Существует другой способ использования STUFF
функции, которая может быть интересна
declare @TableA table (username varchar(1000))
declare @TableB table (username varchar(1000), email_id varchar(2000))
insert into @TableA values
('User, AA # User, BB'),('User, CC # User, DD # User, EE'),('User, FF # User, GG # User, HH')
insert into @TableB values
('User, AA','User.a@gmail.com'),('User, BB','User.b@gmail.com'),('User, CC','User.c@gmail.com'),
('User, DD','User.d@gmail.com'),('User, EE','User.e@gmail.com'),('User, FF','User.f@gmail.com'),
('User, GG','User.g@gmail.com'),('User, HH','User.h@gmail.com')
;with ct as (
select a.username, b.email_id
from @TableB b
inner join @TableA a on charindex(b.username, a.username) > 0
)
select username, abc = STUFF
(
(
SELECT ' # ' + email_id
FROM ct As t2
WHERE t2.username = t1.username
ORDER BY username
FOR XML PATH (''), TYPE
).value('.', 'varchar(max)')
, 1, 1, '')
from ct t1
group by username
Результат