how to combine multiple column of different table into one table - sql

Question

I have three tables a, b and c and need to arrange these table data as target table and all of these tables (a, b, c) are not in database they are fetched from from single table using queries as alias and need to arrange these tables into target table using query. How to do that
table a | table b | table c
| |
id | a_vals | id | b_vals | id | c_vals
------------ | -------------- | -------------
1 | 123 | 1 | 123 | 1 | 123
2 | 124 | 2 | 142 | 2 | 142
3 | 234 | 4 | 234 | 5 | 234
target table
id | a_val| b_val| c_val
1 | 123 | 123 | 123
2 | 124 | 142 | 142
3 | 234 | - | -
4 | - | 234 | -
5 | - | | 234

Answer 1

Since a, b and c share the same name for the column you want to join, you could use "USING" to avoid duplicate keys in the resulting table:
SELECT *
FROM a
FULL OUTER JOIN b USING (id)
FULL OUTER JOIN c USING (id);
Alternativly, since a, b and c's value column all have distinct names you could use NATURAL JOIN:
SELECT *
FROM a
NATURAL FULL OUTER JOIN b
NATURAL FULL OUTER JOIN c;
Be careful not to accidentally rename any of the other columns tho, since natural join removes any duplicate columns.
You can also omit the "OUTER" keyword if you like, but i would leave it for clarity, (since LEFT, RIGHT, and FULL imply an outer join).
See https://www.postgresql.org/docs/10/static/queries-table-expressions.html for details

Answer 2

Please try this:
select aa.id, a_val, b_val, c_val from
(select distinct id as id from table_a
union
select distinct id as id from table_b
union
select distinct id as id from table_c)aa
left join (select id, a_val from table_a)bb on aa.id = bb.id
left join (select id, b_val from table_b)cc on aa.id = cc.id
left join (select id, c_val from table_c)dd on aa.id = dd.id order by aa.id;

Answer 3

Try this code
SELECT
CASE
WHEN t1.id IS not null THEN t1.id
WHEN t2.id IS not null THEN t2.id
ELSE t3.id
END
AS id,
t1.a_vals AS a_val,
t2.b_vals as b_val,
t3.c_vals as c_val
FROM a t1 FULL OUTER JOIN b t2 ON t1.id=t2.id FULL OUTER JOIN c t3 ON
CASE
WHEN t1.id IS not null THEN t1.id
ELSE t2.id
END = t3.id
OR
SELECT COALESCE(t1.id, t2.id, t3.id) as id ,
t1.a_vals AS a_val,
t2.b_vals as b_val,
t3.c_vals as c_val
FROM a t1 FULL OUTER JOIN b t2 ON t1.id=t2.id
FULL OUTER JOIN c t3 ON COALESCE(t1.id, t2.id) = t3.id

Answer 4

You are looking for the ANSI-standard FULL OUTER JOIN:
select coalesce(a.id, b.id, c.id) as id, a.val, b.val, c.val
from a full join
b
on a.id = b.id full join
c
on c.id = coalesce(a.id, b.id);
You can also implement this with union all/group by:
select id, max(a_val) as a_val, max(b_val) as b_val, max(c_val) as c_val
from ((select id, val as a_val, null as b_val, null as c_val
from a
) union all
(select id, null as a_val, val as b_val, null as c_val
from b
) union all
(select id, null as a_val, null as b_val, val as c_val
from c
)
) abc
group by id;

Answer 5

This is probably better done in the 'front end' e.g. this is the kind of thing a reporting tool is designed for.
Avoiding nulls and outer joins (because they by definition produce nulls):
SELECT a_val, b_val, c_val
FROM a
NATURAL JOIN b
NATURAL JOIN c
UNION
SELECT a_val, '-' AS b_val, '-' AS c_val
FROM a
WHERE id NOT IN ( SELECT id FROM b )
AND id NOT IN ( SELECT id FROM c )
UNION
SELECT '-' AS a_val, b_val, '-' AS c_val
FROM b
WHERE id NOT IN ( SELECT id FROM a )
AND id NOT IN ( SELECT id FROM c )
UNION
SELECT '-' AS a_val, '-' AS b_val, c_val
FROM c
WHERE id NOT IN ( SELECT id FROM a )
AND id NOT IN ( SELECT id FROM b );