Seperti apa sub-kueri?

Meningkatkan Kinerja Kueri di SQL Server

Dean Smith

Founder, Atamai Analytics

Seperti apa sub-kueri?

Sub-kueri

Seperti apa sub-kueri?

Sub-kueri dengan kueri luar

Sub-kueri dengan 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-kueri dengan 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-kueri dengan 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
...	...	...

Jenis sub-kueri

Sub-kueri tidak berkorelasi

SELECT CustomerID
       ,CompanyName 
FROM Customers
WHERE CustomerID IN

         (SELECT CustomerID 
         FROM Orders 
         WHERE Freight > 800);

Sub-kueri tidak mereferensikan kueri luar
Sub-kueri bisa berjalan mandiri
Dipakai dengan WHERE dan FROM

Sub-kueri berkorelasi

SELECT CustomerID, 
       CompanyName,

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

FROM Customers c;

Sub-kueri mencantumkan referensi ke kueri luar
Sub-kueri tidak bisa berjalan mandiri
Dipakai dengan WHERE dan SELECT

Kinerja sub-kueri

Berkorelasi

Sub-kueri dieksekusi untuk tiap baris pada kueri luar

Tidak berkorelasi

Sub-kueri dijalankan sekali lalu mengirim hasil ke kueri luar

Sub-kueri vs. INNER JOIN

Sub-kueri berkorelasi

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;

Ayo berlatih!

Meningkatkan Kinerja Kueri di SQL Server