Check constraint using a sub-select to make sure that not more than 10 employees are in one department

create table employees
(
   emp_id           integer not null primary key,
   last_name        varchar(100) not null,
   first_name       varchar(100),
   dept_id integer  not null,
   constraint check_dept_size 
     check ( (select count(*) from emp e where e.dept_id = dept_id) <= 5 )
);

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