Creating relationships between tables that where not specified in the given UML diagram
UML class diagrams do not include relations in a class' attributes list, so when creating database tables representing those classes, the database designer must identify and specify them, much like the software developer must create the pointers and lists to support the related objects.

In your diagram, Commande relates to Client but does not explicitly specify a IDClient attribute. When creating the Commande table, you must include this attribute along with a foreign key to Client.

You can find further information about this in Database Design - UML/SQL:


  The UML representation of the Order class contains only its own descriptive attributes. The UML association tells which customer placed an order. In the database, we will need a different way to identify the customer; that will be part of the relation scheme (below).