T-SQL : 하위 쿼리

Question

안녕을 여러 필드를 선택하는 방법은 내가 고객 테이블의 참조가있는 INVOICE 테이블에 두 테이블 INVOICE 및 고객 (를 고객 ID) 내가 필요가 나는 송장보기가 하나 개의 동일한 하위 쿼리에

NB를 NAME 및 이메일 및 기타 필드를 가져 오지 만들 때 : 나는 CA NNOT 사용은 내가 지금하고 내가

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

가능한 서브 쿼리의 종류입니다 무엇이 필요

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

무엇

가입?

Answer 1

올바른 방법은 join입니다. 말하자면, 키보드의 "J"키가 깨진 경우 apply를 사용하여이 표현할 수 :

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

Answer 2

이

오른쪽 사물의 영업 이익의 진술에 대응하기 위해 내가 안입니다 그것을 사용하고 싶다.

왜 는 JOIN을 사용하지 않는 것일까? 예를 들어 ... 아주 글자 그대로 끔찍한 생각 서브 쿼리이다 사용의 마이크로 소프트에서 WideWorldImporters DATBASE에 아주 간단한 쿼리를 보자 :

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; 

를 통계 시간과의 IO, 이것은 결과 :

를

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. 

약 예정입니다. 이제 하위 쿼리를 사용하여 메서드를 사용합시다 :

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; 

그리고 비용은?

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. 

글쎄, 놀랄는이 가 3000에서 9000으로,을 세 겹으로 읽지 않습니다! 또한 실행 시간이 두 배 이상입니다. 각 열에 대해 하위 쿼리를 사용했기 때문입니다.

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; 

결과 :

지금 바로 추가 점을 증명하기 위해, 우리는,이 어떻게되는지 "심하게"을 참조한다 고객 테이블에 가져 보자?

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. 

실행 시간을 살펴보십시오! 2 초 이상! JOIN을 사용하여 동일한 쿼리를 수행하는 데 9ms가 걸렸습니다. 그리고 그 읽기를보세요!Orders 테이블의 15000 및 Customers 테이블의 80000 JOIN을 사용하면 각각 3000과 40에 해당합니다.

그래서 다시 묻습니다. "왜 JOIN을 사용하고 싶지 않으십니까?" 그렇게 할 이유가 없습니다. Keys와 인덱스가 없어도 성능은 기하 급수적으로 향상됩니다. 정직하게 당신이하는 일은 끔찍한 실수입니다. 하위 쿼리 사용에 대해 다시 생각해주십시오.