Question

在查询中实现子查询，该查询使用max（dateOut）返回DVD的上一次借出。

该查询在借贷表中给出了正确的回报，但是当尝试实现为子查询时会出现错误，您知道应该怎么做？ max（dateout）部分。测试时出现如下错误。

SELECT 
    DVDTitle.DVDTitle, 
    Loan.LoanNumber, 
    Member.MemberNumber, 
    Member.MemberFirstName, 
    Member.MemberLastName, 
    (SELECT max(DateOut) FROM Loan, DVDCopy GROUP BY DVDCopy.CopyNumber), 
    Loan.DateDue, Loan.DateReturned
FROM 
    DVDCopy 
INNER JOIN 
    Loan 
    ON DVDCopy.CopyNumber = Loan.CopyNumber 
INNER JOIN 
    Member 
    ON Loan.MemberNumber = Member.MemberNumber 
INNER JOIN 
    DVDTitle 
    ON DVDCopy.DVDNumber = DVDTitle.DVDNumber
WHERE  
    (DVDCopy.CopyNumber = @CopyNumber)

错误是：

执行查询时出错。请检查命令的语法以及参数的类型和值（如果存在），并确保它们正确无误。子查询返回的值超过1。当子查询遵循=，！=，<，<=，>，> =或将子查询用作表达式时，不允许这样做。

预期结果应基于下拉列表从网格表中的借出表和DVD复制表中基于借出号返回特定DVD的最后借出（此处选择了借出号）。任何想法，谢谢。

Answer 1

如何将SELECT中的子查询移动到INNER JOIN：

SELECT 
    DVDTitle.DVDTitle, 
    Loan.LoanNumber, 
    Member.MemberNumber, 
    Member.MemberFirstName, 
    Member.MemberLastName, 
    mdo.MaxDateOut, 
    Loan.DateDue, Loan.DateReturned
FROM 
    DVDCopy 
INNER JOIN 
    Loan 
    ON DVDCopy.CopyNumber = Loan.CopyNumber 
INNER JOIN 
    Member 
    ON Loan.MemberNumber = Member.MemberNumber 
INNER JOIN 
    DVDTitle 
    ON DVDCopy.DVDNumber = DVDTitle.DVDNumber
INNER JOIN
(SELECT CopyNumber,max(DateOut) as "MaxDateOut" FROM Loan, DVDCopy GROUP BY DVDCopy.CopyNumber) mdo 
    ON DVDCopy.CopyNumber = mdo.CopyNumber
WHERE  
    (DVDCopy.CopyNumber = @CopyNumber)

子查询返回多个值，=，！=，<，<=，>，> =

1 个答案: