How do I convert a number to a numeric, comma-separated formatted string?

Quick & dirty for int to nnn,nnn...

declare @i int = 123456789
select replace(convert(varchar(128), cast(@i as money), 1), '.00', '')
>> 123,456,789

The reason you aren't finding easy examples for how to do this in T-SQL is that it is generally considered bad practice to implement formatting logic in SQL code. RDBMS's simply are not designed for presentation. While it is possible to do some limited formatting, it is almost always better to let the application or user interface handle formatting of this type.

But if you must (and sometimes we must!) use T-SQL, cast your int to money and convert it to varchar, like this:

select convert(varchar,cast(1234567 as money),1)

If you don't want the trailing decimals, do this:

select replace(convert(varchar,cast(1234567 as money),1), '.00','')

Good luck!

For SQL Server 2012, or later, an easier solution is to use FORMAT ()^{Documentation}.
EG:

SELECT Format(1234567.8, '##,##0') 

Results in: 1,234,568

Not sure it works in tsql, but some platforms have to_char():

test=#select to_char(131213211653.78, '9,999,999,999,999.99');
        to_char        
-----------------------
    131,213,211,653.78
test=# select to_char(131213211653.78, '9G999G999G999G999D99');
        to_char        
-----------------------
    131,213,211,653.78
test=# select to_char(485, 'RN');
     to_char     
-----------------
         CDLXXXV

As the example suggests, the format's length needs to match that of the number for best results, so you might want to wrap it in a function (e.g. number_format()) if needed.

---

Converting to money works too, as point out by the other repliers.

test=# select substring(cast(cast(131213211653.78 as money) as varchar) from 2);
     substring      
--------------------
 131,213,211,653.78