Exercise 1: Postgres's Unified Role Concept vs. MySQL's Separate User/Privilege Grouping — Possible Solution
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MYSQL'S APPROACH
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Per this chapter, "in MySQL, there are USER accounts, and a
separately-evolved mechanism (roles, added only in MySQL 8.0) for
grouping privileges — roles are a relatively recent addition layered
on top of a user-centric model." MySQL's core model is user accounts
first, with role-based grouping bolted on afterward as a later
addition rather than the original foundation of its access-control
system.

POSTGRES'S APPROACH
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Per this chapter, "in Postgres, there has only ever been one core
concept: the role. A role can behave as a login-capable user, as a
pure privilege-grouping 'group,' or both at once — the distinction
comes entirely from a single attribute." Rather than two separate
mechanisms bolted together, Postgres has a single mechanism (the
role) that can be configured, via the LOGIN attribute, to serve
either purpose — or, per this chapter's own example, both purposes
combined.

A CONCRETE EXAMPLE OF ROLE MEMBERSHIP
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Using this chapter's own syntax:

  CREATE ROLE alice LOGIN PASSWORD 'secret';
  CREATE ROLE reporting_team;
  GRANT reporting_team TO alice;

Here, alice is a login-capable role (a "user"), and reporting_team is
a pure group role with no LOGIN attribute, meaning it can't connect
to the database directly — it exists purely to hold a set of
privileges. The final GRANT statement makes alice a MEMBER of
reporting_team, which means alice automatically inherits every
privilege that has been (or will be) granted to reporting_team,
without those privileges needing to be granted to alice individually.
If a second employee, bob, is later added the same way (GRANT
reporting_team TO bob;), both alice and bob share the same set of
privileges through their shared membership in reporting_team — the
same underlying mechanism (roles and role membership) handles both
individual login and group-based privilege organization.

WHY THIS WORKS AS AN ANSWER
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It contrasts MySQL's two-mechanism, user-first history with Postgres's
single unified role concept using the chapter's own wording, and
demonstrates role membership concretely using the chapter's own
CREATE ROLE/GRANT example rather than describing it only abstractly.