sql-server - 如果 SELECT 语句没有结果，则使用 CASE 返回字符串

Question

如果我的 SELECT 语句没有结果，是否可以使用 CASE 返回某个字符串？

例子：

DECLARE @accountnumber AS VARCHAR(10)

SET @accountnumber = 'account number to search'

SELECT 
    CASE
    WHEN account IS NOT NULL
    THEN 'We Have Records of this Customer'
    WHEN account IS NULL
    THEN 'We Do Not Have Records For This Customer'
    END AS 'result'
FROM call_records
WHERE account = @accountnumber
GROUP BY account

上述方法不起作用，因为如果我正在搜索的帐号不在我的日志表中，则不会有任何结果，并且消息“我们没有此客户的记录”将永远不会实现。
我可以使用带有 PRINT 命令的纯 T-SQL 来完成我想要实现的目标，但我正在使用第 3 方应用程序，结果必须采用表格形式（因此仅限 SELECT 语句）。

Answer 1

您可以使用EXISTS：

SELECT 
    CASE
    WHEN EXISTS(
        SELECT 1 FROM call_records
        WHERE account = @accountnumber
    ) 
    THEN  'We Have Records of this Customer'
    ELSE  'We Do Not Have Records For This Customer'
    END AS 'result';

Answer 2

尝试：

DECLARE @accountnumber AS VARCHAR(10)

SET @accountnumber = 'account number to search'

SELECT 
    CASE
    WHEN account IS NOT NULL
    THEN 'We Have Records of this Customer'
    WHEN account IS NULL
    THEN 'We Do Not Have Records For This Customer'
    END AS 'result'
FROM (select 1 dummy) dummy
left join call_records ON account = @accountnumber
GROUP BY account