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select * from dept; select * from emp;
+--------+----------+
...
|      2 |      2 |
|      3 |   NULL |
+--------+--------+

my $query = qq{select distinct dept.deptID, emp.deptID 
    from dept left join emp using(deptID)};
...
deptID, deptID
--- fetchrow_hashref
deptID