Failed to add the foreign key constraint, MySQL - mysql

Question

I have this error on PopSQL :
Error: Failed to add the foreign key constraint. Missing index for constraint 'etape__ibfk_1' in the referenced table 'etapexprojet_'
Error Code: ER_FK_NO_INDEX_PARENT
This is my code, i'm only creating tables and it has a problem in the "Etape_" part, i really, don't know why
I'm really, really new to SQL. Like... this is my first time doing this
--Client
CREATE TABLE IF NOT EXISTS jegere.client_ (
idClient INT PRIMARY KEY,
nomClient VARCHAR(45) NOT NULL,
adresse VARCHAR(45) NOT NULL UNIQUE,
telephone VARCHAR(45) NOT NULL UNIQUE,
adresseCourriel VARCHAR(45) NOT NULL UNIQUE
);
--Employés
CREATE TABLE IF NOT EXISTS jegere.Employe_ (
idEmploye INT PRIMARY KEY,
nomEmploye VARCHAR(45) NOT NULL,
adresse VARCHAR(45) NOT NULL UNIQUE,
telephone VARCHAR(45) NOT NULL UNIQUE,
adresseCourriel VARCHAR(45) NOT NULL UNIQUE
);
--Projet
CREATE TABLE IF NOT EXISTS jegere.Projet_ (
idProjet INT PRIMARY KEY,
idClient INT ,
nomProjet VARCHAR(45) NOT NULL ,
dateDebut DATE NOT NULL ,
dateFin DATE,
idResponsable INT NOT NULL ,
FOREIGN KEY (idClient ) REFERENCES jegere.Client_ (idClient ),
FOREIGN KEY (idResponsable ) REFERENCES jegere.Employe_ (idEmploye )
);
--RessourcesProjet
CREATE TABLE IF NOT EXISTS jegere.RessourcesProjet_ (
idProjet INT NOT NULL,
idEmploye INT NOT NULL,
nbrHeure INT NOT NULL,
PrixHeure FLOAT NOT NULL,
PRIMARY KEY(idProjet, idEmploye),
FOREIGN KEY(idProjet) REFERENCES projet_(idProjet),
FOREIGN KEY(idEmploye) REFERENCES employe_(idEmploye)
);
--Etape X Projet
CREATE TABLE IF NOT EXISTS jegere.EtapexProjet_ (
idEtape INT NOT NULL,
idProjet INT NOT NULL,
dateDebut DATE NOT NULL ,
dateFin DATE,
PRIMARY KEY(idProjet, idEtape),
FOREIGN KEY(idProjet) REFERENCES projet_(idProjet)
);
--Etapes
CREATE TABLE IF NOT EXISTS jegere.Etape_ (
idEtape INT,
nomEtape VARCHAR(45) NOT NULL,
Livrable VARCHAR(100) NOT NULL,
PRIMARY KEY(idEtape),
FOREIGN KEY(idEtape) REFERENCES etapexprojet_(idEtape)
);

Answer 1

The referenced column in a foreign key needs to be indexed, so you need to add an index on the idEtape column in etapexprojet:
--Etape X Projet
CREATE TABLE IF NOT EXISTS jegere.EtapexProjet_ (
idEtape INT NOT NULL,
idProjet INT NOT NULL,
dateDebut DATE NOT NULL ,
dateFin DATE,
PRIMARY KEY(idProjet, idEtape),
INDEX (idEtape),
FOREIGN KEY(idProjet) REFERENCES projet_(idProjet)
);
Having it as part of the primary key isn't sufficient. A prefix of an index is also an index, but idEtape is a later part of the primary key index, so it's not indexed by itself.
Another solution is to change the order of the primary key:
PRIMARY KEY(idEtape, idProject),

Answer 2

Your last table etape references only the column idetape, but all referenced columns need an index , primary key or UNIQUE constraint.
so add a KEY to your EtapexProjet_ for the column idetape, and you can run the code
CREATE TABLE IF NOT EXISTS EtapexProjet_ (
idEtape INT NOT NULL,
idProjet INT NOT NULL,
dateDebut DATE NOT NULL ,
dateFin DATE,
PRIMARY KEY(idProjet, idEtape),
KEY(idEtape),
FOREIGN KEY(idProjet) REFERENCES Projet_(idProjet)
);
see https://dbfiddle.uk/z9QFoH0c