子查詢返回多個值。當子查詢遵循=，！=，<,<=,>，> =或當子查詢用作表達式時，這是不允許的。

Question

我有一個存儲過程select * from book table，使用子查詢我的查詢是

USE [library] 
GO 

/****** Object: StoredProcedure [dbo].[report_r_and_l] Script Date: 04/17/2013 12:42:39 ******/ 

SET ANSI_NULLS ON 
GO 

SET QUOTED_IDENTIFIER ON 
GO 

ALTER procedure [dbo].[report_r_and_l] 
@fdate date, 
@tdate date, 
@key varchar(1) 
as 

if(@key='r') 

    select * 
    from dbo.books 
    where isbn =(select isbn from dbo.lending where (act between @fdate and @tdate) and (stat ='close')) 

else if(@key='l') 

    select * 
    from dbo.books 
    where isbn =(select isbn from dbo.lending where lended_date between @fdate and @tdate) 

我知道子查詢是返回多個查詢主要查詢，但我不知道如何避免這個錯誤，任何人都可以幫助我嗎？

Answer 1

的問題是，這兩個查詢每個返回不止一個行：

select isbn from dbo.lending where (act between @fdate and @tdate) and (stat ='close') 
select isbn from dbo.lending where lended_date between @fdate and @tdate 

根據您的預期結果，您有兩種選擇。您可以用東西是保證返回一個單行（例如，通過使用SELECT TOP 1）替換上述疑問，或者您可以將您的=到IN和返回多行，像這樣：

select * from dbo.books where isbn IN (select isbn from dbo.lending where (act between @fdate and @tdate) and (stat ='close')) 

Answer 2

您可以使用IN操作符如下

select * from dbo.books where isbn IN 
(select isbn from dbo.lending where lended_date between @fdate and @tdate) 

Answer 3

使用In而不是=

select * from dbo.books 
where isbn in (select isbn from dbo.lending 
       where act between @fdate and @tdate 
       and stat ='close' 
       ) 

，或者您可以使用Exists

SELECT t1.*,t2.* 
FROM books t1 
WHERE EXISTS (SELECT * FROM dbo.lending t2 WHERE t1.isbn = t2.isbn and 
       t2.act between @fdate and @tdate and t2.stat ='close')