Sub-queries

Improving Query Performance in SQL Server

Dean Smith

Founder, Atamai Analytics

How do sub-queries look?

Sub-query

How do sub-queries look?

Sub-query with outer-query

Sub-query with FROM

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

Sub-query with WHERE

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

Sub-query with SELECT

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
...	...	...

Types of sub-queries

Uncorrelated sub-query

SELECT CustomerID
       ,CompanyName 
FROM Customers
WHERE CustomerID IN

         (SELECT CustomerID 
         FROM Orders 
         WHERE Freight > 800);

Sub-query does not contain a reference to the outer query
Sub-query can run independently of the outer query
Used with WHERE and FROM

Correlated sub-query

SELECT CustomerID, 
       CompanyName,

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

FROM Customers c;

Sub-query contains a reference to the outer query
Sub-query cannot run independently of the outer query
Used with WHERE and SELECT

Sub-query performance

Correlated

Sub-query executes for each row in the outer query

Uncorrelated

Sub-query executes only once and returns the results to the outer query

Sub-query vs. INNER JOIN

Correlated sub-query

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;

Let's practice!

Improving Query Performance in SQL Server