Query in MySQL to Find Distinct Status Based on Primary Key - mysql

Question

My question is regarding a MySQL query that I am trying to write. I have written some psuedo-code to help illustrate what query I am trying to write:
SELECT *
FROM persons AS p
INNER JOIN person_info AS pi
ON p.person_id = pi.person_id
WHERE status MAY INCLUDE lost, missing, or found
WHAT person_id has no instances of the found status
I'd like to know for each person_id (which can have multiple statuses), which do not have any instance of the status "found." I'm not concerned with just the records of lost and missing. I want to find the unique cases where there is no "found" status based on each unique, distinct person_id.

Answer 1

If I'm understand correctly, one option is to use not in:
select *
from persons
where personid not in (
select personid
from person_info
where status = 'found'
)
This will return all records from the persons table that don't have a matching record in the person_info table with status = 'found'.
Alternatively you can use left join/null check. Not exists can work, but may be slower with mysql. There are some potential issues with null checks as well. Depends on desired results at that point.

Answer 2

This is as far as I took it #sgeddes. In writing it I realized it just makes peoples eyes glaze over.
SQL NOT IN () danger
create table mStatus
( id int auto_increment primary key,
status varchar(10) not null
);
insert mStatus (status) values ('single'),('married'),('divorced'),('widow');
create table people
( id int auto_increment primary key,
fullName varchar(100) not null,
status varchar(10) null
);
Chunk1:
truncate table people;
insert people (fullName,status) values ('John Henry','single');
select * from mstatus where status not in (select status from people);
** 3 rows, as expected **
Chunk2:
truncate table people;
insert people (fullName,status) values ('John Henry','single'),('Kim Billings',null);
select * from mstatus where status not in (select status from people);
no rows, huh?
Obviously this is 'incorrect'. It arises from SQL's use of three-valued logic,
driven by the existence of NULL, a non-value indicating missing (or UNKNOWN) information.
With NOT IN, Chunk2 it is translated like this:
status NOT IN ('married', 'divorced', 'widowed', NULL)
This is equivalent to:
NOT(status='single' OR status='married' OR status='widowed' OR status=NULL)
The expression "status=NULL" evaluates to UNKNOWN and, according to the rules of three-valued logic,
NOT UNKNOWN also evaluates to UNKNOWN. As a result, all rows are filtered out and the query returns an empty set.
Possible solutions include:
select s.status
from mstatus s
left join people p
on p.status=s.status
where p.status is null
or use not exists