SQL Server 2017

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CREATE TABLE Employee (ID int, Name varchar(50), ManagerID int);
INSERT INTO Employee VALUES
(1, 'John', null),
(2, 'Peter', null),
(3, 'Joe', 1),
(4, 'Helen', 3);
4 rows affected

SELECT e1.ID,
       e1.Name,
       e1.ManagerID,
       e2.Name AS ManagerName
FROM   Employee e1
JOIN   Employee e2
ON     e2.ID = e1.ManagerID;


ID
Name
ManagerID
ManagerName


3
Joe
1
John

4
Helen
3
Joe


SELECT      e1.ID,
            e1.Name,
            e1.ManagerID,
            e2.Name AS ManagerName
FROM        Employee e1
LEFT JOIN   Employee e2
ON          e2.ID = e1.ManagerID;


ID
Name
ManagerID
ManagerName


1
John
null
null

2
Peter
null
null

3
Joe
1
John

4
Helen
3
Joe