How to make a list of T-SQL results with comma’s between them?

Suppose we have a simple query like this:

SELECT x 
FROM t
WHERE t.y = z

If we have one record in the result set, I want to set variable @v to that one value. If we have two or more records, I’d like the results to be separated by a comma and a space. What is the best way to write this T-SQL code?

Example:

result set of 1 record:

Value1

result set of 2 records:

Value1, Value2

result set of 3 records:

Value1, Value2, Value3

Answer

this will give you the list of values in a comma separated list

create table #temp
(
    y int,
    x varchar(10)
)

insert into #temp values (1, 'value 1')
insert into #temp values (1, 'value 2')
insert into #temp values (1, 'value 3')
insert into #temp values (1, 'value 4')

DECLARE @listStr varchar(255)

SELECT @listStr = COALESCE(@listStr+', ', '') + x
FROM #temp
WHERE #temp.y = 1

SELECT @listStr as List

drop table #temp