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

Diagram
How to read	One employee may manage one other employee, but each employee is managed by exactly one 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,
   <e_other>,
   foreign key (mgr_id) references employee);

Since this relationship is optional in one direction, this is the only way this relationship should be represented in the database.

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