Considering only basic relational algebra operations, how can we implement the full outer-join ?

  • $\begingroup$ A simple google search yields this $\endgroup$
    – Sagnik
    Feb 16, 2018 at 6:29
  • $\begingroup$ @Sagnik well, I need to ignore any type of join and only consider basic operations (the solution comes with inner join). $\endgroup$
    – fhm
    Feb 16, 2018 at 13:43

1 Answer 1


Full outer join returns null in the columns of rows from one table that don't match the other table -- that's a description of SQL.

Standard Relational Algebra doesn't have nulls. Here explains what would be the consequences.

Short answer: 'we' don't want to implement outer joins. If you think you do, think some more until you see why it leads to incoherence of queries.


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