Electronic health records (EHRs) are becoming increasingly accessible as hospitals set up health data warehouses. Those data come with many promises for applications in clinical, epidemiologic, and translational research, but also with particularities in the data access, the data collection process, and the data format that must be accounted for in the preparation and the subsequent analyses. This brief article shares our practical experience building a solid and exhaustive cohort of diabetic patients in the Greater Paris University Hospitals Clinical Data Warehouse (AP-HP EDS). This ambitious project's substantial engineering and computing resources should be balanced with an adequate sharing and reusing policy for future studies.