Sorry for not being clear...i mean tools to create models of our databases.
Sybase PowerDesigner, CA ER-Win Data Modeler, Embarcadero ER Studio, Microsoft Visio
I suppose you are asking about how to go about modelling your database,for a simple db, you can start with database diagrams in sql server, and if it is large then go for something like erwin from ca or Embarcadero studio or any other tool that you like.
I've used both CA ER-Win and ModelRight.
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