Create variable in SQL script before doing queries - mysql

Question

I have the following snippet
select user_id into #selected_user from users where mail = 'mymail#gmail.com';
select * from kc_network_user where user_id = 'previous_id';
which I want to modify to be parametric. I found the following resources:
Using variables in SQL script
How to declare a variable in MySQL?
Declare variables - MySQL Workbench gives me an error with DECLARE
That's what I did:
select #user_mail := 'mymail#gmail.com'; #1
select #user_mail; #2
select user_id into #selected_user from user where mail = #user_mail; #3
select #selected_user;
select * from user_network where user_id = #selected_user; #4
In select #1 I tried also the following:
set #user_mail = 'mymail#gmail.com';
The select #2 returns the correct value, so the variable is initialized.
The select #3 returns the following error:
Error Code: 1267. Illegal mix of collations
(utf8mb4_unicode_ci,IMPLICIT) and (utf8mb4_general_ci,IMPLICIT) for
operation '='
While the select #4 runs correctly (if I set the mail parameter with the literal value).
What's the difference between those queries?
How can I set a variable before doing a query?

Answer 1

I don't understand why not simply
select *
from user_network
join user using (user_id)
where user.mail = 'mymail#gmail.com';
If you need to set mentioned variables additionally (but for what reason?) then use
select *, (#selected_user := user.user_id)
from user_network
join user using (user_id)
where user.mail = (#user_mail := 'mymail#gmail.com');