Why aliases are required in self joins

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Accepted Answer

Without aliases, every column reference is ambiguous - the database cannot tell whether 'employees.id' refers to the left copy or the right copy of the table. The query will fail at parse time with an error like 'table name employees is specified more than once' or 'column reference is ambiguous'. Aliases (e.g. e1, e2 or emp, mgr) are not optional in a self join - they are syntactically required.