Issue while using 'case when ' in 'where' clause sql server - sql-server

While using case when in where clause in sql query it's not working.
Problem :
I have two tables named TblEmployee and TblAssociate.Both tables contains common columns PeriodId, EmpId and AssociateId. My requirement is to fetch values from
TblEmployee with combination of EmpId and AssociateId from TblAssociate should be excluded.And the exclusion should be based on PeriodId condition.`
If(#PeriodID<50)
BEGIN
SELECT *
FROM TblEmployee
WHERE (EmpId+AssociateId) NOT IN (SELECT EmpId+AssociateId FROM TblAssociate)
END
ELSE
BEGIN
SELECT *
FROM TblEmployee
WHERE (EmpId) NOT IN (SELECT EmpId FROM TblAssociate)
END
The above code is working, but I need to avoid that IF-ELSE condition and I wish to use 'case when' in where clause.Please help


Try this:
SELECT *
FROM TblEmployee
WHERE (EmpId + CASE WHEN #PeriodID<50 THEN AssociateId ELSE 0 END) NOT IN
(SELECT EmpId + CASE WHEN #PeriodID<50 THEN AssociateId ELSE 0 END FROM TblAssociate)
You say your code is working but this is rather odd, since it doesn't make much sense to add together id values. In any case, the above statement produces a result that is equivalent to the code originally posted.


You could use AND-OR combination in the WHERE clause. Additionally, you should not be using + as it may lead to incorrect result. You can rewrite your query as:
SELECT e.*
FROM TblEmployee e
WHERE
(
#PeriodID < 50
AND NOT EXISTS(
SELECT 1
FROM TblAssociate a
WHERE
a.EmpId = e.EmpId
AND a.AssociateId = e.AssociateId
)
)
OR
(
#PeriodID >= 50
AND NOT EXISTS(
SELECT 1
FROM TblAssociate a
WHERE a.EmpId = e.EmpId
)
)


The addition of IDs do not guarantee uniqueness. For instance, if EmpId is 5 and AssociateId is 6, then EmpId + AssociateId = 11, while EmpId + AssociateId = 11 even if EmpId is 6 and AssociateId is 5. In the query below, I made sure that the subquery will stop searching when the first record is found and will return a single record, having the value of 1. We select the employee if and only if 1 is among the results. In the subquery we check the operand we are sure of first and then check if we are not in a period where AssociateId must be checked, or it matches.
select *
from TblEmployee
where 1 in (select top 1 1
from TblAssociate
where TblEmployee.EmpId = TblAssociate.EmpId and
(#PeriodID >= 50 or TblEmployee.AssociateId = TblAssociate.AssociateId))

Related

Compare previous date and current date and update the table on basis on condition

I want to update my attendance table on basis of the following condition.
NonWorking type is 1
If its previous day or next attendance type is Absent then I want to mark NonWorking type is LWP in DAOthers Column.

I think you can use LAG() and LEAD() here to peek at the preceding and proceeding values of the attendance type. Then, if one of those should be absent, mark the NonWorking column appropriately.
WITH cte AS (
SELECT *,
LAG(AttendanceType, 1, 'Present') OVER (ORDER BY ADate) AS lag_at,
LEAD(AttendanceType, 1, 'Present') OVER (ORDER BY ADate) AS lead_at
FROM yourTable
)
UPDATE cte
SET NonWorking = 1
WHERE lag_at = 'Absent' OR lead_at = 'Absent'

I am not sure whether you want an sql query to update existing data or a solution which is needed while making an entry.
Use below query to update existing data:
Update AttendanceTable set DaOthers =
(select top 1 'LWP' from AttendanceTable at1
where AttendanceTable.EmployeeId = at1.EmployeeId
and DATEADD(day, -1,AttendanceTable.ADate) = at1.ADate
and at1.NonWorking = 1)
Table befor executing above query:
Table after executing above query:
To update at the time of inserting record:
If you want to update while inserting data then you may need to set a variable first then use that variable while inserting. In the first query you need to use ADate and EmployeeID.The Nonworking is always 1.
DECLARE #DaOthers nvarchar(20) = (select top 1 'LWP' from AttendanceTable at
where DATEADD(day, 1, at.ADate) ='2017-02-04' and at.NonWorking = 1 and EmployeeId = 1)
insert into AttendanceTable
(NonWorking, ADate, AttendanceType, EmployeeId, DaOthers)
values
(0,'2017-02-04', 'Present', 1,#DaOthers)

With CTE as
(
SELECT *,
DATEADD(DAY, 1, Lag(ADate, 1,ADate)
OVER (PARTITION BY DAttendanceId ORDER BY ADate ASC)) AS EndDate
FROM tbl_DailyAttendance where EmployeeId = 1001 and AMonth = 2 and AYear = 2017 and AttendanceType = 'Absent' and NonWorking = 0
)
--select * from CTE
select * from tbl_DailyAttendance tda inner join CTE c on tda.ADate = c.EndDate where tda.EmployeeID = 1001 and tda.NonWorking = 1 order by tda.ADate
This is how i do for checking the conditions

since you hvn't provided sample data,so try to understand my query and correct if anything minor.
;WITH CTE as
(
select *
,isnull((select 1 from tbl_DailyAttendance tdb
where ((tdb.adate=DATEADD(day,-1,tda.adate))
or (tdb.adate=DATEADD(day,1,tda.adate)))
and attendancetype='Absent'
),0) NewnonWorkingType
from tbl_DailyAttendance tda
)
--Testing purpose
--select * from cte
update tda
set nonworking=b.NewnonWorkingType
,daOther=case when b.NewnonWorkingType=1 then 'LWP'
else null end
from tbl_DailyAttendance tda
inner join cte b on tda.id=b.id

How to develop a recursive CTE in T-SQL?

I am new to recursive CTEs. I am trying to develop a CTE which will return all of the employees under each manager name. So I have two tables: people_rv and staff_rv
People_rv table contains all of the people, both managers and employees. Staff_rv only contains manager information. Uniqueidentifier staff values are stored in Staff_rv. Uniqueidentifier employee values are stored in people_rv. People_rv contains varchar first and last name values for both managers and employees.
But when I run the following CTE I get an error:
WITH
cteStaff (ClientID, FirstName, LastName, SupervisorID, EmpLevel)
AS
(
SELECT p.people_id, p.first_name, p.last_name, s.supervisor_id,1
FROM people_rv p JOIN staff_rv s on s.people_id = p.people_id
WHERE s.supervisor_id = '95E16819-8C3A-4098-9430-08F0E3B764E1'
UNION ALL
SELECT p2.people_id, p2.first_name, p2.last_name, s2.supervisor_id, r.EmpLevel + 1
FROM people_rv p2 JOIN staff_rv s2 on s2.people_id = p2.people_id
INNER JOIN cteStaff r on s2.staff_id = r.ClientID
)
SELECT
FirstName + ' ' + LastName AS FullName,
EmpLevel,
(SELECT first_name + ' ' + last_name FROM people_rv p join staff_rv s on s.people_id = p.people_id
WHERE s.staff_id = cteStaff.SupervisorID) AS Manager
FROM cteStaff
OPTION (MAXRECURSION 0);
My output is:
Barbara G 1 Melanie K
Dawn P 1 Melanie K
Garrett M 1 Melanie K
Stephanie P 1 Melanie K
Amanda F 1 Melanie K
Amanda T 1 Melanie K
Stephanie G 1 Melanie K
Carlos H 1 Melanie K
So it is not iterating any more than the first level. Why not?
Melanie is the top most supervisor, but each of the persons in the leftmost column are also supervisors. So this query should also return level 2.

You may be in an infinite loop with your join. I would check how many levels you expect the table to actually go down. Generally you join a recursion on something similar to do
ID = ParentID
of something either contained in a table or in an expression. Keep in mind you can also create a CTE prior to a recursive CTE if you have to make up your relationship.
Here is an example that will self execute, it may help.
Declare #table table ( PersonId int identity, PersonName varchar(512), Account int, ParentId int, Orders int);
insert into #Table values ('Brett', 1, NULL, 1000),('John', 1, 1, 100),('James', 1, 1, 200),('Beth', 1, 2, 300),('John2', 2, 4, 400);
select
PersonID
, PersonName
, Account
, ParentID
from #Table
; with recursion as
(
select
t1.PersonID
, t1.PersonName
, t1.Account
--, t1.ParentID
, cast(isnull(t2.PersonName, '')
+ Case when t2.PersonName is not null then '\' + t1.PersonName else t1.PersonName end
as varchar(255)) as fullheirarchy
, 1 as pos
, cast(t1.orders +
isnull(t2.orders,0) -- if the parent has no orders than zero
as int) as Orders
from #Table t1
left join #Table t2 on t1.ParentId = t2.PersonId
union all
select
t.PersonID
, t.PersonName
, t.Account
--, t.ParentID
, cast(r.fullheirarchy + '\' + t.PersonName as varchar(255))
, pos + 1 -- increases
, r.orders + t.orders
from #Table t
join recursion r on t.ParentId = r.PersonId
)
, b as
(
select *, max(pos) over(partition by PersonID) as maxrec -- I find the maximum occurrence of position by person
from recursion
)
select *
from b
where pos = maxrec -- finds the furthest down tree
-- and Account = 2 -- I could find just someone from a different department

Your problem as far as I can tell is is you have no join connecting managers to their employees.
This join
INNER JOIN cteStaff r on r.StaffID = s2.staff_id
Just joins the same initial level 1 staffer back to himself.
UPDATE:
Still not quite right! You have a supervisor_id, but again you're still not actually using that to join back to the CTE.
So for each recursion of this CTE you need to (excluding the name join):
select {Level 1 Boss}, NULL (no supervisor)
union
select {new employee}, {that employee's boss}
So the join must connect the CTE's ClientID (the level 1 boss) to the second UNION query's supervisor field, which looks to be supervisor_id , not staff_id.
The JOIN to accomplish this second task is (from what I can tell of your staff_rv table schema):
SELECT p2.people_id, p2.first_name, p2.last_name, s2.supervisor_id, r.EmpLevel + 1
FROM people_rv p2 JOIN staff_rv s2 on s2.people_id = p2.people_id
INNER JOIN cteStaff r on s2.supervisor_id = r.ClientID
Note the bottom join joins the r.ClientID (the level 1 boss) to the staffer's supervisor_id field.
(NB: I think your staff_id and supervisor_id's mimic your people_id values from the people_rv table, so this join should work fine. But if they are different (i.e. a staffer's supervisor_id isn't that supervisor's people_id) then you'll need to write the join such that the staffer's supervisor_id can be joined to their people_id you're storing as ClientID in the CTE.)

Here's a good simple Recursive CTE to review (it may not be the answer, but someone else searching on how to make a recursive CTE may need it):
-- Recursive CTE
;
WITH Years ( myYear )
AS (
-- Base case
SELECT DATEPART(year, GETDATE())
UNION ALL
-- Recursive
SELECT Years.myYear - 1
FROM Years
WHERE Years.myYear >= 2002
)
SELECT *
FROM Years

Note that this probably won't solve your problem, but is a means to hopefully seeing where you're going wrong in the original query.
The default is 100 levels of recursion - you can set it to unlimited by using the MAXRECURSION query hint where you're selecting from your CTE:
...
FROM cteStaff
OPTION (MAXRECURSION 0);
From MSDN:
MAXRECURSION number
Specifies the maximum number of recursions allowed for this query. number is a nonnegative integer between 0 and 32767. When 0 is
specified, no limit is applied. If this option is not specified, the
default limit for the server is 100.
When the specified or default number for MAXRECURSION limit is reached during query execution, the query is ended and an error is
returned.
Because of this error, all effects of the statement are rolled back. If the statement is a SELECT statement, partial results or no
results may be returned. Any partial results returned may not include
all rows on recursion levels beyond the specified maximum recursion
level.

SQL Server - Insert into with select and union - duplicates being inserted

When I execute my "select union select", I get the correct number or rows (156)
Executed independently, select #1 returns 65 rows and select #2 returns 138 rows.
When I use this "select union select" with an Insert into, I get 203 rows (65+138) with duplicates.
I would like to know if it is my code structure that is causing this issue ?
INSERT INTO dpapm_MediaObjectValidation (mediaobject_id, username, checked_date, expiration_date, notified)
(SELECT FKMediaObjectId, CreatedBy,#checkdate,dateadd(ww,2,#checkdate),0
FROM dbo.gs_MediaObjectMetadata
LEFT JOIN gs_MediaObject mo
ON gs_MediaObjectMetadata.FKMediaObjectId = mo.MediaObjectId
WHERE UPPER([Description]) IN ('CAPTION','TITLE','AUTHOR','DATE PHOTO TAKEN','KEYWORDS')
AND FKMediaObjectId >=
(SELECT TOP 1 MediaObjectId
FROM dbo.gs_MediaObject
WHERE DateAdded > #lastcheck
ORDER BY MediaObjectId)
GROUP BY FKMediaObjectId, CreatedBy
HAVING count(*) < 5
UNION
SELECT FKMediaObjectId, CreatedBy,getdate(),dateadd(ww,2,getdate()),0
FROM gs_MediaObjectMetadata yt
LEFT JOIN gs_MediaObject mo
ON yt.FKMediaObjectId = mo.MediaObjectId
WHERE UPPER([Description]) = 'KEYWORDS'
AND FKMediaObjectId >=
(SELECT TOP 1 MediaObjectId
FROM dbo.gs_MediaObject
WHERE DateAdded > #lastcheck
ORDER BY MediaObjectId)
AND NOT EXISTS
(
SELECT *
FROM dbo.fnSplit(Replace(yt.Value, '''', ''''''), ',') split
WHERE split.item in (SELECT KeywordEn FROM gs_Keywords) or split.item in (SELECT KeywordFr FROM gs_Keywords)
)
)
I would appreciate any clues into resolving this problem ...
Thank you !

The UNION keyword should only return distinct records between the two queries. However, if I recall correctly, this is only true if the datatypes are the same. The date variables might be throwing that off. Depending on the collation type, whitespace might be handled differently as well. You might want to do a SELECT DISTINCT on the dpapm_MediaObjectValidation table after doing your insert, but be sure to trim whitespace from both sides in your comparison. Another approach is to do your first insert, then on your second insert, forgo the UNION altogether and do a manual EXISTS check to see if the items to be inserted already exist.

SQL Server 2005: Update rows in a specified order (like ORDER BY)?

I want to update rows of a table in a specific order, like one would expect if including an ORDER BY clause, but SQL Server does not support the ORDER BY clause in UPDATE queries.
I have checked out this question which supplied a nice solution, but my query is a bit more complicated than the one specified there.
UPDATE TableA AS Parent
SET Parent.ColA = Parent.ColA + (SELECT TOP 1 Child.ColA
FROM TableA AS Child
WHERE Child.ParentColB = Parent.ColB
ORDER BY Child.Priority)
ORDER BY Parent.Depth DESC;
So, what I'm hoping that you'll notice is that a single table (TableA) contains a hierarchy of rows, wherein one row can be the parent or child of any other row. The rows need to be updated in order from the deepest child up to the root parent. This is because TableA.ColA must contain an up-to-date concatenation of its own current value with the values of its children (I realize this query only concats with one child, but that is for the sake of simplicity - the purpose of the example in this question does not necessitate any more verbosity), therefore the query must update from the bottom up.
The solution suggested in the question I noted above is as follows:
UPDATE messages
SET status=10
WHERE ID in (SELECT TOP (10) Id
FROM Table
WHERE status=0
ORDER BY priority DESC
);
The reason that I don't think I can use this solution is because I am referencing column values from the parent table inside my subquery (see WHERE Child.ParentColB = Parent.ColB), and I don't think two sibling subqueries would have access to each others' data.
So far I have only determined one way to merge that suggested solution with my current problem, and I don't think it works.
UPDATE TableA AS Parent
SET Parent.ColA = Parent.ColA + (SELECT TOP 1 Child.ColA
FROM TableA AS Child
WHERE Child.ParentColB = Parent.ColB
ORDER BY Child.Priority)
WHERE Parent.Id IN (SELECT Id
FROM TableA
ORDER BY Parent.Depth DESC);
The WHERE..IN subquery will not actually return a subset of the rows, it will just return the full list of IDs in the order that I want. However (I don't know for sure - please tell me if I'm wrong) I think that the WHERE..IN clause will not care about the order of IDs within the parentheses - it will just check the ID of the row it currently wants to update to see if it's in that list (which, they all are) in whatever order it is already trying to update... Which would just be a total waste of cycles, because it wouldn't change anything.
So, in conclusion, I have looked around and can't seem to figure out a way to update in a specified order (and included the reason I need to update in that order, because I am sure I would otherwise get the ever-so-useful "why?" answers) and I am now hitting up Stack Overflow to see if any of you gurus out there who know more about SQL than I do (which isn't saying much) know of an efficient way to do this. It's particularly important that I only use a single query to complete this action.
A long question, but I wanted to cover my bases and give you guys as much info to feed off of as possible. :)
Any thoughts?

You cannot succeed this in one query, because your updates are correlated (ie. level N depends on the updated value of level N+1). Relational engines frown on this very explicitly because of the Halloween Problem. The query plan will go out of its way to ensure that the updates occur as if they had two stages: one in which the current state was read, and then one in which the updated state was applied. If necessary, they'll spool intermediate tables just to preserve this apparent execution order (read all->write all). Since your query, if I understand correctly, tries to break this very premise I don't see any way you'll succeed.

UPDATE statements will be executed as a single query, not as a step by step result.
You need to either use a while loop/cursor (uhhgg) or maybe make use of a CTE expression view to achieve what you are trying, which gives you the recursice possibility.
Have a look at
Using Common Table Expressions
Recursive Queries Using Common Table
Expressions

Here is a one line SQL solution. If you ever relax the requirement that it need be one update statement you can factor out some of the complexity
CREATE TABLE [TableA](
[ID] [int] NOT NULL,
[ParentID] [int] NULL,
[ColA] [varchar](max) NOT NULL,
[Priority] [varchar](50) NOT NULL,
[Depth] [int] NOT NULL)
go
INSERT TableA
SELECT 1, NULL, 'p', 'Favorite', 0 UNION ALL
SELECT 2, 1, 'm', 'Favorite', 1 UNION ALL
SELECT 3, 1, 'o', 'Likeable', 1 UNION ALL
SELECT 4, 2, 'v', 'Favorite', 2 UNION ALL
SELECT 5, 2, 'v', 'Likeable', 2 UNION ALL
SELECT 6, 2, 'd', 'Likeable', 2 UNION ALL
SELECT 7, 6, 'c', 'Red-headed Stepchild', 3 UNION ALL
SELECT 8, 6, 's', 'Likeable', 3 UNION ALL
SELECT 9, 8, 'n', 'Favorite', 4 UNION ALL
SELECT 10, 6, 'c', 'Favorite', 3 UNION ALL
SELECT 11, 5, 'c', 'Favorite', 3 UNION ALL
SELECT 12, NULL, 'z', 'Favorite', 0 UNION ALL
SELECT 13, 3, 'e', 'Favorite', 2 UNION ALL
SELECT 14, 8, 'k', 'Likeable', 4 UNION ALL
SELECT 15,4, 'd', 'Favorite', 3
;WITH cte AS (
SELECT a.i, a.Depth, a.maxd, a.mind, a.maxc, a.di, a.ci, a.cdi, a.ID, a.y, CAST('' AS varchar(max))z
FROM(
SELECT DISTINCT i = 1
,p.Depth
,maxd = (SELECT MAX(Depth) FROM TableA)
,mind = (SELECT MIN(Depth) FROM TableA)
,maxc = (SELECT MAX(c) FROM (SELECT COUNT(*) OVER(PARTITION BY ParentID) FROM TableA)f(c))
,di = (SELECT MIN(Depth) FROM TableA)
,ci = 1
,cdi = (SELECT MIN(Depth) FROM TableA)
,p.ID
,CAST(p.ID AS varchar(max)) + p.ColA + SPACE(1) + CASE WHEN g IS NULL THEN '' ELSE '(' END
+ ISNULL(g,'') + CASE WHEN g IS NULL THEN '' ELSE ')' END y
FROM TableA p
LEFT JOIN TableA c ON (c.ParentID = p.ID)
CROSS APPLY (SELECT SPACE(1) + CAST(c2.ID AS varchar(max)) + ColA + SPACE(1)
FROM TableA c2 WHERE ParentID = p.ID
ORDER BY Priority
FOR XML PATH(''))f(g)
)a
UNION ALL
SELECT r.i, r.Depth, r.maxd, r.mind, r.maxc, r.di, r.ci, r.cdi, r.ID
,CASE WHEN di = cdi
THEN REPLACE(r.y,LEFT(r.z,CHARINDEX(SPACE(1),r.z,2)), r.z)
ELSE r.y END [y]
,r.z
FROM(
SELECT i = i + 1
,Depth
,[maxd]
,[mind]
,[maxc]
,CASE WHEN ci = maxc AND cdi = maxd
THEN di + 1
ELSE di
END [di]
,CASE WHEN cdi = [maxd]
THEN CASE WHEN ci + 1 > maxc
THEN 1
ELSE ci + 1
END
ELSE ci
END [ci]
,CASE WHEN cdi + 1 > maxd
THEN mind
ELSE cdi + 1
END [cdi]
,id,y
,CAST(ISNULL((SELECT y FROM(
SELECT p.Depth,p.ID
,SPACE(1) + CAST(p.ID AS varchar(max)) + p.ColA + SPACE(1) +
CASE WHEN g IS NULL THEN '' ELSE '(' END + ISNULL(g,'')
+ CASE WHEN g IS NULL THEN '' ELSE ')' END y
,r1 = DENSE_RANK() OVER(ORDER BY p.ID) --child number
,r2 = ROW_NUMBER() OVER(PARTITION BY p.ID ORDER BY p.ID) --DISTINCT not allowed in recursive section
FROM TableA p
JOIN TableA c ON (c.ParentID = p.ID)
CROSS APPLY (SELECT SPACE(1)+CAST(c2.ID AS varchar(max))+ColA+SPACE(1)
FROM TableA c2
WHERE ParentID = p.ID
ORDER BY Priority
FOR XML PATH(''))f(g)
WHERE p.Depth = cdi AND cdi < di AND p.ID <> cte.ID
)v
WHERE r1 = ci
AND r2 = 1
AND cte.y LIKE '%' + LEFT(v.y,CHARINDEX(SPACE(1),v.y,2) ) + '%'),'') AS varchar(max)) z
FROM cte
WHERE [di]<[maxd] or [ci]<[maxc] or [cdi]<[maxd]
)r
)--cte
UPDATE t
SET ColA = REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE
(y,SPACE(1),''),'1',''),'2',''),'3',''),'4',''),'5',''),'6',''),'7',''),'8',''),'9',''),'0','')
FROM cte
JOIN TableA t ON (t.ID = cte.ID)
WHERE di = (SELECT MAX(Depth) FROM TableA)
AND cdi = (SELECT MAX(Depth) FROM TableA)
AND ci = (SELECT MAX(c) FROM (SELECT COUNT(*) OVER(PARTITION BY ParentID) FROM TableA)f(c))
OPTION(maxrecursion 0)
SELECT * FROM TableA
DROP TABLE TableA

JMTyler-
1 What kind of data is in ColA? What does it look like?
2 How is/should that column be originally populated? I ask this because you would only be able to run the update once since the value in that column would be modified from a previous run. Any additional runs would just concatenate more data. Which makes me believe there is another ColC with the original value for ColA (a person's name?)
3 Will a row ever be deleted orphaning it's children? If yes what should their ParentColB then point to? NULL? Does their depth then get set to 0 so they are now at the top of the hierarchy?
If you can answer this I can give you a solution
Thanks

Function to Calculate Median in SQL Server

According to MSDN, Median is not available as an aggregate function in Transact-SQL. However, I would like to find out whether it is possible to create this functionality (using the Create Aggregate function, user defined function, or some other method).
What would be the best way (if possible) to do this - allow for the calculation of a median value (assuming a numeric data type) in an aggregate query?

If you're using SQL 2005 or better this is a nice, simple-ish median calculation for a single column in a table:
SELECT
(
(SELECT MAX(Score) FROM
(SELECT TOP 50 PERCENT Score FROM Posts ORDER BY Score) AS BottomHalf)
+
(SELECT MIN(Score) FROM
(SELECT TOP 50 PERCENT Score FROM Posts ORDER BY Score DESC) AS TopHalf)
) / 2 AS Median

2019 UPDATE: In the 10 years since I wrote this answer, more solutions have been uncovered that may yield better results. Also, SQL Server releases since then (especially SQL 2012) have introduced new T-SQL features that can be used to calculate medians. SQL Server releases have also improved its query optimizer which may affect perf of various median solutions. Net-net, my original 2009 post is still OK but there may be better solutions on for modern SQL Server apps. Take a look at this article from 2012 which is a great resource: https://sqlperformance.com/2012/08/t-sql-queries/median
This article found the following pattern to be much, much faster than all other alternatives, at least on the simple schema they tested. This solution was 373x faster (!!!) than the slowest (PERCENTILE_CONT) solution tested. Note that this trick requires two separate queries which may not be practical in all cases. It also requires SQL 2012 or later.
DECLARE #c BIGINT = (SELECT COUNT(*) FROM dbo.EvenRows);
SELECT AVG(1.0 * val)
FROM (
SELECT val FROM dbo.EvenRows
ORDER BY val
OFFSET (#c - 1) / 2 ROWS
FETCH NEXT 1 + (1 - #c % 2) ROWS ONLY
) AS x;
Of course, just because one test on one schema in 2012 yielded great results, your mileage may vary, especially if you're on SQL Server 2014 or later. If perf is important for your median calculation, I'd strongly suggest trying and perf-testing several of the options recommended in that article to make sure that you've found the best one for your schema.
I'd also be especially careful using the (new in SQL Server 2012) function PERCENTILE_CONT that's recommended in one of the other answers to this question, because the article linked above found this built-in function to be 373x slower than the fastest solution. It's possible that this disparity has been improved in the 7 years since, but personally I wouldn't use this function on a large table until I verified its performance vs. other solutions.
ORIGINAL 2009 POST IS BELOW:
There are lots of ways to do this, with dramatically varying performance. Here's one particularly well-optimized solution, from Medians, ROW_NUMBERs, and performance. This is a particularly optimal solution when it comes to actual I/Os generated during execution – it looks more costly than other solutions, but it is actually much faster.
That page also contains a discussion of other solutions and performance testing details. Note the use of a unique column as a disambiguator in case there are multiple rows with the same value of the median column.
As with all database performance scenarios, always try to test a solution out with real data on real hardware – you never know when a change to SQL Server's optimizer or a peculiarity in your environment will make a normally-speedy solution slower.
SELECT
CustomerId,
AVG(TotalDue)
FROM
(
SELECT
CustomerId,
TotalDue,
-- SalesOrderId in the ORDER BY is a disambiguator to break ties
ROW_NUMBER() OVER (
PARTITION BY CustomerId
ORDER BY TotalDue ASC, SalesOrderId ASC) AS RowAsc,
ROW_NUMBER() OVER (
PARTITION BY CustomerId
ORDER BY TotalDue DESC, SalesOrderId DESC) AS RowDesc
FROM Sales.SalesOrderHeader SOH
) x
WHERE
RowAsc IN (RowDesc, RowDesc - 1, RowDesc + 1)
GROUP BY CustomerId
ORDER BY CustomerId;

In SQL Server 2012 you should use PERCENTILE_CONT:
SELECT SalesOrderID, OrderQty,
PERCENTILE_CONT(0.5)
WITHIN GROUP (ORDER BY OrderQty)
OVER (PARTITION BY SalesOrderID) AS MedianCont
FROM Sales.SalesOrderDetail
WHERE SalesOrderID IN (43670, 43669, 43667, 43663)
ORDER BY SalesOrderID DESC
See also : http://blog.sqlauthority.com/2011/11/20/sql-server-introduction-to-percentile_cont-analytic-functions-introduced-in-sql-server-2012/

My original quick answer was:
select max(my_column) as [my_column], quartile
from (select my_column, ntile(4) over (order by my_column) as [quartile]
from my_table) i
--where quartile = 2
group by quartile
This will give you the median and interquartile range in one fell swoop. If you really only want one row that is the median then uncomment the where clause.
When you stick that into an explain plan, 60% of the work is sorting the data which is unavoidable when calculating position dependent statistics like this.
I've amended the answer to follow the excellent suggestion from Robert Ševčík-Robajz in the comments below:
;with PartitionedData as
(select my_column, ntile(10) over (order by my_column) as [percentile]
from my_table),
MinimaAndMaxima as
(select min(my_column) as [low], max(my_column) as [high], percentile
from PartitionedData
group by percentile)
select
case
when b.percentile = 10 then cast(b.high as decimal(18,2))
else cast((a.low + b.high) as decimal(18,2)) / 2
end as [value], --b.high, a.low,
b.percentile
from MinimaAndMaxima a
join MinimaAndMaxima b on (a.percentile -1 = b.percentile) or (a.percentile = 10 and b.percentile = 10)
--where b.percentile = 5
This should calculate the correct median and percentile values when you have an even number of data items. Again, uncomment the final where clause if you only want the median and not the entire percentile distribution.

Even better:
SELECT #Median = AVG(1.0 * val)
FROM
(
SELECT o.val, rn = ROW_NUMBER() OVER (ORDER BY o.val), c.c
FROM dbo.EvenRows AS o
CROSS JOIN (SELECT c = COUNT(*) FROM dbo.EvenRows) AS c
) AS x
WHERE rn IN ((c + 1)/2, (c + 2)/2);
From the master Himself, Itzik Ben-Gan!

MS SQL Server 2012 (and later) has the PERCENTILE_DISC function which computes a specific percentile for sorted values. PERCENTILE_DISC (0.5) will compute the median - https://msdn.microsoft.com/en-us/library/hh231327.aspx

Simple, fast, accurate
SELECT x.Amount
FROM (SELECT amount,
Count(1) OVER (partition BY 'A') AS TotalRows,
Row_number() OVER (ORDER BY Amount ASC) AS AmountOrder
FROM facttransaction ft) x
WHERE x.AmountOrder = Round(x.TotalRows / 2.0, 0)

If you want to use the Create Aggregate function in SQL Server, this is how to do it. Doing it this way has the benefit of being able to write clean queries. Note this this process could be adapted to calculate a Percentile value fairly easily.
Create a new Visual Studio project and set the target framework to .NET 3.5 (this is for SQL 2008, it may be different in SQL 2012). Then create a class file and put in the following code, or c# equivalent:
Imports Microsoft.SqlServer.Server
Imports System.Data.SqlTypes
Imports System.IO
<Serializable>
<SqlUserDefinedAggregate(Format.UserDefined, IsInvariantToNulls:=True, IsInvariantToDuplicates:=False, _
IsInvariantToOrder:=True, MaxByteSize:=-1, IsNullIfEmpty:=True)>
Public Class Median
Implements IBinarySerialize
Private _items As List(Of Decimal)
Public Sub Init()
_items = New List(Of Decimal)()
End Sub
Public Sub Accumulate(value As SqlDecimal)
If Not value.IsNull Then
_items.Add(value.Value)
End If
End Sub
Public Sub Merge(other As Median)
If other._items IsNot Nothing Then
_items.AddRange(other._items)
End If
End Sub
Public Function Terminate() As SqlDecimal
If _items.Count <> 0 Then
Dim result As Decimal
_items = _items.OrderBy(Function(i) i).ToList()
If _items.Count Mod 2 = 0 Then
result = ((_items((_items.Count / 2) - 1)) + (_items(_items.Count / 2))) / 2#
Else
result = _items((_items.Count - 1) / 2)
End If
Return New SqlDecimal(result)
Else
Return New SqlDecimal()
End If
End Function
Public Sub Read(r As BinaryReader) Implements IBinarySerialize.Read
'deserialize it from a string
Dim list = r.ReadString()
_items = New List(Of Decimal)
For Each value In list.Split(","c)
Dim number As Decimal
If Decimal.TryParse(value, number) Then
_items.Add(number)
End If
Next
End Sub
Public Sub Write(w As BinaryWriter) Implements IBinarySerialize.Write
'serialize the list to a string
Dim list = ""
For Each item In _items
If list <> "" Then
list += ","
End If
list += item.ToString()
Next
w.Write(list)
End Sub
End Class
Then compile it and copy the DLL and PDB file to your SQL Server machine and run the following command in SQL Server:
CREATE ASSEMBLY CustomAggregate FROM '{path to your DLL}'
WITH PERMISSION_SET=SAFE;
GO
CREATE AGGREGATE Median(#value decimal(9, 3))
RETURNS decimal(9, 3)
EXTERNAL NAME [CustomAggregate].[{namespace of your DLL}.Median];
GO
You can then write a query to calculate the median like this:
SELECT dbo.Median(Field) FROM Table

I just came across this page while looking for a set based solution to median. After looking at some of the solutions here, I came up with the following. Hope is helps/works.
DECLARE #test TABLE(
i int identity(1,1),
id int,
score float
)
INSERT INTO #test (id,score) VALUES (1,10)
INSERT INTO #test (id,score) VALUES (1,11)
INSERT INTO #test (id,score) VALUES (1,15)
INSERT INTO #test (id,score) VALUES (1,19)
INSERT INTO #test (id,score) VALUES (1,20)
INSERT INTO #test (id,score) VALUES (2,20)
INSERT INTO #test (id,score) VALUES (2,21)
INSERT INTO #test (id,score) VALUES (2,25)
INSERT INTO #test (id,score) VALUES (2,29)
INSERT INTO #test (id,score) VALUES (2,30)
INSERT INTO #test (id,score) VALUES (3,20)
INSERT INTO #test (id,score) VALUES (3,21)
INSERT INTO #test (id,score) VALUES (3,25)
INSERT INTO #test (id,score) VALUES (3,29)
DECLARE #counts TABLE(
id int,
cnt int
)
INSERT INTO #counts (
id,
cnt
)
SELECT
id,
COUNT(*)
FROM
#test
GROUP BY
id
SELECT
drv.id,
drv.start,
AVG(t.score)
FROM
(
SELECT
MIN(t.i)-1 AS start,
t.id
FROM
#test t
GROUP BY
t.id
) drv
INNER JOIN #test t ON drv.id = t.id
INNER JOIN #counts c ON t.id = c.id
WHERE
t.i = ((c.cnt+1)/2)+drv.start
OR (
t.i = (((c.cnt+1)%2) * ((c.cnt+2)/2))+drv.start
AND ((c.cnt+1)%2) * ((c.cnt+2)/2) <> 0
)
GROUP BY
drv.id,
drv.start

The following query returns the median from a list of values in one column. It cannot be used as or along with an aggregate function, but you can still use it as a sub-query with a WHERE clause in the inner select.
SQL Server 2005+:
SELECT TOP 1 value from
(
SELECT TOP 50 PERCENT value
FROM table_name
ORDER BY value
)for_median
ORDER BY value DESC

Although Justin grant's solution appears solid I found that when you have a number of duplicate values within a given partition key the row numbers for the ASC duplicate values end up out of sequence so they do not properly align.
Here is a fragment from my result:
KEY VALUE ROWA ROWD
13 2 22 182
13 1 6 183
13 1 7 184
13 1 8 185
13 1 9 186
13 1 10 187
13 1 11 188
13 1 12 189
13 0 1 190
13 0 2 191
13 0 3 192
13 0 4 193
13 0 5 194
I used Justin's code as the basis for this solution. Although not as efficient given the use of multiple derived tables it does resolve the row ordering problem I encountered. Any improvements would be welcome as I am not that experienced in T-SQL.
SELECT PKEY, cast(AVG(VALUE)as decimal(5,2)) as MEDIANVALUE
FROM
(
SELECT PKEY,VALUE,ROWA,ROWD,
'FLAG' = (CASE WHEN ROWA IN (ROWD,ROWD-1,ROWD+1) THEN 1 ELSE 0 END)
FROM
(
SELECT
PKEY,
cast(VALUE as decimal(5,2)) as VALUE,
ROWA,
ROW_NUMBER() OVER (PARTITION BY PKEY ORDER BY ROWA DESC) as ROWD
FROM
(
SELECT
PKEY,
VALUE,
ROW_NUMBER() OVER (PARTITION BY PKEY ORDER BY VALUE ASC,PKEY ASC ) as ROWA
FROM [MTEST]
)T1
)T2
)T3
WHERE FLAG = '1'
GROUP BY PKEY
ORDER BY PKEY

In a UDF, write:
Select Top 1 medianSortColumn from Table T
Where (Select Count(*) from Table
Where MedianSortColumn <
(Select Count(*) From Table) / 2)
Order By medianSortColumn

Justin's example above is very good. But that Primary key need should be stated very clearly. I have seen that code in the wild without the key and the results are bad.
The complaint I get about the Percentile_Cont is that it wont give you an actual value from the dataset.
To get to a "median" that is an actual value from the dataset use Percentile_Disc.
SELECT SalesOrderID, OrderQty,
PERCENTILE_DISC(0.5)
WITHIN GROUP (ORDER BY OrderQty)
OVER (PARTITION BY SalesOrderID) AS MedianCont
FROM Sales.SalesOrderDetail
WHERE SalesOrderID IN (43670, 43669, 43667, 43663)
ORDER BY SalesOrderID DESC

Using a single statement - One way is to use ROW_NUMBER(), COUNT() window function and filter the sub-query. Here is to find the median salary:
SELECT AVG(e_salary)
FROM
(SELECT
ROW_NUMBER() OVER(ORDER BY e_salary) as row_no,
e_salary,
(COUNT(*) OVER()+1)*0.5 AS row_half
FROM Employee) t
WHERE row_no IN (FLOOR(row_half),CEILING(row_half))
I have seen similar solutions over the net using FLOOR and CEILING but tried to use a single statement. (edited)

Median Finding
This is the simplest method to find the median of an attribute.
Select round(S.salary,4) median from employee S
where (select count(salary) from station
where salary < S.salary ) = (select count(salary) from station
where salary > S.salary)

See other solutions for median calculation in SQL here:
"Simple way to calculate median with MySQL" (the solutions are mostly vendor-independent).

Building on Jeff Atwood's answer above here it is with GROUP BY and a correlated subquery to get the median for each group.
SELECT TestID,
(
(SELECT MAX(Score) FROM
(SELECT TOP 50 PERCENT Score FROM Posts WHERE TestID = Posts_parent.TestID ORDER BY Score) AS BottomHalf)
+
(SELECT MIN(Score) FROM
(SELECT TOP 50 PERCENT Score FROM Posts WHERE TestID = Posts_parent.TestID ORDER BY Score DESC) AS TopHalf)
) / 2 AS MedianScore,
AVG(Score) AS AvgScore, MIN(Score) AS MinScore, MAX(Score) AS MaxScore
FROM Posts_parent
GROUP BY Posts_parent.TestID

For a continuous variable/measure 'col1' from 'table1'
select col1
from
(select top 50 percent col1,
ROW_NUMBER() OVER(ORDER BY col1 ASC) AS Rowa,
ROW_NUMBER() OVER(ORDER BY col1 DESC) AS Rowd
from table1 ) tmp
where tmp.Rowa = tmp.Rowd

Frequently, we may need to calculate Median not just for the whole table, but for aggregates with respect to some ID. In other words, calculate median for each ID in our table, where each ID has many records. (based on the solution edited by #gdoron: good performance and works in many SQL)
SELECT our_id, AVG(1.0 * our_val) as Median
FROM
( SELECT our_id, our_val,
COUNT(*) OVER (PARTITION BY our_id) AS cnt,
ROW_NUMBER() OVER (PARTITION BY our_id ORDER BY our_val) AS rnk
FROM our_table
) AS x
WHERE rnk IN ((cnt + 1)/2, (cnt + 2)/2) GROUP BY our_id;
Hope it helps.

For large scale datasets, you can try this GIST:
https://gist.github.com/chrisknoll/1b38761ce8c5016ec5b2
It works by aggregating the distinct values you would find in your set (such as ages, or year of birth, etc.), and uses SQL window functions to locate any percentile position you specify in the query.

To get median value of salary from employee table
with cte as (select salary, ROW_NUMBER() over (order by salary asc) as num from employees)
select avg(salary) from cte where num in ((select (count(*)+1)/2 from employees), (select (count(*)+2)/2 from employees));

I wanted to work out a solution by myself, but my brain tripped and fell on the way. I think it works, but don't ask me to explain it in the morning. :P
DECLARE #table AS TABLE
(
Number int not null
);
insert into #table select 2;
insert into #table select 4;
insert into #table select 9;
insert into #table select 15;
insert into #table select 22;
insert into #table select 26;
insert into #table select 37;
insert into #table select 49;
DECLARE #Count AS INT
SELECT #Count = COUNT(*) FROM #table;
WITH MyResults(RowNo, Number) AS
(
SELECT RowNo, Number FROM
(SELECT ROW_NUMBER() OVER (ORDER BY Number) AS RowNo, Number FROM #table) AS Foo
)
SELECT AVG(Number) FROM MyResults WHERE RowNo = (#Count+1)/2 OR RowNo = ((#Count+1)%2) * ((#Count+2)/2)

--Create Temp Table to Store Results in
DECLARE #results AS TABLE
(
[Month] datetime not null
,[Median] int not null
);
--This variable will determine the date
DECLARE #IntDate as int
set #IntDate = -13
WHILE (#IntDate < 0)
BEGIN
--Create Temp Table
DECLARE #table AS TABLE
(
[Rank] int not null
,[Days Open] int not null
);
--Insert records into Temp Table
insert into #table
SELECT
rank() OVER (ORDER BY DATEADD(mm, DATEDIFF(mm, 0, DATEADD(ss, SVR.close_date, '1970')), 0), DATEDIFF(day,DATEADD(ss, SVR.open_date, '1970'),DATEADD(ss, SVR.close_date, '1970')),[SVR].[ref_num]) as [Rank]
,DATEDIFF(day,DATEADD(ss, SVR.open_date, '1970'),DATEADD(ss, SVR.close_date, '1970')) as [Days Open]
FROM
mdbrpt.dbo.View_Request SVR
LEFT OUTER JOIN dbo.dtv_apps_systems vapp
on SVR.category = vapp.persid
LEFT OUTER JOIN dbo.prob_ctg pctg
on SVR.category = pctg.persid
Left Outer Join [mdbrpt].[dbo].[rootcause] as [Root Cause]
on [SVR].[rootcause]=[Root Cause].[id]
Left Outer Join [mdbrpt].[dbo].[cr_stat] as [Status]
on [SVR].[status]=[Status].[code]
LEFT OUTER JOIN [mdbrpt].[dbo].[net_res] as [net]
on [net].[id]=SVR.[affected_rc]
WHERE
SVR.Type IN ('P')
AND
SVR.close_date IS NOT NULL
AND
[Status].[SYM] = 'Closed'
AND
SVR.parent is null
AND
[Root Cause].[sym] in ( 'RC - Application','RC - Hardware', 'RC - Operational', 'RC - Unknown')
AND
(
[vapp].[appl_name] in ('3PI','Billing Rpts/Files','Collabrent','Reports','STMS','STMS 2','Telco','Comergent','OOM','C3-BAU','C3-DD','DIRECTV','DIRECTV Sales','DIRECTV Self Care','Dealer Website','EI Servlet','Enterprise Integration','ET','ICAN','ODS','SB-SCM','SeeBeyond','Digital Dashboard','IVR','OMS','Order Services','Retail Services','OSCAR','SAP','CTI','RIO','RIO Call Center','RIO Field Services','FSS-RIO3','TAOS','TCS')
OR
pctg.sym in ('Systems.Release Health Dashboard.Problem','DTV QA Test.Enterprise Release.Deferred Defect Log')
AND
[Net].[nr_desc] in ('3PI','Billing Rpts/Files','Collabrent','Reports','STMS','STMS 2','Telco','Comergent','OOM','C3-BAU','C3-DD','DIRECTV','DIRECTV Sales','DIRECTV Self Care','Dealer Website','EI Servlet','Enterprise Integration','ET','ICAN','ODS','SB-SCM','SeeBeyond','Digital Dashboard','IVR','OMS','Order Services','Retail Services','OSCAR','SAP','CTI','RIO','RIO Call Center','RIO Field Services','FSS-RIO3','TAOS','TCS')
)
AND
DATEADD(mm, DATEDIFF(mm, 0, DATEADD(ss, SVR.close_date, '1970')), 0) = DATEADD(mm, DATEDIFF(mm,0,DATEADD(mm,#IntDate,getdate())), 0)
ORDER BY [Days Open]
DECLARE #Count AS INT
SELECT #Count = COUNT(*) FROM #table;
WITH MyResults(RowNo, [Days Open]) AS
(
SELECT RowNo, [Days Open] FROM
(SELECT ROW_NUMBER() OVER (ORDER BY [Days Open]) AS RowNo, [Days Open] FROM #table) AS Foo
)
insert into #results
SELECT
DATEADD(mm, DATEDIFF(mm,0,DATEADD(mm,#IntDate,getdate())), 0) as [Month]
,AVG([Days Open])as [Median] FROM MyResults WHERE RowNo = (#Count+1)/2 OR RowNo = ((#Count+1)%2) * ((#Count+2)/2)
set #IntDate = #IntDate+1
DELETE FROM #table
END
select *
from #results
order by [Month]

This works with SQL 2000:
DECLARE #testTable TABLE
(
VALUE INT
)
--INSERT INTO #testTable -- Even Test
--SELECT 3 UNION ALL
--SELECT 5 UNION ALL
--SELECT 7 UNION ALL
--SELECT 12 UNION ALL
--SELECT 13 UNION ALL
--SELECT 14 UNION ALL
--SELECT 21 UNION ALL
--SELECT 23 UNION ALL
--SELECT 23 UNION ALL
--SELECT 23 UNION ALL
--SELECT 23 UNION ALL
--SELECT 29 UNION ALL
--SELECT 40 UNION ALL
--SELECT 56
--
--INSERT INTO #testTable -- Odd Test
--SELECT 3 UNION ALL
--SELECT 5 UNION ALL
--SELECT 7 UNION ALL
--SELECT 12 UNION ALL
--SELECT 13 UNION ALL
--SELECT 14 UNION ALL
--SELECT 21 UNION ALL
--SELECT 23 UNION ALL
--SELECT 23 UNION ALL
--SELECT 23 UNION ALL
--SELECT 23 UNION ALL
--SELECT 29 UNION ALL
--SELECT 39 UNION ALL
--SELECT 40 UNION ALL
--SELECT 56
DECLARE #RowAsc TABLE
(
ID INT IDENTITY,
Amount INT
)
INSERT INTO #RowAsc
SELECT VALUE
FROM #testTable
ORDER BY VALUE ASC
SELECT AVG(amount)
FROM #RowAsc ra
WHERE ra.id IN
(
SELECT ID
FROM #RowAsc
WHERE ra.id -
(
SELECT MAX(id) / 2.0
FROM #RowAsc
) BETWEEN 0 AND 1
)

For newbies like myself who are learning the very basics, I personally find this example easier to follow, as it is easier to understand exactly what's happening and where median values are coming from...
select
( max(a.[Value1]) + min(a.[Value1]) ) / 2 as [Median Value1]
,( max(a.[Value2]) + min(a.[Value2]) ) / 2 as [Median Value2]
from (select
datediff(dd,startdate,enddate) as [Value1]
,xxxxxxxxxxxxxx as [Value2]
from dbo.table1
)a
In absolute awe of some of the codes above though!!!

This is as simple an answer as I could come up with. Worked well with my data. If you want to exclude certain values just add a where clause to the inner select.
SELECT TOP 1
ValueField AS MedianValue
FROM
(SELECT TOP(SELECT COUNT(1)/2 FROM tTABLE)
ValueField
FROM
tTABLE
ORDER BY
ValueField) A
ORDER BY
ValueField DESC

The following solution works under these assumptions:
No duplicate values
No NULLs
Code:
IF OBJECT_ID('dbo.R', 'U') IS NOT NULL
DROP TABLE dbo.R
CREATE TABLE R (
A FLOAT NOT NULL);
INSERT INTO R VALUES (1);
INSERT INTO R VALUES (2);
INSERT INTO R VALUES (3);
INSERT INTO R VALUES (4);
INSERT INTO R VALUES (5);
INSERT INTO R VALUES (6);
-- Returns Median(R)
select SUM(A) / CAST(COUNT(A) AS FLOAT)
from R R1
where ((select count(A) from R R2 where R1.A > R2.A) =
(select count(A) from R R2 where R1.A < R2.A)) OR
((select count(A) from R R2 where R1.A > R2.A) + 1 =
(select count(A) from R R2 where R1.A < R2.A)) OR
((select count(A) from R R2 where R1.A > R2.A) =
(select count(A) from R R2 where R1.A < R2.A) + 1) ;

DECLARE #Obs int
DECLARE #RowAsc table
(
ID INT IDENTITY,
Observation FLOAT
)
INSERT INTO #RowAsc
SELECT Observations FROM MyTable
ORDER BY 1
SELECT #Obs=COUNT(*)/2 FROM #RowAsc
SELECT Observation AS Median FROM #RowAsc WHERE ID=#Obs

I try with several alternatives, but due my data records has repeated values, the ROW_NUMBER versions seems are not a choice for me. So here the query I used (a version with NTILE):
SELECT distinct
CustomerId,
(
MAX(CASE WHEN Percent50_Asc=1 THEN TotalDue END) OVER (PARTITION BY CustomerId) +
MIN(CASE WHEN Percent50_desc=1 THEN TotalDue END) OVER (PARTITION BY CustomerId)
)/2 MEDIAN
FROM
(
SELECT
CustomerId,
TotalDue,
NTILE(2) OVER (
PARTITION BY CustomerId
ORDER BY TotalDue ASC) AS Percent50_Asc,
NTILE(2) OVER (
PARTITION BY CustomerId
ORDER BY TotalDue DESC) AS Percent50_desc
FROM Sales.SalesOrderHeader SOH
) x
ORDER BY CustomerId;

For your question, Jeff Atwood had already given the simple and effective solution. But, if you are looking for some alternative approach to calculate the median, below SQL code will help you.
create table employees(salary int);
insert into employees values(8); insert into employees values(23); insert into employees values(45); insert into employees values(123); insert into employees values(93); insert into employees values(2342); insert into employees values(2238);
select * from employees;
declare #odd_even int; declare #cnt int; declare #middle_no int;
set #cnt=(select count(*) from employees); set #middle_no=(#cnt/2)+1; select #odd_even=case when (#cnt%2=0) THEN -1 ELse 0 END ;
select AVG(tbl.salary) from (select salary,ROW_NUMBER() over (order by salary) as rno from employees group by salary) tbl where tbl.rno=#middle_no or tbl.rno=#middle_no+#odd_even;
If you are looking to calculate median in MySQL, this github link will be useful.

Resources