Hi, @Chehade Claudia (XC-AD/ESW4) Welcome to Microsoft Q&A, Thanks for posting your question.
MySQL Flexible Server with private access provides the highest level of network isolation in Azure. All communications and connections to the MySQL Flexible Server with private access can only happen from the same virtual network or a peered virtual network. This minimizes any potential threats to your MySQL server.
Microsoft Defender for Cloud provides a set of advanced database security capabilities, including database Vulnerability Assessment and Advanced Threat Protection whereas private access is for providing network traffic isolation. The purpose and use case are different.
Defender is not a necessary instrument but a good to have instrument at your organizational level to identify potential blind spots which can become a potential threat. With MySQL Flexible Server running with private access and by following the best practices for virtual network outlined in the documentation here, you will ensure to isolate and secure network access with the highest available layer of protection in Azure.
Please let us know if the information helps.
Regards
Geetha