double hits to those tables. It is doing two executions under the covers. However, this still may be the best way as I haven't found a truly scalable 1-query design.
A less scalable 1-query design is to dump a completed ordered list into a #tablevariable , SELECT ##ROWCOUNT to get the full count, and select from #tablevariable where row number between X and Y. This works well for <10000 rows, but with results in the millions of rows, populating that #tablevariable gets expensive.
A hybrid approach is to populate this temp/variable up to 10000 rows. If not all 10000 rows are filled up, you're set. If 10000 rows are filled up, you'll need to rerun the search to get the full count. This works well if most of your queries return well under 10000 rows. The 10000 limit is a rough approximation, you can play around with this threshold for your case.
Write "AS" after the CTE table name Paging as below:
with Paging AS (RowNo, ID, Name, TotalOccurrences) as
(
ROW_NUMBER() over (order by TotalOccurrences desc) as RowNo, V.ID, V.Name, R.TotalOccurrences FROM dbo.Videos V INNER JOIN ....
)
SELECT * FROM Paging WHERE RowNo BETWEEN 1 and 50
SELECT COUNT(*) FROM Paging
I'm trying to do something like :
SELECT * FROM table LIMIT 10,20
or
SELECT * FROM table LIMIT 10 OFFSET 10
but using SQL Server
The only solution I found looks like overkill:
SELECT * FROM (
SELECT *, ROW_NUMBER() OVER (ORDER BY name) as row FROM sys.databases
) a WHERE row > 5 and row <= 10
I also found:
SELECT TOP 10 * FROM stuff;
... but it's not what I want to do since I can't specify the starting limit.
Is there another way for me to do that ?
Also, just curious, is there a reason why doesn't SQL Server support the LIMIT function or something similar? I don't want to be mean, but that really sounds like something a DBMS needs ... If it does, then I'm sorry for being so ignorant! I've been working with MySQL and SQL+ for the past 5 years so...
For SQL Server 2012 + you can use.
SELECT *
FROM sys.databases
ORDER BY name
OFFSET 5 ROWS
FETCH NEXT 5 ROWS ONLY
The LIMIT clause is not part of standard SQL. It's supported as a vendor extension to SQL by MySQL, PostgreSQL, and SQLite.
Other brands of database may have similar features (e.g. TOP in Microsoft SQL Server), but these don't always work identically.
It's hard to use TOP in Microsoft SQL Server to mimic the LIMIT clause. There are cases where it just doesn't work.
The solution you showed, using ROW_NUMBER() is available in Microsoft SQL Server 2005 and later. This is the best solution (for now) that works solely as part of the query.
Another solution is to use TOP to fetch the first count + offset rows, and then use the API to seek past the first offset rows.
See also:
"Emulate MySQL LIMIT clause in Microsoft SQL Server 2000"
"Paging of Large Resultsets in ASP.NET"
as you found, this is the preferred sql server method:
SELECT * FROM (
SELECT *, ROW_NUMBER() OVER (ORDER BY name) as row FROM sys.databases
) a WHERE a.row > 5 and a.row <= 10
How about this?
SET ROWCOUNT 10
SELECT TOP 20 *
FROM sys.databases
ORDER BY database_id DESC
It gives you the last 10 rows of the first 20 rows. One drawback is that the order is reversed, but, at least it's easy to remember.
If you are using SQL Server 2012+ vote for Martin Smith's answer and use the OFFSET and FETCH NEXT extensions to ORDER BY,
If you are unfortunate enough to be stuck with an earlier version, you could do something like this,
WITH Rows AS
(
SELECT
ROW_NUMBER() OVER (ORDER BY [dbo].[SomeColumn]) [Row]
, *
FROM
[dbo].[SomeTable]
)
SELECT TOP 10
*
FROM
Rows
WHERE Row > 10
I believe is functionaly equivalent to
SELECT * FROM SomeTable LIMIT 10 OFFSET 10 ORDER BY SomeColumn
and the best performing way I know of doing it in TSQL, before MS SQL 2012.
If there are very many rows you may get better performance using a temp table instead of a CTE.
Unfortunately, the ROW_NUMBER() is the best you can do. It's actually more correct, because the results of a limit or top clause don't really have meaning without respect to some specific order. But it's still a pain to do.
Update: Sql Server 2012 adds a limit -like feature via OFFSET and FETCH keywords. This is the ansi-standard approach, as opposed to LIMIT, which is a non-standard MySql extension.
SELECT TOP 10 *
FROM TABLE
WHERE IDCOLUMN NOT IN (SELECT TOP 10 IDCOLUMN FROM TABLE)
Should give records 11-20.
Probably not too efficient if incrementing to get further pages, and not sure how it might be affected by ordering.
Might have to specify this in both WHERE statements.
A good way is to create a procedure:
create proc pagination (#startfrom int ,#endto int) as
SELECT * FROM (
SELECT *, ROW_NUMBER() OVER (ORDER BY name desc) as row FROM sys.databases
) a WHERE a.row > #startfrom and a.row <= #endto
just like limit 0,2
///////////////
execute pagination 0,4
Just for the record solution that works across most database engines though might not be the most efficient:
Select Top (ReturnCount) *
From (
Select Top (SkipCount + ReturnCount) *
From SourceTable
Order By ReverseSortCondition
) ReverseSorted
Order By SortCondition
Pelase note: the last page would still contain ReturnCount rows no matter what SkipCount is. But that might be a good thing in many cases.
The equivalent of LIMIT is SET ROWCOUNT, but if you want generic pagination it's better to write a query like this:
;WITH Results_CTE AS
(
SELECT
Col1, Col2, ...,
ROW_NUMBER() OVER (ORDER BY SortCol1, SortCol2, ...) AS RowNum
FROM Table
WHERE <whatever>
)
SELECT *
FROM Results_CTE
WHERE RowNum >= #Offset
AND RowNum < #Offset + #Limit
select * from (select id,name,ROW_NUMBER() OVER (ORDER BY id asc) as row
from tableName1) tbl1
where tbl1.row>=10 and tbl1.row<=15
Will print rows from 10 to 15.
So far this format is what is working for me (not the best performance though):
SELECT TOP {desired amount of rows} *
FROM (SELECT *, ROW_NUMBER() OVER (ORDER BY {order columns} asc)__row__ FROM {table})tmp
WHERE __row__ > {offset row count}
A note on the side, paginating over dynamic data can lead to strange/unexpected results.
From the MS SQL Server online documentation (http://technet.microsoft.com/en-us/library/ms186734.aspx
), here is their example that I have tested and works, for retrieving a specific set of rows. ROW_NUMBER requires an OVER, but you can order by whatever you like:
WITH OrderedOrders AS
(
SELECT SalesOrderID, OrderDate,
ROW_NUMBER() OVER (ORDER BY OrderDate) AS RowNumber
FROM Sales.SalesOrderHeader
)
SELECT SalesOrderID, OrderDate, RowNumber
FROM OrderedOrders
WHERE RowNumber BETWEEN 50 AND 60;
Use all SQL server:
;with tbl as (SELECT ROW_NUMBER() over(order by(select 1)) as RowIndex,* from table)
select top 10 * from tbl where RowIndex>=10
SELECT * FROM users WHERE Id Between 15 and 25
it will print from 15 to 25 as like limit in MYSQl