find who the manager using boolean datatype(true, false) in ismanager column

There are set of data listed under table employee like empid, empname, position, managerid, ismanager.

Here some employees are manager and some not. If that employee is manager it has to show 1 in ismanager column, if that employee is not manager it has to show 0 in ismanager column, if empid is listed in managerid then ismanager should be updated to 1 else it should be 0.

create table usertable (
    empid int primary key,
    empname varchar(40),
    position varchar(40),
    managerid int,  
    foreign key (managerid) references usertable(empid),
    ismanager bit(1)
);

This is what I have tried:

update usertable  set ismanager=
case
when empid  in (select managerid from usertable) then 1
else 0
end;

As they are all set to 0 currently, you only need to update the ones who are managers:

UPDATE usertable u1
JOIN (SELECT DISTINCT managerid FROM usertable WHERE managerid IS NOT NULL) u2
    ON u1.empid = u2.managerid
SET u1.ismanager = 1;

If they are all set to null and you need to set both the 1s and 0s:

UPDATE usertable u1
LEFT JOIN (SELECT DISTINCT managerid FROM usertable WHERE managerid IS NOT NULL) u2
    ON u1.empid = u2.managerid
SET u1.ismanager = (u2.managerid IS NOT NULL);

Of course, you could ditch the redundant ismanager column and retrieve it dynamically:

SELECT *, EXISTS (SELECT 1 FROM usertable WHERE managerid = u.empid) AS ismanager
FROM usertable u;