How to update one table based on the count value of another table while matching ids?

UPDATE SUBJECT
LEFT JOIN (
SELECT count(*) AS num, subject_id
FROM class
GROUP BY subject_id
where date_time > NOW()) AS t ON SUBJECT.subject_id = t.subject_id
SET SUBJECT.current_class_count = coalesce( t.num, 0 )

UPDATE Geo_location AS g 
JOIN users_data AS u ON u.member_num = g.member_num 
SET g.public = ud.acceptinClients

-- query open event counts by contact id
SELECT ID, COUNT(*) AS OpenEventCount
FROM Events
WHERE EventType = 'Open'
GROUP BY ID

UPDATE c
SET c.Tier = CASE
        WHEN COALESCE(ec.OpenEventCount, 0) > 2 then 'Max'
        WHEN COALESCE(ec.OpenEventCount, 0) = 2 then 'High'
        ELSE 'Low' 
    END
FROM Contacts c
    LEFT OUTER JOIN ( -- left join to update contacts with no open events
        SELECT ID, COUNT(*) AS OpenEventCount
        FROM Events
        WHERE EventType = 'Open'
        GROUP BY ID
    ) ec ON ec.ID = c.ID

select source, action,
       count(*) as total_received,
       sum(case when status = 'Closed' then 1 else 0 end) as total_closed,
       sum(case when status = 'Open' and DueDate < Today() then 1 else 0 end) as overdue
from t
group by source, action;

UPDATE [Insight].[dbo].[Rev Old New]
SET [Rev Old New].[Calls] = (SELECT COUNT(*) FROM [Insight].[dbo].[Calls] t1
                  WHERE t1.Date BETWEEN [Rev Old New].[Start] AND [Rev Old New].[Finish])