Question

I have a database which is including two tables, Labs and LabUsers
How do I join a column from LabUsers into a cell in Labs.

To be specific, I would like to have the user names of a lab to be concatenated and separated with a bullet ( • is Alt+0149), and to have these usernames sorted alphabetically left to right.

Here is an example.

The Labs table looks like this:

LabID  LabName     LabStudents
-----  ----------  -----------
1      North       NULL
2      North East  NULL
3      South West  NULL

and the LabUsers looks like this:

LabUserID   LabUserName   LabID
---------   -----------   -----    
1           Diana         1
2           Paul          2
3           Paula         2
4           Romeo         1
5           Julia         1
6           Rose          2
7           Diana         2

I would like to get this outcome in the Labs table:

LabID  LabName     LabUsers
-----  ----------  ---------------------
1      North       Diana•Julia•Romeo
2      North East  Diana•Paul•Paula•Rose
3      South West  NULL

Here is the script to create the tables:

USE [tempdb];
GO
CREATE TABLE [dbo].[LabUsers]
(
    [LabUserID] [int] PRIMARY KEY CLUSTERED,
    [LabUserName] [nvarchar](50) NOT NULL,
    [LabID] [int] NOT NULL
);
GO
INSERT [dbo].[LabUsers] SELECT 1, N'Diana', 1;
INSERT [dbo].[LabUsers] SELECT 2, N'Paul',  2;
INSERT [dbo].[LabUsers] SELECT 3, N'Paula', 2;
INSERT [dbo].[LabUsers] SELECT 4, N'Romeo', 1;
INSERT [dbo].[LabUsers] SELECT 5, N'Julia', 1;
INSERT [dbo].[LabUsers] SELECT 6, N'Rose',  2;
INSERT [dbo].[LabUsers] SELECT 7, N'Diana', 2;

CREATE TABLE [dbo].[Labs]
(
    [LabID] [int] PRIMARY KEY CLUSTERED,
    [LabName] [nvarchar](50) NOT NULL,
    [LabUsers] [nvarchar](max) NULL
);
GO
INSERT [dbo].[Labs] SELECT 1, N'North',      NULL;
INSERT [dbo].[Labs] SELECT 2, N'North East', NULL;
INSERT [dbo].[Labs] SELECT 3, N'South West', NULL;

Answer 1

SELECT l.LabID, l.LabName, LabUsers = STUFF((SELECT N'•' + lu.LabUserName
  FROM dbo.LabUsers AS lu 
  WHERE lu.LabID = l.LabID
  ORDER BY lu.LabUserName
  FOR XML PATH(''), 
  TYPE).value(N'./text()[1]', N'nvarchar(max)'), 1, 1, N'')
FROM dbo.Labs AS l;

I see absolutely no reason to store this in the table, since you can always generate the code at runtime when you run a query. If you store it in the table, then you have to update it every single time you change any row in the table.

However, if I can’t convince you not to do this (it really is bad to store redundant data like this), you can try this way:

;WITH x AS
(
    SELECT l.LabID, l.LabName, x = STUFF((SELECT N'•' + lu.LabUserName
      FROM dbo.LabUsers AS lu 
      WHERE lu.LabID = l.LabID
      ORDER BY lu.LabUserName
      FOR XML PATH(''), 
      TYPE).value(N'./text()[1]', N'nvarchar(max)'), 1, 1, N'')
    FROM dbo.Labs AS l
)
UPDATE l
SET LabUsers = x.x
FROM dbo.Labs AS l
INNER JOIN x ON l.LabID = x.LabID;

As for the performance tests, I’d compare the above version with this variation:

SELECT l.LabID, l.LabName, LabUsers = STUFF((SELECT N'•' + lu.LabUserName
  FROM dbo.LabUsers AS lu 
  WHERE lu.LabID = l.LabID
  ORDER BY lu.LabUserName
  FOR XML PATH('')), 1, 1, '')
FROM dbo.Labs AS l;

On my system I see the initial version at the top of this answer to be far more expensive. Also note that stuffing (no pun intended) these approaches into a user-defined function will bring it closer to the concatenation method @RThomas proposed.