Question

I need to full join two tables with multiple keys

As an example, I have two tables, one looks like:

id1 | foreignerkey | name | value
 1         5         name1    1
 2         6         name2    2
 3         7         name4    3

as t01; the other looks like:

id2 | foreignerkey | name | value
 1         5         name1  1
 2         7         name2  2
 3         8         name2  3

as t02.

I need “select t01.* , to2.* from t01 full join t02 on t01.name = t02.name“,
but I need also “select t01.* , t02.* from t01 join t02 where t01.foreignerkey = t02.foreignerkey“, the result I need is like this:

 id1 | foreignerkey | name | value | id2 | foreignerkey | name | value
  1         5         name1    1      1         5         name1    1
  2         6         name2    2     null     null        null    null
  3         7         name4    3     null     null        null    null
 null     null        null    null    2         7         name2    2
 null     null        null    null    3         8         name2    3

The problem is, as you can see, at first, I put my query string like this,
“select t01.* , t02.* from t01 full join t02 on t01.name = t02.name where
t01.foreignerkey = t02.foreignerkey“, but then when t01.name = "name4" (which is not in t02), it won’t show up in the results.

So, my final query string:

select a.* from
    (select t01.id1, t01.foreignerkey as foreignerkey1, t01.name, t01.value,
            t02.id2, t02.foreignerkey as foreignerkey2, t02.name, t02.value 
    from t01 
    full join t02 
    on t01.name = t02.name) a
where a.foreignerkey1 = a.foreignerkey2 
      or a.foreignerkey1 is null 
      or a.foreignerkey2 is null

It costs too much to get the results, is there a better solution?

Answer 1

I can’t verify that this is faster as your original SQL doesn’t work on the version of Oracle I’m using but you could try this:

SELECT *
  FROM t01, t02
 WHERE t01.NAME = t02.NAME
   AND t01.foreignerkey = t02.foreignerkey
UNION ALL
SELECT t01.*, NULL, NULL, '', ''
  FROM t01
 WHERE NOT EXISTS (SELECT NULL
          FROM t02
         WHERE t01.NAME = t02.NAME
           AND t01.foreignerkey = t02.foreignerkey)
UNION ALL
SELECT NULL, NULL, '', '', t02.*
  FROM t02
 WHERE NOT EXISTS (SELECT NULL
          FROM t01
         WHERE t01.NAME = t02.NAME
           AND t01.foreignerkey = t02.foreignerkey)

• Use an inner join to t01.NAME = t02.NAME AND t01.foreignerkey = t02.foreignerkey
• Use NOT EXISTS to get the records from t01 that dont have a match in t01
• Use NOT EXISTS to get the records from t02 that dont have a match in t01
• Use UNION to join the result sets in this case ALL is safe as we know that the three result sets are mutually exclusive

Or you can use an outer join as @kedar kamthe has sugested:

SELECT *
  FROM t01, t02
 WHERE t01.NAME = t02.NAME(+)
   AND t01.foreignerkey = t02.foreignerkey(+)
UNION
SELECT NULL, NULL, '', '', t02.*
  FROM t02
 WHERE NOT EXISTS (SELECT NULL FROM t01 WHERE t01.NAME = t02.NAME  AND  t01.foreignerkey = t02.foreignerkey)

But you still need the second query to return the results from t02