列循环并使用另一个表中的COUNT（）值更新另一列

Question

This is my table events

event name  countBus
ev12  test  NULL
ev16  paris NULL

and table klient

event name  bus
ev12  bob   True
ev12  Alice True
ev12  John  False
ev16  Tom   True
ev16  Peter False

What I'm trying to do is to loop through all rows of table events and update amount of rows that is found in table klient for particular event and bus=True . Then back update that value to particular row in table events column countBus .

Output of table event would be

ev12  test  2
ev16  paris 1

I'm sorry for not providing my attempt as I don't even know where to start.

Thanks for spending your free time on this.

Answer 1

You could di it with a sub-select like this:

UPDATE events
SET countBus = (SELECT count(*) 
                FROM klient k 
                WHERE events.event = k.event 
                AND k.bus = 'True')

Or with a join, like this:

UPDATE e
SET countBus = t.countBus2
FROM events e
INNER JOIN (SELECT event, bus, COUNT(*) countBus2
            FROM klient
            WHERE bus = 'True'
            GROUP BY event, bus) AS t
ON e.event = k.event 

Answer 2

So to break this up and explain it a little you need to do two things really:

1. Find a query that will give you the count per event
2. Write an update query based on the logic of the query in step 1

Query to get the count per event

select e.event, e.name, count(bus)
from klient k
inner join events e on k.event = e.event 
where k.bus = N'True'
group by e.event, e.name

Working Sample: SQL Fiddle

The Update Query

update events
set countBus = (select count(bus)
                from klient k
                where k.event = events.event and k.bus = N'True')
-- view the results
select * from events

Working Sample: SQL Fiddle