SQL：如何使用 group by 从表中选择不在另一个表中的行？

Question

Suppose that I have a table A with a column id (PK) and a table B with a foreign key to_id that refers to id.

I want to select all rows from A that aren't refered from B.

I thought about the following approach:

select * from A group by id having id not in (select to_id from B);

But I'd like to know if there is an approach that doens't use "NOT IN".

Something like:

select * from A, B group by id having id = to_id and count(to_id) = 0;

This one doesn't work, but I'd like something like this if possible.

Answer 1

I want to select all rows from A that aren't refered from B.

This reads like not exists :

select a.*
from a
where not exists (select 1 from b where b.to_id = a.id)

There is no need to group by whatsoever.

Another typical approach is an anti- left join :

select a.*
from a
left join b on b.to_id = a.id
where b.to_id is null

Answer 2

Just a left join and a where condition to pull only those which have a null as the b id.

select a.*
from a
left join b on b.to_id = a.id
where b.to_id is null