SQL不存在自联接

Question

I've got a table which stores all letters sent out to customers. I need to run a query to select all account_id's that have received letter_2 but not letter_1. So for example the below results should be accounts 2 & 3.

Account_ID Letter_Type
1          Letter_1
1          Letter_2
1          Letter_3
1          Letter_2
2          Letter_2
2          Letter_3
3          Letter_2

Thanks in advance!

Answer 1

A straightforward way of doing this, is to use except :

select account_id 
from the_table 
where letter_type = 'Letter_2' 
except 
select account_id 
from the_table 
where letter_type = 'Letter_1'

An anti-join using not exists would look like this:

select t1.account_id 
from the_table t1
where t1.letter_type = 'Letter_2' 
  and not exists (select *
                  from the_table t2
                  where t2.letter_type = 'Letter_1' and 
                    and t1.account_id = t2.account_id);

You didn't state your DBMS, but the above is standard ANSI SQL