T-SQL：如何在SubQuery中选择多个字段

问题描述 投票：0回答：2

嗨我在INVOICE表中有两个表INVOICE和CUSTOMER我有一个CUSTOMER表（CustomerId）的引用，当我创建发票视图时，我需要在一个相同的子查询中检索NAME和EMAIL以及其他字段

注意：我不能使用加入

我现在在做什么

SELECT 
InvoiceId, 
(SELECT FullName FROM Customer WHERE CustomerId = CustomerId) AS CustomerFullName, 
(SELECT Email FROM Customer WHERE CustomerId = CustomerId) AS CustomerEmail 
...
FROM
InVoice

我需要的是

SELECT 
InvoiceId, 
(SELECT FullName, Email, ....  FROM Customer WHERE CustomerId = CustomerId) AS CustomerFullName, CustomerEmail, ....  
FROM
InVoice

这种子查询可能吗？

sql sql-server tsql
2个回答
2
投票

正确的方法是使用join。例如，如果键盘上的“j”键坏了，您可以使用apply表达：

SELECT i.InvoiceId, c.*
FROM InVoice i OUTER APPLY
     (SELECT c.FullName, c.Email, . . .
      FROM Customer c
      WHERE c.CustomerId = i.CustomerId
     ) c;
2
投票

回应OP的声明

正确的事情是我不想使用它。

你为什么不想使用JOIN？从字面上看，使用子查询是一个糟糕的想法...例如，让我们对来自Microsoft的WideWorldImporters数据库进行非常简单的查询：

SELECT I.InvoiceID,
       O.OrderID,
       O.OrderDate, O.ExpectedDeliveryDate
FROM sales.Invoices I
     JOIN sales.Orders O ON I.OrderID = O.OrderID
WHERE I.InvoiceID BETWEEN 1 AND 1000;

启用统计时间和IO，会导致：

Table 'Orders'. Scan count 0, logical reads 3000, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

Table 'Invoices'. Scan count 1, logical reads 182, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 


SQL Server Execution Times:
CPU time = 4 ms, elapsed time = 4 ms.

那时候即将到来。现在，让我们使用您的方法（使用子查询）：

SELECT I.InvoiceID,
       (SELECT O.OrderID FROM sales.Orders O WHERE I.OrderID = O.OrderID) AS OrderID,
       (SELECT O.OrderDate FROM sales.Orders O WHERE I.OrderID = O.OrderID) AS OrderDate,
       (SELECT O.ExpectedDeliveryDate FROM sales.Orders O WHERE I.OrderID = O.OrderID) AS ExpectedDeliveryDate
FROM sales.Invoices I     
WHERE I.InvoiceID BETWEEN 1 AND 1000;

和成本？

Table 'Orders'. Scan count 0, logical reads 9000, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

Table 'Invoices'. Scan count 1, logical reads 182, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 


SQL Server Execution Times:
CPU time = 10 ms, elapsed time = 10 ms.

嗯，毫不奇怪，读数翻了三倍，从3000到9000！此外，执行时间超过两倍。仅仅因为我们为每列使用了子查询。

现在，为了证明更进一步的观点，让我们来看看客户表，看看这是多么“糟糕”：

PRINT 'Using JOIN';
SELECT I.InvoiceID,
       C.CustomerName,
       C.DeliveryPostalCode,
       O.OrderID,
       O.OrderDate, O.ExpectedDeliveryDate
FROM sales.Invoices I
     JOIN sales.Orders O ON I.OrderID = O.OrderID
     JOIN sales.Customers C ON O.CustomerID = C.CustomerID
WHERE I.InvoiceID BETWEEN 1 AND 1000;

PRINT 'Using Subqueries'
SELECT I.InvoiceID,
       (SELECT C.Customername FROM sales.Customers C WHERE C.CustomerID = (SELECT O.CustomerID FROM sales.Orders O WHERE I.OrderID = O.OrderID)) AS CustomerName,
       (SELECT C.DeliveryPostalCode FROM sales.Customers C WHERE C.CustomerID = (SELECT O.CustomerID FROM sales.Orders O WHERE I.OrderID = O.OrderID)) AS DeliveryPostalCode,
       (SELECT O.OrderID FROM sales.Orders O WHERE I.OrderID = O.OrderID) AS OrderID,
       (SELECT O.OrderDate FROM sales.Orders O WHERE I.OrderID = O.OrderID) AS OrderDate,
       (SELECT O.ExpectedDeliveryDate FROM sales.Orders O WHERE I.OrderID = O.OrderID) AS ExpectedDeliveryDate
FROM sales.Invoices I     
WHERE I.InvoiceID BETWEEN 1 AND 1000;

结果？

Using JOIN 


SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 0 ms. 

    (1000 rows affected) 

    Table 'Workfile'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

    Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

    Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

    Table 'Customers'. Scan count 1, logical reads 40, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

    Table 'Orders'. Scan count 0, logical reads 3000, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

    Table 'Invoices'. Scan count 1, logical reads 182, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 


SQL Server Execution Times:
CPU time = 9 ms, elapsed time = 8 ms. 

    Using Subqueries 


SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 0 ms. 

    (1000 rows affected) 

    Table 'Orders'. Scan count 0, logical reads 15000, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

    Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

    Table 'Customers'. Scan count 2000, logical reads 80000, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 

    Table 'Invoices'. Scan count 1, logical reads 182, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0. 


SQL Server Execution Times:
CPU time = 2028 ms, elapsed time = 2029 ms.

看看那个执行时间！超过2秒！使用JOIN进行相同的查询花了9ms。看看那些读物！订单表的15000和Customers表的80000。使用JOIN意味着它们分别为3000和40。

所以，我不得不再次问：“为什么你不想使用JOIN？”没有充分的理由这样做。即使没有Keys和索引，性能也会呈指数级增长。诚实地说，你所做的是一个可怕的错误。请，请重新考虑使用子查询。

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