But Some Queries are Difficult to Unnest

select distinct struct ( D: d, E: ( select distinct e

from e in Employees

where e.dno = d.dno ) )

from d in Departments;

?{ < D = d, E = ?{ e | e ? Employees, e.dno = d.dno } >

| d ? Departments }

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But there are queries that cannot be unnesting by normalization alone. Here is a comprehension that contains another comprehension in its head. This query cannot be unnested by normalization.