sql-server - 子查询返回超过 1 个值。当子查询跟随 =,!=,<,<=,>,>= 或子查询用作表达式时，这是不允许的

Question

我有一个存储过程select * from book table，使用子查询我的查询是

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)

我知道子查询是向主查询返回多个查询，但我不知道如何避免此错误，有人可以帮助我吗？

Answer 1

问题是这两个查询都返回不止一行：

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

您有两种选择，具体取决于您想要的结果。您可以将上述查询替换为保证返回单行的内容（例如，使用SELECT TOP 1），或者您可以切换=到IN并返回多行，如下所示：

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

Answer 2

使用In代替=

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

或者你可以使用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' )

Answer 3

您可以使用 IN 运算符，如下所示

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

Answer 4

Using operator 'IN' helps

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

else if(@key='l')

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