子查询

改进 SQL Server 中的查询性能

Dean Smith

Founder, Atamai Analytics

子查询长什么样？

子查询

子查询长什么样？

带外层查询的子查询

带 FROM 的子查询

SELECT OrderID, 
       CustomerID, 
       NumDays
FROM 
    (SELECT *, 
     DATEDIFF(DAY,OrderDate,ShippedDate) AS NumDays 
     FROM Orders) AS o
WHERE NumDays >= 35;

OrderID	CustomerID	NumDays
10380	HUNGO	35
10427	PICCO	35
10545	LAZYK	35
10593	LEHMS	35
10660	HUNGC	37
10777	GOURL	37
10924	BERGS	35

带 WHERE 的子查询

SELECT CustomerID
       ,CompanyName 
FROM Customers
WHERE CustomerID 
         IN (SELECT CustomerID 
         FROM Orders 
         WHERE Freight > 800);

CustomerID	CompanyName
QUEEN	Queen Cozinha
QUICK	QUICK-Stop
SAVEA	Save-a-lot Markets

带 SELECT 的子查询

SELECT CustomerID, 
       CompanyName, 
        (SELECT AVG(Freight) 
        FROM Orders o 
        WHERE c.CustomerID = o.CustomerID) AS AvgFreight
FROM Customers c;

CustomerID	CompanyName	AvgFreight
ALFKI	Alfreds Futterkiste	37.6
ANATR	Ana Trujillo Emparedados y helados	24.4
ANTON	Antonio Moreno Taquería	38.4
...	...	...

子查询的类型

非相关子查询

SELECT CustomerID
       ,CompanyName 
FROM Customers
WHERE CustomerID IN

         (SELECT CustomerID 
         FROM Orders 
         WHERE Freight > 800);

子查询不引用外层查询
子查询可独立运行
常与 WHERE 和 FROM 搭配

相关子查询

SELECT CustomerID, 
       CompanyName,

        (SELECT AVG(Freight) 
        FROM Orders o 
        WHERE c.CustomerID = o.CustomerID) AS AvgFreight

FROM Customers c;

子查询引用外层查询
子查询不能独立运行
常与 WHERE 和 SELECT 搭配

子查询的性能

子查询 vs. INNER JOIN

相关子查询

SELECT CustomerID, 
       CompanyName, 
        (SELECT AVG(Freight) 
        FROM Orders o 
        WHERE c.CustomerID = o.CustomerID) AS AvgFreight
FROM Customers c;

INNER JOIN

SELECT c.CustomerID,
       c.CompanyName, 
       AVG(o.Freight)
FROM Customers c
INNER JOIN Orders o
    ON c.CustomerID = o.CustomerID
GROUP BY c.CustomerID,
         c.CompanyName;

Passons à la pratique !

改进 SQL Server 中的查询性能