# here is the test table.create table cdr ( call_id bigint auto_increment primary key, call_cid varchar(32), call_csid varchar(32), call_start timestamp, call_end timestamp, index call_start_i (call_start), index call_end_i (call_end));# Paste this by hand about 80 times to generate sample data.set @ts:=(now() - interval cast(rand()*1000 as SIGNED) second); insert into cdr VALUES (null, concat('360555', lpad((cast(rand()*1000 as SIGNED)),4,'0')), concat('206555', lpad((cast(rand()*1000 as SIGNED)),4,'0')), @ts, (@ts + interval cast(rand()*1000 as SIGNED) second) );# then query it :)select a.call_start, (select count(call_id) from cdr as b where ( b.call_start <= a.call_start and b.call_end >= a.call_start) ) as 'Simultanious' from cdr as a group by call_start order by Simultanious;