Created models for the database.

models.py

 from flask import Flask
 from flask_sqlalchemy import SQLAlchemy
 
-db = SQLAlchemy()
+app = Flask(__name__)
+app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
+db = SQLAlchemy(app)
 
 
-class Element(db.Model)
+element_label = db.Table('Element_Label', db.Model.metadata,
+                         db.Column('id_element', db.Integer, db.ForeignKey('element.id')),
+                         db.Column('id_label', db.Integer, db.ForeignKey('label.id'))
+                         )
+
+class Element(db.Model):
     id = db.Column(db.INTEGER, primary_key=True)
     element_name = db.Column(db.String(50), unique=True, nullable=False)
+    labels = db.relationship(
+        "Label",
+        secondary=element_label,
+        back_populates="elements")
 
-class Label(db.Model)
+class Label(db.Model):
     id = db.Column(db.INTEGER, primary_key=True)
     label_name = db.Column(db.String(50), unique=True, nullable=False)
     father_id = db.Column(db.INTEGER, db.ForeignKey('label.id'))
+    children = db.relationship("Label",
+                    backref=db.backref('parent', remote_side=[id])
+                    )
+    aliases = db.relationship('Alias', back_populates='label')
+    elements = db.relationship(
+        "Element",
+        secondary=element_label,
+        back_populates="labels")
+
+class Alias(db.Model):
+    alias_name = db.Column(db.String(100), primary_key=True)
+    id_label = db.Column(db.INTEGER, db.ForeignKey('label.id'), nullable=False)
+    label = db.relationship('Label', back_populates='aliases')
+
+
+