The lecture SQL - Outer Joins, Self Joins by Martin Holzke is from the course SQL Database for Beginners. It contains the following chapters:
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... results in records without manager assignment to be excluded. 7) Write a query returning a list of persons including, if assigned, their respective manager. Answer: select p.firstname, p.lastname, m.firstname, m.lastname from person p left join person m on p.managerid=m.id 8) Combine the previous two queries to ensure persons without manager assignment are included in the result. ...