drop table if exists t1;
create table t1 (kill_id int);
insert into t1 values(connection_id());
select kill_id-kill_id from t1;
kill_id-kill_id
0
flush tables with read lock;
select ((@id := kill_id) - kill_id) from t1;
((@id := kill_id) - kill_id)
0
kill connection @id;
drop table t1;