如何使用CASE语句判断客户至少有1张未付发票时显示'debtor'

如何使用CASE实现：当客户至少有一张发票未支付（Invoice_Status = 1）时显示'debtor'？
*单个客户对应多张发票
现有查询可返回Id_Customer及其名下多张发票的状态，但会针对该客户的每一张发票重复展示同一条Id_Customer记录：

SELECT Id_Customer, 
    CASE Invoice_Status
        WHEN 1 THEN 'Debtor'
        ELSE 'No debts'
    END AS Status
FROM Tb_Invoices
ORDER BY Id_Customer
GO

现有查询是按单条发票维度返回结果，所以同一个客户会出现多条重复记录。要实现每个客户仅返回1条状态记录，只要存在任意未支付发票就标记为debtor，可以参考以下两种实现：

方案1：GROUP BY + 聚合判断（写法最简洁） #

SELECT 
    Id_Customer,
    CASE 
        WHEN MAX(Invoice_Status) = 1 THEN 'debtor'
        ELSE 'No debts'
    END AS Status
FROM Tb_Invoices
GROUP BY Id_Customer
ORDER BY Id_Customer
GO

逻辑说明：只要客户名下存在任意一张状态为1的未支付发票，MAX(Invoice_Status)返回的结果就为1，符合CASE判断条件返回debtor；只有该客户所有发票都已支付时，才会返回No debts。GROUP BY Id_Customer保证每个客户仅返回一条记录。

方案2：EXISTS子查询（大数据量下性能更优） #

如果数据量较大，不需要全量扫描单个客户的所有发票，可以用EXISTS子查询，只要找到一张未支付发票就会终止判断，执行效率更高：

SELECT 
    DISTINCT t1.Id_Customer,
    CASE 
        WHEN EXISTS (
            SELECT 1 FROM Tb_Invoices t2 
            WHERE t2.Id_Customer = t1.Id_Customer 
            AND t2.Invoice_Status = 1
        ) THEN 'debtor'
        ELSE 'No debts'
    END AS Status
FROM Tb_Invoices t1
ORDER BY t1.Id_Customer
GO

内容的提问来源于stack exchange，提问作者Rohalt