Concat Names against row_number() or similar function - tsql

Question

my data repeats rows for individual relationships between people. For example, the below states that John Smith is known by 3 employees:
Person EmployeeWhoKnowsPerson
John Smith Derek Jones
John Smith Adrian Daniels
John Smith Peter Low
I am looking to do the following:
1) Count the number of people who know John Smith. I have done this via the row_number() function and it appears to be behaving:
select Person, MAX(rowrank) as rowrank
from (
select Person, EmployeeWhoKnowsPerson, rowrank=ROW_NUMBER() over (partition by Person order by EmployeeWhoKnowsPerson desc)
from Data
) as t
group by Person
Which returns:
Person rowrank
John Smith 3
But now i am looking at concatenating the EmployeeWhoKnowsPerson column to return and was wondering how this might be possible:
Person rowrank EmployeesWhoKnow
John Smith 3 Derek Jones, Adrian Daniels, Peter Low

Answer 1

For SQL Server 2017 +
select
person,
count(*) as KnowsCount,
string_agg(EmployeeWhoKnowsPerson, ',') WITHIN GROUP (ORDER BY EmployeeWhoKnowsPerson ASC) AS EmployeesWhoKnowPerson
from
data
group by person;
For prior versions:
select
person,
count(*) as KnowsCount,
stuff((select ',' + EmployeeWhoKnowsPerson
from data as dd
where dd.Person = d.Person
order by EmployeeWhoKnowsPerson
for xml path('')), 1, 1, '') AS EmployeesWhoKnowPerson
from
data as d
group by person;
And you're overthinking that whole count of who knows piece.
Here's a SQL Fiddle Demo with an extra name thrown in.

Answer 2

If 2017+, you can use string_agg() in a simple group by
Example
Declare #YourTable Table ([Person] varchar(50),[EmployeeWhoKnowsPerson] varchar(50)) Insert Into #YourTable Values
('John Smith','Derek Jones')
,('John Smith','Adrian Daniels')
,('John Smith','Peter Low')
Select Person
,rowrank = sum(1)
,[EmployeeWhoKnowsPerson] = string_agg([EmployeeWhoKnowsPerson],', ')
From #YourTable
Group By Person
Returns
Person rowrank EmployeeWhoKnowsPerson
John Smith 3 Derek Jones, Adrian Daniels, Peter Low
If <2017 ... use the stuff()/xml approach
Select Person
,rowrank = sum(1)
,[EmployeeWhoKnowsPerson] = stuff((Select ', ' + [EmployeeWhoKnowsPerson]
From #YourTable
Where Person=A.Person
For XML Path ('')),1,2,'')
From #YourTable A
Group By Person