Check if a user exists in a SQL Server database

I'm working with SQL Server 2012. I want to check if a user exists before adding it to a database.

This is what I have tested:

USE [MyDatabase]
GO

IF NOT EXISTS (SELECT name 
                FROM [sys].[server_principals]
                WHERE name = N'IIS APPPOOL\MyWebApi AppPool')
Begin
    CREATE USER [IIS APPPOOL\MyWebApi AppPool] 
    FOR LOGIN [IIS APPPOOL\MyWebApi AppPool] WITH DEFAULT_SCHEMA=[dbo]
end
ALTER ROLE [db_owner] ADD MEMBER [IIS APPPOOL\MyWebApi AppPool]
GO

But, this code SELECT name FROM [sys].[server_principals] doesn't return if that user exists in MyDatabase.

How can I check if an user exists in MyDatabase?

Use sys.database_principals instead of sys.server_principals.

So the final query would look like this (accounting for the user filter):

USE [MyDatabase]
GO

IF NOT EXISTS (SELECT [name]
                FROM [sys].[database_principals]
                WHERE [type] = N'S' AND [name] = N'IIS APPPOOL\MyWebApi AppPool')
Begin
    CREATE USER [IIS APPPOOL\MyWebApi AppPool] 
    FOR LOGIN [IIS APPPOOL\MyWebApi AppPool] WITH DEFAULT_SCHEMA=[dbo]
end
ALTER ROLE [db_owner] ADD MEMBER [IIS APPPOOL\MyWebApi AppPool]
GO

StackExchange Database Administrators Q#125886, answer score: 45