INNER JOIN on a Sub Query - sql

Question

I have a list of tasks in a table called dbo.Task
In the database, each Task can have 1 or more rows in the TaskLine table.
TaskLine has a TaskID to related the Tasklines to the Task.
A TaskLine has a column called TaskHeadingTypeID
I need to return all the tasks, joined to the LAST TaskLine for that Task.
In english, I need to display a task, with the latest TaskLine heading. So, I basically need to join to the TaskLine table, like this (which, is incorrect and maybe inefficient, but hopefully shows what I am trying to do)
SELECT *
FROM #Results r
INNER JOIN (
SELECT TOP 1 TaskID, TaskHeadingTypeID FROM dbo.TaskLine
ORDER BY TaskLineID DESC
) tl
ON tl.TaskID = r.TaskID
However, the issue is, the sub query only brings back the last TaskLine row, which is incorrect.
Edit:
At the moment, it's 'Working' like the code below, but it seems highly inefficient, because for each task row, it has to run two extra queries. And they're both on the same table, just slightly different columns in that table:
(An extract of the columns in the SELECT cause)
SELECT TaskStatusID,
TaskStatus,
(SELECT TOP 1 TaskHeadingTypeID FROM dbo.TaskLine
WHERE TaskID = r.TaskID
ORDER BY TaskLineID DESC) AS TaskHeadingID,
(SELECT TOP 1 LongName FROM dbo.TaskLine tl
INNER JOIN ref.TaskHeadingType tht
ON tht.TaskHeadingTypeID = tl.TaskHeadingTypeID
WHERE TaskID = r.TaskID
ORDER BY TaskLineID DESC) AS TaskHeading,
PersonInCareID,
ICMSPartyID,
CarerID.... FROM...
EDIT:
Thanks to the ideas and comments below, I have ended up with this, using CTE:
;WITH ValidTaskLines (RowNumber, TaskID, TaskHeadingTypeID, TaskHeadingType)
AS
(SELECT
ROW_NUMBER()OVER(PARTITION BY tl.TaskID, tl.TaskHeadingTypeID ORDER BY tl.TaskLineID) AS RowNumber,
tl.TaskID,
tl.TaskHeadingTypeID,
LongName AS TaskHeadingType
FROM dbo.TaskLine tl
INNER JOIN ref.TaskHeadingType tht
ON tht.TaskHeadingTypeID = tl.TaskHeadingTypeID
)
SELECT AssignedByBusinessUserID,
BusinessUserID,
LoginName,
Comments,
r.CreateDate,
r.CreateUser,
r.Deleted,
r.Version,
IcmsBusinessUserID,
r.LastUpdateDate,
r.LastUpdateUser,
OverrrideApprovalBusinessUserID,
PlacementID,
r.TaskID,
TaskPriorityTypeID,
TaskPriorityCode,
TaskPriorityType,
TaskStatusID,
TaskStatus,
vtl.TaskHeadingTypeID AS TaskHeadingID,
vtl.TaskHeadingType AS TaskHeading,
PersonInCareID,
ICMSPartyID,
CarerID,
ICMSCarerEntityID,
StartDate,
EndDate
FROM #Results r
INNER JOIN ValidTaskLines vtl
ON vtl.TaskID = r.TaskID
AND vtl.RowNumber = 1

Answer 1

You could use the ROW_NUMBER() function for this:
SELECT *
FROM #Results r
INNER JOIN (SELECT TaskID
, TaskHeadingTypeID
, ROW_NUMBER()OVER(PARTITION BY TaskID, TaskHeadingTypeID ORDER BY TAskLineID DESC) RN
FROM dbo.TaskLine
) tl
ON tl.TaskID = r.TaskID
AND t1.RN = 1
The ROW_NUMBER() function assigns a number to each row. PARTITION BY is optional, but used to start the numbering over for each value in that group, ie: if you PARTITION BY Some_Date then for each unique date value the numbering would start over at 1. ORDER BY of course is used to define how the counting should go, and is required in the ROW_NUMBER() function.
You may need to adjust the PARTITION BY to suit your query, run the subquery by itself to get an idea of how the ROW_NUMBER() works.