Find people who have the same day of birth - mysql

Question

I'm looking for a way to create a query that gives me the list of people who were born on the same day (the data is in two different tables).
All ideas are welcome. ( I use MySQL)
SQL code :
https://pastebin.com/StNggaYg
CREATE TABLE Professeurs(
ID_Professeur Int Auto_increment NOT NULL ,
Nom_Professeur Varchar (50) NOT NULL ,
Prenom_Professeur Varchar (50) NOT NULL ,
Ville_de_naissance_P Varchar (50) NOT NULL ,
Date_de_naissance_P Date NOT NULL ,
Professeur_Principal Bool NOT NULL
,CONSTRAINT Professeurs_PK PRIMARY KEY (ID_Professeur)
)ENGINE=InnoDB;
#------------------------------------------------------------
# Table: Classes
#------------------------------------------------------------
CREATE TABLE Classes(
ID_Classes Int Auto_increment NOT NULL ,
Lettre_classe Char (5) NOT NULL ,
ID_Professeur Int NOT NULL
,CONSTRAINT Classes_PK PRIMARY KEY (ID_Classes)
,CONSTRAINT Classes_Professeurs_FK FOREIGN KEY (ID_Professeur) REFERENCES Professeurs(ID_Professeur)
)ENGINE=InnoDB;
#------------------------------------------------------------
# Table: Elèves
#------------------------------------------------------------
CREATE TABLE Eleves(
ID_Eleve Int Auto_increment NOT NULL ,
Nom_eleve Varchar (50) NOT NULL ,
Prenom_eleve Varchar (50) NOT NULL ,
Ville_de_naissance_E Varchar (50) NOT NULL ,
Date_de_naissance_E Date NOT NULL ,
ID_Classes Int NOT NULL
,CONSTRAINT Eleves_PK PRIMARY KEY (ID_Eleve)
,CONSTRAINT Eleves_Classes_FK FOREIGN KEY (ID_Classes) REFERENCES Classes(ID_Classes)
)ENGINE=InnoDB;
The column :
Date_de_naissance_E Date NOT NULL ,
Date_de_naissance_P Date NOT NULL ,

Answer 1

First, you would have to get the results of BOTH tables dates via a UNION to find what dates have more than one instance. Its possible to have two teachers with same date, two students with same date, OR a teacher AND student have same date. Once that is done, then you can join back to the respective original source to grab the names.
To simplify the query from rewriting it twice nested within itself, I will use CTE (common table expression). It allows you to write a query that will be used multiple times allowing simple alias name to be used in a subsequent query.
Below is the CTE via "With AllPeople as". This select is the query used in the second half. Notice I am getting ID, name, birth date and also a type "P" as Professeur and "E" as Eleves for the person type. First time through I am getting a count where a given birthdate occurs more than once. THEN, re-join to the same AllPeople again that had that date found as a common birth date.
with AllPeople as
( select
'P' personType,
p.ID_Professeur personID,
p.Nom_Professeur personName,
Date_Format( p.Date_de_naissance_P, "%M %d" ) commonDate
from
Professeurs p
union all
select
'E' personType,
e.ID_Eleve personID,
e.nom_eleve personName,
Date_Format( e.Date_de_naissance_E, "%M %d" ) commonDate
from
Eleves e
)
select
who.PersonType,
who.PersonID,
who.PersonName,
BornOnSameDay.commonDate
from
(select
commonDate
from
AllPeople
group by
commonDate
having
count(*) > 1 ) BornOnSameDay
JOIN
AllPeople Who
on BornOnSameDay.commonDate = Who.commonDate
order by
BornOnSameDay.commonDate,
who.PersonName
Now, you also provided a class table of which teachers were teaching what class, and if a student was in a particular class. If your intention was to find out which classes has a student with the same birthdate as the professor teaching it, that would be a different query.
Please advise on which you meant. The one I have provided is ANY teacher having a same birthday as ANY other teacher OR student. And likewise could be two students having the same birth date.

Answer 2

You can use the GROUP_CONCAT function:
SELECT date_format(birthdate,"%d %b") AS birthday, GROUP_CONCAT(person_name) AS people
FROM persons
GROUP BY MONTH(birthdate), DATE(birthdate)