Okay, so always on teacher, 1 or more Learners.

Are subjects a set thing? That is, only a limited amount of subjects (expanded at will), or can it be anything, that is, just a quick note.

I don't believe you need a session id. As one person can only teach one session at a time, the natural key is Teacher/Time. And for Person, shouldn't it be employee? And, do Employees already have unique id, such as their company email address?

Yes, teacher ID could be used as the session (pair) ID.

Available topics are expected to be quite dynamic. Someone could change the topic they offer each week, or stick to the same topic for long periods of time, or have several topics on offer at the same time.

Person / Employee is not an interesting distinction. This system is unrelated to any "business" database. In fact it's quite likely that visitors will take part in sessions.

Email addresses are stored in the People database as an ID, but can be changed. The autoincremented internal ID is the "authoritative" identifier in the system.

Perl is the programming world's equivalent of English

Someone could change the topic they offer each week

If the same topic comes up twice, would it have the same spelling and case?

it's quite likely that visitors will take part in sessions

That answers that right there. :)

Email addresses are stored in the People database as an ID, but can be changed. The auto incremented internal ID is the "authoritative" identifier in the system.

One reason to sue an email address or the like is so the FKs make sense. No reason to hide information behind a number. As people may have the same name, ids are usually used, but a company email address or the like can help it be more meaningful.

Regardless, i posted some tables and a view below that might help. If you like the approach but want a small change or two, please just say so!

Here's an attempt at it:

CREATE TABLE Employee
(
 Id        NUMBER        PRIMARY KEY AUTOINCREMENT,
 First_Name    TEXT,
 Last_Name    TEXT
);

CREATE TABLE Topic
(
 Name        TEXT CHECK(Name = LOWER(Name)) PRIMARY KEY,
 Description    TEXT
);

CREATE TABLE Session
(
 Teacher    INTEGER                REFERENCES Employee,
 Start        DATETIME,
 Finish        DATETIME,
 Topic        TEXT        NOT NULL    REFERENCES Topic,
 PRIMARY KEY(Teacher, Start)
);

CREATE TABLE Session_Learner
(
 Teacher    NUMBER,
 Start        DATETIME,
 Learner    NUMBER        NOT NULL    REFERENCES Employee,
 --
 FOREIGN KEY(Teacher, Start) REFERENCES Session,
 PRIMARY KEY(Teacher, Start, Learner)
);

CREATE VIEW
    Session_Employee
AS
    WITH
        Session_Teacher_Learner
    AS
        (
         SELECT
             Session.Teacher,
             Session.Start,
             Session.Topic,
             Session_Learner.Learner
         FROM
             Session,
             Session_Learner
         WHERE
             Session_Learner.Teacher    = Session.Teacher
           AND    Session_Learner.Start    = Session.Start
        )
    SELECT
        Teacher,
        Start,
        Topic,
        Teacher Employee
    FROM
        Session_Teacher_Learner
    UNION ALL
    SELECT
        Teacher,
        Start,
        Topic,
        Learner
    FROM
        Session_Teacher_Learner;
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