Checking if a SQL Server login already exists

Here's a way to do this in SQL Server 2005 and later without using the deprecated syslogins view:

IF NOT EXISTS 
    (SELECT name  
     FROM master.sys.server_principals
     WHERE name = 'LoginName')
BEGIN
    CREATE LOGIN [LoginName] WITH PASSWORD = N'password'
END

The server_principals view is used instead of sql_logins because the latter doesn't list Windows logins.

If you need to check for the existence of a user in a particular database before creating them, then you can do this:

USE your_db_name

IF NOT EXISTS
    (SELECT name
     FROM sys.database_principals
     WHERE name = 'Bob')
BEGIN
    CREATE USER [Bob] FOR LOGIN [Bob] 
END

You can use the built-in function:

SUSER_ID ( [ 'myUsername' ] )

via

IF [value] IS NULL [statement]

like:

IF SUSER_ID (N'myUsername') IS NULL
CREATE LOGIN [myUsername] WITH PASSWORD=N'myPassword', 
DEFAULT_LANGUAGE=[us_english], 
CHECK_EXPIRATION=OFF, 
CHECK_POLICY=OFF 
GO

https://technet.microsoft.com/en-us/library/ms176042(v=sql.110).aspx

In order to hande naming conflict between logins, roles, users etc. you should check the type column according to Microsoft sys.database_principals documentation

In order to handle special chacters in usernames etc, use N'<name>' and [<name>] accordingly.

Create login

USE MASTER
IF NOT EXISTS (SELECT 1 FROM master.sys.server_principals WHERE 
[name] = N'<loginname>' and [type] IN ('C','E', 'G', 'K', 'S', 'U'))
    CREATE LOGIN [<loginname>] <further parameters>

Create database user

USE [<databasename>]
IF NOT EXISTS (SELECT 1 FROM sys.database_principals WHERE 
[name] = N'<username>' and [type] IN ('C','E', 'G', 'K', 'S', 'U'))
    CREATE USER [<username>] FOR LOGIN [<loginname>]

Create database role

USE [<databasename>]
IF NOT EXISTS (SELECT 1 FROM sys.database_principals WHERE 
[name] = N'<rolename>' and Type = 'R')
    CREATE ROLE [<rolename>]

Add user to role

USE [<databasename>]
EXEC sp_addrolemember N'<rolename>', N'<username>'

Grant rights to role

USE [<databasename>]
GRANT SELECT ON [<tablename>] TO [<rolename>]
GRANT UPDATE ON [<tablename>] ([<columnname>]) TO [<rolename>]
GRANT EXECUTE ON [<procedurename>] TO [<rolename>]

The SQL is tested on SQL Server 2005, 2008, 2008 R2, 2014, 2016, 2017, 2019

This is for Azure SQL:

IF (EXISTS(SELECT TOP 1 1 FROM sys.sql_logins WHERE [name] = '<login>'))
    DROP LOGIN [<login>];

Source: How to check whether database user already exists in Azure SQL Database

Starting SQL 2016:

DROP USER IF EXISTS [userName]

CREATE USER [userName] FOR LOGIN [loginName]