Select minimum time for the same id from two tables (MySQL) - mysql

Question

Hi I have two tables and for the same contact "email" in the two tables, I want to know the first time the "email" gets in touch.
conversation_and_id
contact
Expected output
So far I only have done outer join to join the two tables, but it removed duplicated "email", which I need it to compare time difference from the same "email".
ALTER TABLE conversation_and_id CHANGE `email` `conversation_and_id_email` VARCHAR(100);
create table conversation_and_id_and_contact
SELECT *
FROM contact AS con
LEFT JOIN conversation_and_id AS ci ON con.email = ci.conversation_and_id_email
UNION
SELECT *
FROM contact AS con
RIGHT JOIN conversation_and_id AS ci ON con.email = ci.conversation_and_id_email;
create table final
select
if (f.email is not null, f.email, f.conversation_and_id_email) as email1,
if (f.atlas_interest is not null, f.atlas_interest, f.created_at) as time1,
if (f.conversation_and_id_email is not null, f.conversation_and_id_email, f.email) as email2,
if (f.created_at is not null, f.created_at, f.atlas_interest) as time2
from conversation_and_id_and_contact f;

Answer 1

The standard solution everywhere is to perform a full outer join with both tables and get the MIN() timestamp.
However, since MySQL does not (yet) support full outer joins you'll need to union a left and right joins to get the result you want. The final [unnecessarily long] query looks like:
select
coalesce(iemail, cemail) as email,
least(created_at, atlas_interest) as expressed_interest_at
from (
select -- this is the left join
i.email as iemail, i.created_at, c.email as cemail, c.atlas_interest
from conversation_and_id i
left join contact c on c.email = i.email
union
select -- this is the right join
i.email as iemail, i.created_at, c.email as cemail, c.atlas_interest
from conversation_and_id i
right join contact c on c.email = i.email
) x