SQL Server 2016 Express, need to add account with Administrator access to a database
If you are a Windows Administrator, follow the procedure documented here.

Or simply run this script from an elevated command prompt on the SQL Server:

net stop mssql$sqlexpress 
net start mssql$sqlexpress /mSQLCMD 
sqlcmd -S (local)\sqlexpress -Q "if not exists(select * from sys.server_principals where name='BUILTIN\administrators') CREATE LOGIN [BUILTIN\administrators] FROM WINDOWS;EXEC master..sp_addsrvrolemember @loginame = N'BUILTIN\administrators', @rolename = N'sysadmin'" 
net stop mssql$sqlexpress
net start mssql$sqlexpress
sqlcmd -S (local)\sqlexpress -Q "if exists( select * from fn_my_permissions(NULL, 'SERVER') where permission_name = 'CONTROL SERVER') print 'You are a sysadmin.'"


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