SQL Server display first, second, and third values with same value - sql-server

Question

I am trying to create a query in SQL Server but having some difficulties. I will try to supply some sample data so it is easier. There are multiple tables I am trying to pull from:
Units:
UnitID
------
101
102
103
104
etc..
Jobs:
JobID
------
1
2
etc.
Job Units:
UnitID | JobID | DispatchDate
-----------------------------
102 | 12 | Dec 12 2015
104 | 14 | Dec 12 2015
102 | 18 | Dec 12 2015
108 | 18 | Dec 12 2015
102 | 11 | Dec 12 2015
104 | 10 | Dec 12 2015
What I would like for a desired outcome would reflect this data set:
UnitID | Job 1 | Job 2 | Job 3
------------------------------
102 | 12 | 18 | 11
103 | | |
104 | 14 | 10 |
105 | | |
106 | | |
107 | | |
108 | 18 | |
So basically, I want to display the job the Unit has been out to up to three jobs, but I still need to show the other units that haven't gone out, or only have gone out once or twice.
I currently am exporting this data set out to three separate listviews and using three separate stored procedures, but this isn't getting the job done and is a mess, so I won't even bother posting my code, but if needed I can.
Any help is greatly appreciated. Thanks!
Edit: As per request of /u/Pasty, here is the code I am working on:
select UnitID, case when (select COUNT(JobID)
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 12 2015') >= 1
then (select top 1 JobID
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 12 2015'
order by JobID asc )
else null
end as 'Job 1', case when (select COUNT(JobID)
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 12 2015') >= 2
then (select top 1 JobID
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 12 2015'
order by JobID desc )
else null
end as 'Job 2', case when (select COUNT(JobID)
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 12 2015') >= 3
then (select top 1 JobID
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 12 2015'
order by JobID asc )
else null
end as 'Job 3'
from Units U
order by UnitID asc

Answer 1

You need to use LEFT JOIN in order to achieve the desired result. If you have table A and B and want to display matches in B and all items from A, LEFT JOIN allow's you to do that. A good visual explanation of SQL JOINS can be found here - A Visual Explanation of SQL Joins.
How can you create the desired output in your case (I am assuming the tables are named Job, Unit ans Unit_has_Job):
Unit | UnitId
-----------
101
102
103
104
105
107
Job | JobId
--------------
1
2
3
4
5
Unit_has_JobId | UnitId | JobId
----------------------------------
101 | 1
101 | 3
101 | 4
102 | 4
105 | 3
select u.UnitId, ISNULL(j.JobId, 0) as [JobId] from unit as u
left join unit_has_job as uhj on u.UnitId = uhj.UnitId
left join job as j on uhj.JobId = j.JobId
The result is:
UnitId | JobId
101 | 1
101 | 3
101 | 4
102 | 4
103 | 0
104 | 0
105 | 3
107 | 0
I have created an example in SQL-Fiddle.
With the ISNULL function you mark the empty slots. The different JobId for every UnitId is equivalent to the columns in your desired output.
Using LINQ2SQL and the GroupBy method you can easily group the jobs per unit from the result and iterate over them:
var jobsPerUnit = result.GroupBy (r => r.UnitId);
foreach (var jobs in jobsPerUnit)
{
Console.WriteLine("Unit: " + jobs.Key);
foreach (var job in jobs)
{
if (job.JobId > 0)
{
Console.WriteLine("Job: " + job.JobId);
}
}
Console.WriteLine("=================");
}
The output:
Unit: 101
Job: 1
Job: 3
Job: 4
=================
Unit: 102
Job: 4
=================
Unit: 103
=================
Unit: 104
=================
Unit: 105
Job: 3
=================
Unit: 107
=================
Columns in SQL are the result of a selection/projection, which can't be created so easily dynamically. One possible solution is to use dynamic SQL in order to create a temp table, fill it and then select from this temp table, but the overhead is probably not worth it. You still need a cursor for this solution.
If you want to do everything on the SQL-side, then one possible solution would be to iterate with a cursor over the result and group by UnitId.

Answer 2

I was just playing around and this is what I came up with:
SQLFiddle
IF OBJECT_ID(N'dbo.UNITS', 'U') IS NOT NULL
DROP TABLE dbo.UNITS
GO
CREATE TABLE UNITS(ID INT IDENTITY(1, 1), UnitID INT, JobID INT, DispatchDate DATETIME)
GO
INSERT INTO UNITS
VALUES(102, 12, 'Dec 12 2015')
,(104, 14, 'Dec 12 2015')
,(102, 18, 'Dec 12 2015')
,(108, 18, 'Dec 12 2015')
,(102, 11, 'Dec 12 2015')
,(104, 10, 'Dec 12 2015')
,(103, NULL, NULL)
,(105, NULL, NULL)
,(106, NULL, NULL)
,(107, NULL, NULL)
GO
SELECT
UnitId
,Job_1 = JobIDs.value('/JobID[1]','INT')
,Job_2 = JobIDs.value('/JobID[2]','INT')
,Job_3 = JobIDs.value('/JobID[3]','INT')
FROM
(
SELECT
UnitID
,JobIDs = CONVERT(XML,'<JobID>'
+ REPLACE(Units.JobIDs, '|', '</JobID><JobID>')
+ '</JobID>')
FROM
(
SELECT DISTINCT
U.UnitId
,JobIDs = STUFF((
SELECT
'|' + CAST(UU.JobID AS NVARCHAR(25))
FROM
UNITS UU
WHERE
UU.UnitID = U.UnitID
ORDER BY
UU.ID, UU.JobID
FOR XML PATH (''), type).value('.', 'nvarchar(max)'), 1, 1, '')
FROM
UNITS U
GROUP BY
U.UnitID) Units) UJ

Answer 3

There is an easier way to get to the same result, where you don't need to run inner selects all the time. This yields a much simpler execution plan and faster execution if you put on the proper indexes.
Note that there is a LEFT JOIN in the inner query (LEFT JOIN JobUnits ju), that will produce an row for each row in Units, regardless if it has related JobUnit.
select UnitID,
SUM(CASE WHEN cnt=1 THEN JobID ELSE 0 END) AS Job1,
SUM(CASE WHEN cnt=2 THEN JobID ELSE 0 END) AS Job2,
SUM(CASE WHEN cnt=3 THEN JobID ELSE 0 END) AS Job3,
SUM(CASE WHEN cnt=4 THEN JobID ELSE 0 END) AS Job4,
SUM(CASE WHEN cnt=5 THEN JobID ELSE 0 END) AS Job5
FROM (
select ju.UnitID, ju.JobID, count(1) as cnt
FROM Units u
LEFT JOIN JobUnits ju on (u.UnitID = ju.UnitID)
LEFT JOIN JobUnits ju2 on (ju.UnitID = ju2.UnitID AND ju.UnitID <= ju2.UnitID)
)
GROUP BY UnitID
The only disadvantage of this SELECT that it will not preserve the order (12, 18, 11 in case of 102), instead it orders it ascending way (11, 12, 18) - I don't know if it is an advantage or disadvantage in your business case.
You can preserve the order if you compare the ROWNUMs, but that requires two more subselects, and I'm not sure if it worth or not.

Answer 4

I ended up solving this scenario with quite a mess of code, but it works and I'm tired, so I am just going to go with it.
select UnitID, case when (select COUNT(JobID)
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 4 2015') >= 1
then (select top 1 JobID
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 4 2015'
order by JobID asc )
else null
end as 'Job 1', case when (select COUNT(JobID)
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 4 2015') >= 1
then (select Customer
from Customers C
full
where CustomerID=())
else null
end as 'Customer 1',
case when (select COUNT(JobID)
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 4 2015') >= 2
then
(SELECT TOP 1 JobID
FROM
(
SELECT TOP 2 JobID
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 4 2015'
ORDER BY JobID desc
) sub
ORDER BY JobID asc)
else null
end as 'Job 2',
case when (select COUNT(JobID)
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 4 2015') >= 3
then
(SELECT TOP 1 JobID
FROM
(
SELECT TOP 3 JobID
from JobUnits
where UnitID=U.UnitID
and DispatchDate='Dec 4 2015'
ORDER BY JobID desc
) sub
ORDER BY JobID desc)
else null
end as 'Job 3'
from Units U
order by UnitID asc