First, the definition
sql server should refer to the sqlserver database that contains a database management system and so on.
navicat for sql server is just a third-party development tools, management tools sqlserver's.
Second, the development kit with different
1, SQLserver: SQLserver can with Microsoft Visual Studio, Microsoft Office System and Development Kit match.
2, Mysql: Mysql not work with Microsoft Visual Studio, Microsoft Office System and Development Kit match.
Third, openness different
1, SQLserver: SQLserver is not open source software
2, Mysql: Mysql is open source software.