检查SQL Server数据库中是否存在用户

我正在使用SQL Server2012。我想在将用户添加到数据库之前检查用户是否存在。

这是我测试过的：

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

但是，SELECT name FROM [sys].[server_principals]如果该用户存在于中，则此代码不会返回MyDatabase。

如何检查用户是否存在MyDatabase？

使用sys.database_principals代替sys.server_principals。

因此，最终查询将如下所示（考虑用户过滤器）：

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

我将SUSER_ID（）和USER_ID（）用于此类事情：

-- Check SQL Server Login
IF SUSER_ID('SomeLogin') IS NULL
    CREATE LOGIN SomeLogin WITH PASSWORD = 'SomePassword';

-- Check database user
IF USER_ID('SomeUser') IS NULL
    CREATE USER SomeUser FOR LOGIN SomeLogin;

进一步完善，因为这样可以使阅读效果更好

USE [MyDatabase]
GO

IF DATABASE_PRINCIPAL_ID('IIS APPPOOL\MyWebApi AppPool') IS NULL
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

如果您使用的是拒绝服务器，则可以一次检查许多服务器，并使用以下命令返回true / false：

SELECT @@servername,    
    CASE 
        WHEN EXISTS(SELECT name FROM sys.database_principals WHERE name = 'LoginName') THEN 1 
        ELSE 0 
    END AS YesNo