MySQL 5.7

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create table sessions (uid int);
insert into sessions values (0),
(0),
(0),
(0),
(0),
(0),
(0),
(0),
(0),
(0),

(2),
(2)
select * from sessions


uid


0

0

0

0

0

0

0

0

0

0

2

2


select count(case when uid=0 then uid end)+ 
count(distinct case when uid!=0 then uid end) 
FROM sessions
/*WHERE last_active >= DATE_SUB(NOW(), INTERVAL 15 MINUTE);*/


count(case when uid=0 then uid end)+ 
count(distinct case when uid!=0 then uid end)


11