Concatenating with Cursor - sql-server

Question

I really want to learn and understand how to concatenate strings with the cursor approach.
Here is my table:
declare #t table (id int, city varchar(15))
insert into #t values
(1, 'Rome')
,(1, 'Dallas')
,(2, 'Berlin')
,(2, 'Rome')
,(2, 'Tokyo')
,(3, 'Miami')
,(3, 'Bergen')
I am trying to create a table that has all cities for each ID within one line sorted alphabetically.
ID City
1 Dallas, Rome
2 Berlin, Rome, Tokyo
3 Bergen, Miami
This is my code so far but it is not working and if somebody could walk me through each step I would be very happy and eager to learn it!
set nocount on
declare #tid int
declare #tcity varchar(15)
declare CityCursor CURSOR FOR
select * from #t
order by id, city
open CityCursor
fetch next from CityCursor into #tid, #tcity
while ( ##FETCH_STATUS = 0)
begin
if #tid = #tid -- my idea add all cities in one line within each id
print cast(#tid as varchar(2)) + ', '+ #tcity
else if #tid <> #tid --when it reaches a new id and we went through all cities it starts over for the next line
fetch next from CityCursor into #tid, #tcity
end
close CityCursor
deallocate CityCursor
select * from CityCursor

Answer 1

First, for future readers: A cursor, as Sean Lange wrote in his comment, is the wrong tool for this job. The correct way to do it is using a subquery with for xml.
However, since you wanted to know how to do it with a cursor, you where actually pretty close. Here is a working example:
set nocount on
declare #prevId int,
#tid int,
#tcity varchar(15)
declare #cursorResult table (id int, city varchar(32))
-- if you are expecting more than two cities for the same id,
-- the city column should be longer
declare CityCursor CURSOR FOR
select * from #t
order by id, city
open CityCursor
fetch next from CityCursor into #tid, #tcity
while ( ##FETCH_STATUS = 0)
begin
if #prevId is null or #prevId != #tid
insert into #cursorResult(id, city) values (#tid, #tcity)
else
update #cursorResult
set city = city +', '+ #tcity
where id = #tid
set #prevId = #tid
fetch next from CityCursor into #tid, #tcity
end
close CityCursor
deallocate CityCursor
select * from #cursorResult
results:
id city
1 Dallas, Rome
2 Berlin, Rome, Tokyo
3 Bergen, Miami
I've used another variable to keep the previous id value, and also inserted the results of the cursor into a table variable.

Answer 2

I have written nested cursor to sync with distinct city id. Although it has performance issue, you can try the following procedure
CREATE PROCEDURE USP_CITY
AS
BEGIN
set nocount on
declare #mastertid int
declare #detailstid int
declare #tcity varchar(MAX)
declare #finalCity varchar(MAX)
SET #finalCity = ''
declare #t table (id int, city varchar(max))
insert into #t values
(1, 'Rome')
,(1, 'Dallas')
,(2, 'Berlin')
,(2, 'Rome')
,(2, 'Tokyo')
,(3, 'Miami')
,(3, 'Bergen')
declare #finaltable table (id int, city varchar(max))
declare MasterCityCursor CURSOR FOR
select distinct id from #t
order by id
open MasterCityCursor
fetch next from MasterCityCursor into #mastertid
while ( ##FETCH_STATUS = 0)
begin
declare DetailsCityCursor CURSOR FOR
SELECT id,city from #t order by id
open DetailsCityCursor
fetch next from DetailsCityCursor into #detailstid,#tcity
while ( ##FETCH_STATUS = 0)
begin
if #mastertid = #detailstid
begin
SET #finalCity = #finalCity + CASE #finalCity WHEN '' THEN +'' ELSE ', ' END + #tcity
end
fetch next from DetailsCityCursor into #detailstid, #tcity
end
insert into #finaltable values(#mastertid,#finalCity)
SET #finalCity = ''
close DetailsCityCursor
deallocate DetailsCityCursor
fetch next from MasterCityCursor into #mastertid
end
close MasterCityCursor
deallocate MasterCityCursor
SELECT * FROM #finaltable
END
If you will face any problem, feel free to write in comment section. Thanks

Answer 3

Using a cursor for this is probably the slowest possible solution. If performance is important then there are three valid approaches. The first approach is FOR XML without special XML character protection.
declare #t table (id int, city varchar(15))
insert into #t values (1, 'Rome'),(1, 'Dallas'),(2, 'Berlin'),(2, 'Rome'),(2, 'Tokyo'),
(3, 'Miami'),(3, 'Bergen');
SELECT
t.id,
city = STUFF((
SELECT ',' + t2.city
FROM #t t2
WHERE t.id = t2.id
FOR XML PATH('')),1,1,'')
FROM #t as t
GROUP BY t.id;
The drawback to this approach is when you add a reserved XML character such as &, <, or >, you will get an XML entity back (e.g. "&amp" for "&"). To handle that you have to modify your query to look like this:
Sample data
IF OBJECT_ID('tempdb..#t') IS NOT NULL DROP TABLE #t;
CREATE TABLE #t (id int, words varchar(20))
INSERT #t VALUES (1, 'blah blah'),(1, 'yada yada'),(2, 'PB&J'),(2,' is good');
SELECT
t.id,
city = STUFF((
SELECT ',' + t2.words
FROM #t t2
WHERE t.id = t2.id
FOR XML PATH(''), TYPE).value('.','varchar(1000)'),1,1,'')
FROM #t as t
GROUP BY t.id;
The downside to this approach is that it will be slower. The good news (and another reason this approach is 100 times better than a cursor) is that both of these queries benefit greatly when the optimizer chooses a parallel execution plan.
The best approach is a new fabulous function available in SQL Server 2017, STRING_AGG. STRING_AGG does not have the problem with special XML characters and is, by far the cleanest approach:
SELECT t.id, STRING_AGG(t.words,',') WITHIN GROUP (ORDER BY t.id)
FROM #t as t
GROUP BY t.id;