windows身份验证："server=.;database=Northwind;Integrated Security=True;"
sql验证："server=.;database=Northwind;uid=sa;pwd=;”
也可以用如下代码：
windows身份验证："Data Source=.;Initial Catalog=Northwind;Integrated Security=True;"
sql验证："Data Source=.;Initial Catalog=Northwind;User ID=sa;Password=;"