MySQL 8.0

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create table users(user_guid int);
create table dogs(user_guid int);
insert into users values(1);
insert into users values(2);
insert into users values(2);
insert into users values(3);
insert into dogs values(2);
insert into dogs values(3);
insert into dogs values(5);
select count(distinct u.user_guid) unique_users    
from users u     
where u.user_guid IN (select d.user_guid from dogs d );


unique_users


2


select count(distinct u.user_guid) unique_users    
from users u     
where exists (select 1 from dogs d where u.user_guid = d.user_guid);


unique_users


2