SQL Self-join, Not Exists, Or Something Else? - sql

Question

many-to-many name and role table --
create table t (name varchar, role varchar) ;
insert into t (name, role) values ('joe', 'husband'), ('joe', 'father'),
('tom', 'husband'), ('neo', 'bachelor') ;
> select * from t;
name | role
------+----------
joe | husband
joe | father
tom | husband
neo | bachelor
need to convert into mapping of name and the role(s) he does not have --
not_a | name
---------+-----------
husband | neo
father | tom
father | neo
bachelor | joe
bachelor | tom
How to achieve that in true SQL without iterating through each role/name?

Answer 1

To get roles that someone doesn't have is a little complicated. You have to generate all pairs of names and roles and then pick out the ones that don't exist. This uses a left outer join.
The following is standard SQL for doing this:
select r.role as not_a, n.name
from (select distinct name from t) n cross join
(select distinct role from t) r left outer join
t
on t.name = n.name and t.role = r.role
where t.name is null;
As a note: never use varchar() without a length when defining variables and columns. The default values may not do what you expect.

Answer 2

Assuming you only have this table you can use:
SELECT r.role AS not_a, n.Name
FROM (SELECT DISTINCT Name FROM T) AS n
CROSS JOIN (SELECT DISTINCT Role FROM T) AS r
WHERE NOT EXISTS
( SELECT 1
FROM t
WHERE t.Name = n.Name
AND t.Role = r.Role
);
Example on SQL Fiddle
The main query will generate all pairs of names/roles, then the not exists will exlcude all the pairs that already exist.
If you actually have a name and role table, then you can replace the subqueries with the actual tables:
SELECT r.role AS not_a, n.Name
FROM Names AS n
CROSS JOIN Roles AS r
WHERE NOT EXISTS
( SELECT 1
FROM t
WHERE t.Name = n.Name
AND t.Role = r.Role
);
You haven't specified a DBMS, so if you are using MySQL, using LEFT JOIN\IS NULL will perform better than NOT EXISTS
SELECT r.role AS not_a, n.Name
FROM (SELECT DISTINCT Name FROM T) AS n
CROSS JOIN (SELECT DISTINCT Role FROM T) AS r
LEFT JOIN t
ON t.Name = n.Name
AND t.Role = r.Role
WHERE t.Name IS NULL;
I am also assuming it was just a demo, but in your table DDL you have used VARCHAR without a length which is not a good idea at all