SQL server Login and User - sql-server

Question

I am confused with Login and User. I found following in articles :
A "Login" grants the principal entry into the SERVER.
A "User" grants a login entry into a single DATABASE.
One "Login" can be associated with many users (one per database).
I can understand it theoretically. But, I think I might not have understood this practically.
I created a Login in my SQL server 2008 management studio by right clicking SERVERNAME=>SECURITY=>LOGIN. The default database was "master". Now, I can log into sql server with this login name and password. I noticed that if I change the default database in Login properties to a specific database, I cannot login again with this credential. I reverted back to "master" and it works. What happened here?
Also, why do we need users? I created a user by right clicking DATABASENAME=>SECURITY=>USERS. I cannot relogin with this user credential. So, what is the purpose we need this for. I can understand the theory of this answer but I need little more explanations to make sense.
Also, I am a .net developer, so I would like to know, what are the credentials provided in sql connection strings. Are they login or user or can be any of these?

Answer 1

The simplest explanation is that the SQL Server login gets you into the server, and the settings on that login control how it works in each database.
Don't worry about the database logins for the moment. You already went to SERVERNAME=>SECURITY=>LOGIN. Let's look at what you do with this login - right click and go into properties if you already created the login. Look under Roles - there are a number of different ones with different purposes on the server. But for an app, generally speaking, the average user should only have the Public role.
As far as the Database login, you go to the Mapping section to point your login to whichever databases it needs access to. When you map the login to the database, this CREATES the database login you saw under DATABASENAME=>SECURITY=>USERS if it does not already exist. The mapping is the most important part though, it's what literally gives the login the ability to see data in the database.
For applications, you're using the server login. If you set up the links to the databases you need in mapping, you don't really need to think about the database level login info.

Answer 2

The login only exists at the server level, which is why it maps automatically to the master database.
Users control access to individual databases. When you create a user, you can map it to a login (see Create User on MSDN for the syntax). If you create a user mapped to the login in the database, you can set it as the default and log in.
One reason this is done is to allow multi-tenant environments where a single server hosts many databases that not everyone who can access the server should be able to access. For example, say we provide services to Company A and Company B, and we host the database for each on the same server. We don't want someone from Company A (or, more importantly, someone who has compromised the credentials of someone from Company A) to be able to access the data for Company B, so we only create a user for the Company A login in the Company A database. Here's a brief code setup for you to experiment with:
-- This script assumes whoever is running it has sysadmin permissions on the instance of
-- SQL Server on which it is running. Do not run this on a production instance.
-- Create a database for each company on the server instance.
create database CompanyA;
create database CompanyB;
go
-- Create a login for each company on the server instance.
-- SQL Server integrated security has it's issues, but it's useful for an example like this.
create login CompanyA_Login with password = 'pa55wOrd1', default_database = CompanyA;
create login CompanyB_Login with password = 'pa55wOrd2', default_database = CompanyB;
go
-- Create a user in the appropriate database for each login.
-- We need to tell the server that we want to use a specific database
use CompanyA;
create user CompanyA_User for login CompanyA_Login;
-- We're granting it dbo for the purposes of our example here;
-- a broad permission set like that is a bad practice.
alter role db_owner add member CompanyA_User;
go
-- Repeat the process...
use CompanyB;
create user CompanyB_User for login CompanyB_Login;
alter role db_owner add member CompanyB_User;
go
-- Create a table in each database and populate it with some data.
use CompanyA;
create table dbo.sensitiveInformation
(
sensitiveInformation NVARCHAR(50) NOT NULL
);
insert dbo.sensitiveInformation (sensitiveInformation)
values ('Oh man, it would be bad if this got out!');
go
use CompanyB;
create table dbo.sensitiveInformation
(
sensitiveInformation NVARCHAR(50) NOT NULL
);
insert dbo.sensitiveInformation (sensitiveInformation)
values ('Oh man, it would be even worse if THIS got out!');
go
-- Now, feel free to log in as either user and see what you can and can't do.
-- You will find that the CompanyA_Login will never be able to access CompanyB's
-- data and vice versa. This allows for secure multi-tenant environments.
-- Once you're done playing around, we'll clean up our samples.
use CompanyB;
drop table dbo.sensitiveInformation;
drop user CompanyB_User;
go
use CompanyA;
drop table dbo.sensitiveInformation;
drop user CompanyA_User;
go
use master;
drop login CompanyB_Login;
drop login CompanyA_Login;
drop database CompanyB;
drop database CompanyA;
If you need separate, discrete security/permission sets, and you need one user to have more than one of these sets, you want to use database roles. This article from Tech Republic gives a decent gloss of the benefits of roles, though I'd recommend checking MSDN for the most current ways of creating them.