Multiple join not joining as expected - sql

Question

I have the following tables
table 2 with a fk to table 1
table 3 with a fk to table 1
In table 2 I have two rows linked to table 1
In table 3 I have one row linked table 1
I am trying to produce a table that has
| table1 pk | table 2 pk | null |
| table1 pk | table 2 pk | null |
| table1 pk | null | table 3 pk |
However when I try the following I get
select tab1.id, tab2.id, tab3.id
from table1 tab1
left join tab2 on tab1.id = tab2.tab1_id
left join tab3 on tab1.id = tab3.tab1_id
gives this table
| table1 pk | table 2 pk | table 3 pk |
| table1 pk | table 2 pk | table 3 pk |
Can anyone help with this SQL please?
Thanks in advance
EDIT
I think I may have simplified this a bit too much. Ideally output would be
| table1 pk | table 2 pk |
| table1 pk | table 3 pk |
| table1 pk | table 3 pk |
Once I get this join working it will be added to another massive query...

Answer 1

One unusual solution is
select coalesce(t2.tab1_id, t3.tab1_id) pk,
t2.pk, t3.pk
from table2 t2
full join table3 t3
on t3.tab1_id = t2.tab1_id
where exists (select * from table1
where pk in (t2.tab1_id, t3.tab1_id)
NOTE (Edit) As noted by Andriy M in comment, the where clause only eliminates rows from table2 and table3 where the FK does not exist in table1, which cannot exist if FK constraints have been properly applied to table2 and table3.

Answer 2

I don't see anything crazy here
WITH TAB1
AS (SELECT 1 AS ID FROM DUAL
UNION ALL
SELECT 2 AS ID FROM DUAL
UNION ALL
SELECT 3 AS ID FROM DUAL),
TAB2
AS (SELECT 1 AS TAB1_ID, 'A' AS ID FROM DUAL
UNION ALL
SELECT 2 AS TAB1_ID, 'B' AS ID
FROM DUAL),
TAB3
AS (SELECT 3 AS TAB1_ID, 'C' AS ID
FROM DUAL)
SELECT
TAB1.ID,
COALESCE ( TAB2.ID,
TAB3.ID )
FROM
TAB1
LEFT JOIN TAB2
ON TAB1.ID = TAB2.TAB1_ID
LEFT JOIN TAB3
ON TAB1.ID = TAB3.TAB1_ID;
1 A
2 B
3 C

Answer 3

You could use a join to the result of a union:
SELECT
t1.table1pk,
t23.table2pk,
t23.table3pk
FROM table1 t1
INNER JOIN
(
SELECT table2pk, NULL AS table3pk, table1fk
FROM table2
UNION ALL
SELECT NULL AS table2pk, table3pk, table1fk
FROM table3
) t23
ON t1.table1pk = t23.table1fk
;
Or you could use a union of two joins' results:
SELECT
t1.table1pk,
t2.table2pk,
NULL AS table3pk
FROM table1 t1
INNER JOIN table2 t2
ON t1.table1pk = t2.table1fk
UNION ALL
SELECT
t1.table1pk,
NULL AS table2pk,
t3.table3pk
FROM table1 t1
INNER JOIN table3 t3
ON t1.table1pk = t3.table1fk
;
Both methods could be adapted to produce the two-column version of the desired output:
a join to a union:
SELECT
t1.table1pk,
t23.otherpk
FROM table1 t1
INNER JOIN
(
SELECT table2pk AS otherpk, table1fk
FROM table2
UNION ALL
SELECT table3pk AS otherpk, table1fk
FROM table3
) t23
ON t1.table1pk = t23.table1fk
;
a union of joins:
SELECT
t1.table1pk,
t2.table2pk AS otherpk
FROM table1 t1
INNER JOIN table2 t2
ON t1.table1pk = t2.table1fk
UNION ALL
SELECT
t1.table1pk,
t3.table3pk AS otherpk
FROM table1 t1
INNER JOIN table3 t3
ON t1.table1pk = t3.table1fk
;