SQL syntax to get all possible combinations when joining 3 tables - sql

Question

I have tables T1, T2, T3 with values
CREATE TABLE #T1
(
[user1] char(20)
)
INSERT INTO #T1 VALUES ('A'), ('B'), ('C'), ('D')
CREATE TABLE #T2
(
[user2] char(20)
)
INSERT INTO #T2 VALUES ('D'), ('E'), ('F'), ('G')
CREATE TABLE #T3
(
[user3] char(20)
)
INSERT INTO #T2 VALUES ('D'), ('E'), ('C'), ('H')
I want the result to by display like
user1 user2 user3
-------------------------
A NULL NULL
B NULL NULL
C NULL C
D D D
NULL E E
NULL F NULL
NULL NULL H
I tried this:
select
user1,
user2,
user3
from
#T1
full join
#T2 on #T1.user1 = #T2.user2
full join
#T3 on #T2.user2 = #T3.user3
But obviously I missing something. Thanks for the help

Answer 1

You just needed to add an extra predicate to the join on #T3. So this:
full outer join #T3 on #T2.user2 = #T3.user3
becomes this:
full outer join #T3 on #T2.user2 = #T3.user3 OR #t3.user3 = #T1.user1
I am assuming your sample data had a typo since you meant to insert into #t3 in the last section. And you posted desired output is also missing NULL, G, NULL
Here's the full corrected code:
CREATE TABLE #T1
(
user1 char(20)
)
insert into #T1 VALUES ('A')
insert into #T1 VALUES ('B')
insert into #T1 VALUES ('C')
insert into #T1 VALUES ('D')
CREATE TABLE #T2
(
user2 char(20)
)
insert into #T2 VALUES ('D')
insert into #T2 VALUES ('E')
insert into #T2 VALUES ('F')
insert into #T2 VALUES ('G')
CREATE TABLE #T3
(
user3 char(20)
)
insert into #T3 VALUES ('D')
insert into #T3 VALUES ('E')
insert into #T3 VALUES ('C')
insert into #T3 VALUES ('H')
select
user1,
user2,
user3
from #T1
full outer join #T2 on #T1.user1 = #T2.user2
full outer join #T3 on #T2.user2 = #T3.user3 OR #t3.user3 = #T1.user1
drop table #T1
drop table #T2
drop table #T3