Left join get all data in both table - sql-server

Question

I have two tables Q and A,
records are A are
QID UserID Value
1 100 A
2 100 B
3 100 C
1 101 AA
2 101 BB
3 101 CC
1 102 AAA
2 102 BBB
As you can see, there is no record for user 102 for QID 3. There is this another table Q.
QID Value
1 Name
2 Email
3 Site
What I want is, for each user, weather they have answered a question or not (that is, weather a entry exits in A table or not) I want all questions for all users and their answers. Something like this.
QID QValue UserID Value
1 Name 100 A
2 Email 100 B
3 Site 100 C
1 Name 101 AA
2 Email 101 BB
3 Site 101 CC
1 Name 102 AAA
2 Email 102 BBB
What the problem is one row is missing from the desired output, and that is
3 Site 102 NULL
Because for user 102 there is no entry in A table. I tried LEFT JOIN, but obviously it won't give the desired result as all the left table are already there. And INNER JOIN doesn't works either.
It is also complete possible for answers table (table A) to have data like this
QID QValue UserID Value
1 Name 100 A
2 Email 101 BB
3 Site 102 CCC
Say, all users just have filled in one record, in this case desired output is something like this
QID QValue UserID Value
1 Name 100 A
2 Email 100 NULL
3 Site 100 NULL
1 Name 101 NULL
2 Email 101 BB
3 Site 101 NULL
1 Name 102 NULL
2 Email 102 NULL
3 Email 102 CCC
If I do a LEFT JOIN on QID it doesn't works. Please suggest what should be done.

Answer 1

Try this:
declare #A table(QID int, UserID int, Value varchar(10))
declare #Q table(QID int, Value varchar(10))
insert into #A values (1, 100, 'A')
insert into #A values (2, 100, 'B')
insert into #A values (3, 100, 'C')
insert into #A values (1, 101, 'AA')
insert into #A values (2, 101, 'BB')
insert into #A values (3, 101, 'CC')
insert into #A values (1, 102, 'AAA')
insert into #A values (2, 102, 'BBB')
insert into #Q values (1, 'Name')
insert into #Q values (2, 'Email')
insert into #Q values (3, 'Site')
select
U.UserID,
Q.QID,
Q.Value as QValue,
A.Value
from
(select distinct UserID from #A) U -- all Users
cross join #Q Q -- all Questions
left outer join #A A on A.UserID = U.UserID and A.QID = Q.QID
So basically you do a cross join between all questions and all users first to get all combinations. Then you take this result and do a left join with all the answers. Missing answers will have NULL values in the Value (the real answer) field.