使用来自一列和另一列的值创建表而没有交集

Question

I have a table like so:

userid | clothesid
-------|-----------
  1    |   1
  1    |   3
  2    |   1
  2    |   4
  2    |   5

What I want from this table is a table like so:

userid | clothesid
-------|-----------
  1    |   4
  1    |   5
  2    |   3

How can I do this?

I've tried it with one entry as:

select distinct r.clothesid from table r where r.clothes not in (select r1.clothes from table r1 where r1.userid=1);

and this returns 4,5, but I'm not sure where to proceed from here

Answer 1

You can cross join the list of userid s and the list of clothesid to generate all combinations, and then use not exists on the original table to identify the missing rows:

select u.userid, c.clothesid
from (select distinct userid from mytable) u
cross join (select distinct clothesid from mytable) c
where not exists(
    select 1 from mytable t on t.userid = u.userid and t.clothesid = c.clothesid
)

Answer 2

I think you want:

select (case when t1.clothesid is not null then 2 else 1 end),
       coalesce(t1.clothesid, t2.clothesid)
from (select t.*
      from t
      where t.userid = 1
     ) t1 full join
     (select t.*
      from t
      where t.userid = 2
     ) t2
     on t1.clothesid = t2.clothesid
where t1.clothesid is null or t2.clothesid is null;

Actually, I think I have a simpler solution:

select (case when min(t.userid) = 1 then 2 else 1 end), clothesid
from t
group by clothesid
having count(*) = 1;

Here is a db<>fiddle.

Answer 3

Left join all the combinations of userid and clothesid to the table and return only the unmatched rows:

select t1.userid, t2.clothesid
from (select distinct userid from tablename) t1
cross join (select distinct clothesid from tablename) t2
left join tablename t on t.userid = t1.userid and t.clothesid = t2.clothesid
where t.userid is null

Or with the operator EXCEPT :

select t1.userid, t2.clothesid
from (select distinct userid from tablename) t1
cross join (select distinct clothesid from tablename) t2
except
select userid, clothesid
from tablename

See the demo .
Results:

> userid | clothesid
> -----: | --------:
>      1 |         4
>      1 |         5
>      2 |         3