SQL: a self join relates a table to itself

Two aliases on the same table let you compare rows to other rows - an employee to a manager, a reading to the previous reading. The aliases are what make the columns unambiguous.

Code

SELECT e.name AS employee, m.name AS manager
  FROM employee e
  LEFT JOIN employee m ON m.id = e.manager_id;

-- A window function is often clearer for row-to-row comparison:
SELECT reading_at, value,
       value - LAG(value) OVER (ORDER BY reading_at) AS delta
  FROM meter;

Output

employee | manager
Ann      | NULL
Bob      | Ann

reading_at | value | delta
09:00      |  100  | NULL
09:05      |  118  |   18

-- LEFT is important in the first query: an INNER JOIN would silently drop
-- anyone without a manager, usually the CEO.

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Published 2026-08-25