id | sender_mail | recipient_mail | username | subject | message | created
1    aa@test.com    bb@test.com        aaa       xxxx     yyy       10
2    bb@test.com    aa@test.com        bbb       xxxx     yyy       11
3    cc@test.com    bb@test.com        ccc       xxxx     yyy       12
4    bb@test.com    cc@test.com        bbb       xxxx     yyy       13
5    dd@test.com    gg@test.com        ddd       xxxx     yyy       14
6    gg@test.com    bb@test.com        ggg       xxxx     yyy       15

id | sender_mail | recipient_mail | username | subject | message | created
2    bb@test.com    aa@test.com        bbb       xxxx     yyy       11
4    bb@test.com    cc@test.com        bbb       xxxx     yyy       13
6    gg@test.com    bb@test.com        ggg       xxxx     yyy       15

select *
from messages as m
where
    m.sender_mail = 'bb@test.com' OR
    m.recipient_mail = 'bb@test.com'

group by m.sender_mail, m.recipient_mail

DrupalCoreDatabaseDatabaseExceptionWrapper: SQLSTATE[42000]:
Syntax error or access violation: 1055 'drupal.m.username' isn't in GROUP BY:
SELECT m.sender_mail, m.recipient_mail, m. username, m.subject, m.message, m.created