Creating and Managing Logins and Users

Creating and managing logins and users in SQL Server is a fundamental aspect of database administration. Properly managing logins and users is critical to ensuring the security and integrity of your databases while enabling authorized access to the system. This detailed guide will cover everything you need to know about creating and managing logins and users in SQL Server, including an explanation of how SQL Server handles logins, users, and permissions, along with best practices for security and troubleshooting.

1. Introduction to SQL Server Logins and Users

In SQL Server, logins and users are essential components of authentication and authorization:

Logins: A login is an authentication principal at the SQL Server level. It represents an identity used to connect to SQL Server. A login is required to access the SQL Server instance itself. Logins are used to verify the identity of a user when they attempt to connect to SQL Server.
Users: A user is an identity within a specific database. A user is associated with a login and is used to grant or deny permissions to database objects (such as tables, views, and stored procedures) within that database.

To access a SQL Server database, a user must first authenticate through a login. Once authenticated, the user is granted specific permissions within the database based on their assigned roles or explicit permissions.

2. Understanding the Relationship Between Logins and Users

In SQL Server, there is a distinction between logins and users, and understanding this relationship is key to proper security management.

Login: A login is an entity used to authenticate access to the SQL Server instance. Logins are created at the server level, and they can be associated with either Windows authentication (using Windows user accounts) or SQL Server authentication (using SQL Server-specific credentials).
User: A user exists within a specific database and represents the identity that interacts with the database’s objects. Users are tied to logins, but they are specific to databases. A single login can map to multiple users in different databases.

Mapping Between Logins and Users

When a login is created in SQL Server, it can be mapped to a user in one or more databases. This allows the login to authenticate at the server level and then access specific databases via the associated user.
In a typical scenario, a login is created at the server level, and then a user is created in each database where access is required. These users are linked to the login to ensure that the login can access the database objects in the context of their user account.

3. Types of Logins in SQL Server

SQL Server supports various types of logins, which can be created depending on the security model of the system and the type of authentication required:

Windows Authentication Login
- This login type uses Windows accounts (both local and domain accounts) for authentication.
- The login is linked directly to a Windows user or group, and SQL Server trusts Windows to handle the authentication.
- This is the preferred authentication method for enhanced security, as it benefits from Windows security policies like password expiration, account lockout, and group memberships.
SQL Server Authentication Login
- This type of login is based on SQL Server-specific usernames and passwords.
- SQL Server handles the authentication of these logins directly, and the credentials are stored in SQL Server’s system tables.
- This method is less secure than Windows Authentication, as it does not support group memberships or the security policies inherent in Windows.
Mixed Mode Authentication
- In Mixed Mode, SQL Server allows both Windows Authentication and SQL Server Authentication logins to access the system.
- This provides flexibility for environments where both types of logins are needed, but it comes with additional security risks because of the potential weaknesses in SQL Server Authentication.
Contained Database Users
- SQL Server 2012 and later versions introduced contained databases, which allow users to authenticate within the database without relying on server-level logins.
- Contained users simplify database portability and reduce the dependency on server-level security.

4. Creating Logins in SQL Server

Creating logins in SQL Server is done using the SQL Server Management Studio (SSMS) or Transact-SQL (T-SQL). Let’s cover the steps to create logins using both methods:

4.1. Creating a Login via SSMS

Open SQL Server Management Studio (SSMS).
Connect to the SQL Server instance where the login will be created.
In the Object Explorer, expand the Security folder.
Right-click the Logins folder and select New Login.
In the Login – New dialog box:
- Select either Windows Authentication or SQL Server Authentication.
- For SQL Server Authentication, enter the Login Name and Password.
- Under Default Database, choose the default database for the login.
- Optionally, set other options such as Default Language and Server Roles.
Click OK to create the login.

4.2. Creating a Login via T-SQL

To create a login using T-SQL, use the following SQL syntax:

Creating a SQL Server Authentication Login:

CREATE LOGIN [login_name] WITH PASSWORD = 'password';

Creating a Windows Authentication Login:

CREATE LOGIN [domain\username] FROM WINDOWS;

In both cases, you can also specify options such as default database, default language, and server roles.

4.3. Configuring Login Options

When creating a login, you can specify various options:

Default Database: This is the database the user will be connected to by default.
Default Language: Sets the language to be used for system messages.
Password Expiration: Enforces password expiration and complexity policies for SQL Server Authentication logins.
Server Roles: Server roles can be assigned to logins to grant permissions for managing the server.

5. Creating Users in SQL Server

After creating a login, you need to create a user in the database to give that login access to specific database objects. Here are the steps to create a user and associate it with a login:

5.1. Creating a User via SSMS

In SSMS, expand the Databases node in the Object Explorer.
Right-click the database where the user will be created and select Properties.
In the Database Properties dialog, go to the Permissions page.
Click Search and select the login that you wish to associate with a user.
Choose the User Mapping tab, and then click the Map checkbox to create the user.
Click OK to create the user in the database.

5.2. Creating a User via T-SQL

To create a user for an existing login, use the following T-SQL syntax:

CREATE USER [username] FOR LOGIN [login_name];

This command creates a user in the current database and links it to an existing login.

5.3. Managing User Roles and Permissions

Once a user is created, you can assign various roles and permissions to manage what actions the user can perform within the database. SQL Server has several predefined roles, such as db_owner, db_datareader, db_datawriter, and db_securityadmin, which can be assigned to users for more efficient role-based security management.

Granting Permissions to a User:

GRANT SELECT, INSERT ON [Table_Name] TO [username];

You can also revoke permissions or deny them entirely:

Revoking Permissions:

REVOKE SELECT, INSERT ON [Table_Name] FROM [username];

Denying Permissions:

DENY SELECT, INSERT ON [Table_Name] TO [username];

6. Managing Permissions and Roles for Users

Permissions in SQL Server control the actions that users can perform on database objects. These permissions can be granted directly to a user or through the assignment of roles. There are two primary types of roles in SQL Server:

Fixed Server Roles: These roles apply to the entire SQL Server instance, such as sysadmin, serveradmin, and securityadmin.
Fixed Database Roles: These roles apply within a specific database, such as db_owner, db_datareader, and db_datawriter.

6.1. Granting Server-Level Roles

You can assign a user to a server-level role to grant permissions for server-wide tasks. For example, to grant a user the sysadmin role:

ALTER SERVER ROLE [sysadmin] ADD MEMBER [username];

6.2. Granting Database-Level Roles

To grant a user a specific database-level role, such as db_owner:

ALTER ROLE [db_owner] ADD MEMBER [username];

6.3. Best Practices for Assigning Permissions

Use roles rather than assigning individual permissions directly to users. This simplifies management and ensures consistency.
Regularly review the roles and permissions assigned to users to ensure they have the minimum necessary access (principle of least privilege).
Use contained users for scenarios where portability and isolation of user authentication are needed.

7. Deleting Logins and Users

When a login or user is no longer needed, it is essential to clean up these entities to avoid unnecessary access or clutter in your system. Deleting logins and users should be done with caution to ensure that no unintended data access occurs.

7.1. Deleting a Login

To delete a login, use the following command:

DROP LOGIN [login_name];

This will remove the login from SQL Server. However, if the login is associated with any users in databases, those users will still exist in their respective databases.

7.2. Deleting a User

To delete a user from a specific database:

DROP USER [username];

This will remove the user from the database but not the associated login. The login will still be able to connect to the server but will no longer have access to the database unless a user is created for that login.

8. Best Practices for Creating and Managing Logins and Users

Use Windows Authentication Whenever Possible: This is more secure and easier to manage than SQL Server Authentication.
Apply the Principle of Least Privilege: Only give users the minimum permissions necessary to perform their jobs.
Regularly Review Logins and Users: Ensure that only authorized users have access to databases and that old or unused accounts are removed.
Use Roles to Simplify Permission Management: Group users by roles and assign permissions to those roles rather than assigning permissions to individual users.
Enable Auditing and Logging: Track login attempts, permission changes, and access to sensitive data.

9. Troubleshooting Logins and Users

If users are having trouble logging in or accessing certain data, you can perform the following troubleshooting steps:

Check Authentication Mode: Ensure the server is configured to allow the correct authentication mode (Windows or SQL Server Authentication).
Verify User Permissions: Ensure that the user has the appropriate roles and permissions for the tasks they need to perform.
Check for Login Failures: Review error logs and login failure reports to understand why a login might have failed.

Creating and managing logins and users in SQL Server is a fundamental part of database security. By understanding the relationship between logins and users, the different types of logins, and how to configure and manage permissions, you can ensure that your database remains secure while providing appropriate access to authorized users. Following best practices for login and user management will help maintain the integrity and security of your SQL Server environment.