One (mandatory) to One (mandatory) [recursive]

Diagram
How to read	One employee must manage exactly one other employee. Each employee must be managed by exactly one other employee.
Relevant relations	None.

The table structure is as follows:

employee(__id__, name, mgr_id*, <e_other>)

The SQL create statement is as follows:

create table employee
  (id type primary key,
   name type,
   mgr_id type not null unique,
   <e_other>,
   foreign key (mgr_id) references employee);

The table could also have the following structure:

employee(__id__, name, emp_id*, <e_other>)

The SQL create statement for this version is as follows:

create table employee
  (id type primary key,
   name type,
   emp_id type not null unique,
   <e_other>,
   foreign key (emp_id) references employee);

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