SQL Server: aggregate error on grouping - sql

Question

I have a table called tasks, which lists different tasks a worker can complete. Then i have a relationship table that links a completed task to a worker.
I'm trying to write query that groups the tasks into a list based on the worker id, but the query gives me the following error (see below).
Column 'mater.dbo.worker_task_completion.FK_task_id' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
Tables
CREATE TABLE [dbo].[tasks]
(
[task_id] [int] IDENTITY(1,1) NOT NULL,
[name] [nvarchar](50) NOT NULL,
[icon] [nvarchar](max) NULL,
[isActive] [int] NOT NULL,
[time] [int] NOT NULL,
CONSTRAINT [PK_tasks] PRIMARY KEY CLUSTERED
(
[task_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
CREATE TABLE [dbo].[worker_task_completion]
(
[FK_worker_id] [int] NOT NULL,
[FK_task_id] [int] NOT NULL,
[update_date] [datetime] NOT NULL,
CONSTRAINT [PK_worker_task_completion] PRIMARY KEY CLUSTERED
(
[FK_worker_id] ASC,
[FK_task_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
Query
SELECT top 100 wtc.FK_worker_id,
tasks = Stuff((SELECT ', ' + dbo.tasks.NAME
FROM dbo.tasks
WHERE dbo.tasks.task_id =
wtc.FK_task_id
FOR xml path ('')), 1, 1, '')
FROM dbo.worker_task_completion AS wtc
LEFT JOIN dbo.tasks AS tc
ON tc.task_id = wtc.fk_task_id
-- WHERE wtc.FK_worker_id IN ()
GROUP BY wtc.FK_worker_id

Answer 1

Hmmm. You cannot use a non-aggregated column for the correlation clause. The solution is to move the JOIN into the subquery:
SELECT top 100 wtc.FK_worker_id,
stuff((SELECT ', ' + t.NAME
FROM dbo.worker_task_completion wtc2 JOIN
dbo.tasks t
ON t.task_id = wtc2.FK_task_id
WHERE wtc2.FK_worker_id = wtc.FK_worker_id
FOR xml path ('')
), 1, 2, ''
) as tasks
FROM (SELECT DISTINCT wtc.FK_worker_id
FROM dbo.worker_task_completion wtc
) wtc
-- WHERE wtc.FK_worker_id IN ()
Note that I changed the second argument for STUFF(). Presumably, you want to remove the space as well as the comma.

Answer 2

;WITH CTE_worker_task_completion
AS (SELECT [FK_worker_id]
,[FK_task_id]
FROM [dbo].[worker_task_completion])
------------------------------------------
SELECT [WT].[FK_worker_id]
,[Task]=Stuff(
(
SELECT ', '+[T].[Name]
FROM [dbo].[tasks] AS [T]
INNER JOIN CTE_worker_task_completion AS [WTC]
ON [T].[Task_ID]=[WTC].[Task_ID]
Where [WT].[Worker_ID]=[WTC].[Worker_ID]
ORDER BY [T].[Name] FOR XML PATH('')
),1,1,'')
FROM CTE_worker_task_completion AS [WT]
GROUP BY [WT].[FK_worker_id];