Many-to-one with table records requiring one another - sql

Question

I have two tables: Talks and Days. Talks looks something like:
+----+----------------------------------+--------+
| Id | Name | Leader |
+----+----------------------------------+--------+
| 1 | How to improve revenue for tacos | Tacob |
| 2 | Improving sales potential | Bocat |
+----+----------------------------------+--------+
and the Days:
+--------+-----+
| TalkId | Day |
+--------+-----+
| 1 | Mon |
| 1 | Tue |
| 1 | Thu |
| 2 | Mon |
| 2 | Tue |
+--------+-----+
TalkId is a foreign key referencing the Talks table.
The foreign key enforces the relationship of "A Day requires a Talk". However, I would like to also enforce the reverse relationship "A Talk requires at least a Day".
I know that this constraint is similar to a Many-to-many relationship, where both records depend on each other. However, in this case, many days reference a talk but only one talk references many days.
Another problem is that after creating such a constraint, how would one insert both records at once?
I have searched for other questions and only found cases of Many-to-many relationships which will turn out like so:
+----+----------------------------------+--------+
| Id | Name | Leader |
+----+----------------------------------+--------+
| 1 | How to improve revenue for tacos | Tacob |
| 2 | Improving sales potential | Bocat |
+----+----------------------------------+--------+
+----+-----+
| Id | Day |
+----+-----+
| 1 | Mon |
| 2 | Tue |
| 3 | Thu |
| 4 | Mon |
| 5 | Tue |
+----+-----+
+--------+-------+
| TalkId | DayId |
+--------+-------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 4 |
| 2 | 5 |
+--------+-------+
Where TalkId references Talks's Id and DayId references Days's Id.
Edit:
Ignore what I requested for above.
What I hope to be able to do:
SELECT all valid Talks
SELECT all valid Days
What I hope to be unable to do:
INSERT a Talk without a Day
INSERT a Day without a Talk

Answer 1

It sounds like you want a simple foreign key relationship:
alter table days add constraint fk_days_talkid foreign key (talkid) references talks(talkid);
This guarantees that talkid is valid. Then you declare days.talkid to be not null and you are guaranteeing the relationship you describe.

Answer 2

-- Day named (TheDay) exists.
--
Calendar {TheDay}
PK {TheDay}
-- Talk (TalkID) titled (TalkName), presented by (Leader) is by default
-- scheduled on (DefaultDay).
Talk {TalkID, TalkName, Leader, DefaultDay}
PK {TalkID}
AK {TalkName}
FOREIGN KEY {DefaultDay} REFERENCES Calendar {TheDay}
--Talk (TalkID) is also scheduled on (TheDay).
--
TalkDay {TalkID, TheDay}
PK {TalkID, TheDay}
FOREIGN KEY {TalkID} REFERENCES Talk {TalkID}
FOREIGN KEY {TheDay} REFERENCES Calendar {TheDay}
Note PK = primary key
AK = alternate key (unique)
All attributes (columns) NOT NULL
Select all days for a specific talk:
select TalkName, DefaultDay as TalkDay
from Talk
where TalkName = 'How to improve revenue for tacos'
union
select TalkName, b.TheDay as TalkDay
from Talk as a
join TalkDay as b on b.TalkID = a.TalkID
where a.TalkName = 'How to improve revenue for tacos'
Select all talks on a specific day:
select TalkName, DefaultDay as TalkDay
from Talk
where DefaultDay = 'Tue'
union
select TalkName, b.TheDay as TalkDay
from Talk as a
join TalkDay as b on b.TalkID = a.TalkID
where b.TheDay = 'Tue'