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I am looking for a sophisticated software package which will allow me to design Entity Relationship Diagrams (ERDs) for relational databases. Is there any free software out there which will allow me to do this? I tend to find that planning your databases out first during the design process been implementing is a better process through the software life cycle.

I am designing a databse for MS SQL Server so once the ERD has been designed, if the software could export SQL code to create the database then this is the type of software I am looking for.

Feel free to suggest any software packages which you might know of.