Subquery returned more than 1 value.This is not permitted when the subquery follows =,!=,<,<=,>,>= or when the subquery is used as an expression

Question

I have a stored procedure that select * from book table , using sub query my query is

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)

I know sub query is return more than one query to main query , But i don't know how to avoid this error, can any one help me ?

Answer 1

The problem is that these two queries are each returning more than one row:

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

You have two choices, depending on your desired outcome. You can either replace the above queries with something that's guaranteed to return a single row (for example, by using SELECT TOP 1), OR you can switch your = to IN and return multiple rows, like this:

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

Answer 2

Use In instead of =

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

or you can use 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' )