Self-Join with missing rows on both sides

Question

I have a table containing people joining and leaving an organization.

Sample data: Sql Fiddle

| pId |     pName |  Unit | YearIn | Yea         


        

Answer 1

I think you want a subquery and aggregation:

SELECT Year, Unit, SUM(NumIn) as NumIn, SUM(NumOut) as NumOut
FROM (SELECT YearIn AS Year, Unit, COUNT(pId) AS NumIn, NULL AS NumOut
      FROM People
      WHERE YearIn >= 2013
      GROUP BY YearIn, Unit
      UNION ALL
      SELECT YearOut AS Year, Unit, NULL AS NumIn, COUNT(pId) AS NumOut
      FROM People
      WHERE YearOut >= 2013
      GROUP BY YearOut, Unit
     ) yu
GROUP BY Year, Unit;