You can join the characters table twice:
select c.name, k.name as killer from films f inner join deaths d on f.id = d.id_film inner join characters c on c.id = d.id_character inner join characters k on k.id = d.id_killer where f.episode = 'Episode III'
Use standard joins! Implicit joins (with commas in the
fromclause) are legacy syntax from (literaly) decades ago, that should not be used in new code
Use single quotes for literal strings; in standard SQL, double quotes stand for identifiers (although MySQL makes things differently)
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