CRM "Customer Relationship Management". So, managing all the interactions between a company and its existing or potential customers.
CRM is a software that stores all customer data and interactions, facilitates tracking activities, aggregates contacts, creates reports, sends emails, and automates business processes.
The goal of CRM is to keep relationships with customers by offering tailored reactions. 
Satisfy the customer (price, quality, deadlines, purchase route, experience)
Improve the way it responds to customer expectations
Maintain a competitive advantage in its market
CRM ER Diagram example
Here is an example of a CRM Database model, this ER diagram below is done with the ERBuilder data modeler.
The documentation of this CRM database is presented here. This documentation is generated by ERBuilder modeling tool. The project file can be downloadedhere.
A Contact may have one or more Opportunity
An Opportunity can be for one or more Contact
A Contact may have one or more Account
An Account belongs to a single Contact
An Opportunity can have one or more Account
An Account belongs to a single Opportunity
A Contact may belong to one or more Campaigns
A campaign may have zero or more Contact and zero or more Leads
ERBuilder is a GUI data modeling tool that allows you to visualize, design and model databases by using entity relationship diagrams and automatically generates the most popular SQL databases. Generate and share the data Model documentation with your team. Optimize your data model by using advanced features such as test data generation, schema compare and schema synchronization.
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