Window function in T-Sql

Window syntax in T-SQL

Create the window with OVER clause
PARTITION BY creates the frame
If you do not include PARTITION BY , the entire table is used as the frame.
To arrange the result, use ORDER BY
Allows aggregations to be created at the same time with the window.

SELECT OrderID, TerritoryName, 
       -- Total price for each partition
       SUM(OrderPrice) 
       -- Create the window and partitions
       OVER(PARTITION BY TerritoryName) AS TotalPrice
FROM Orders

There are four window functions; FIRST_VALUE(), LAST_VALUE(), LAG() and LEAD(). FIRST_VALUE(): returns the first value in a window. LAST_VALUE(): returns the last value in a window. LAG(): returns the previous value in partitioned condition. Note that the lag value will always be null for the first value. Screenshot 2022-07-30 at 19.17.24.png

LEAD(): returns the next value in the partitioned condition. Note that the lead value will always be NULL for the last window.

Screenshot 2022-07-30 at 19.16.33.png Use cases;

SELECT TerritoryName, OrderDate, 
       #Select the first value in each partition
       FIRST_VALUE(OrderDate) 
       #Create the partitions and arrange the rows
       OVER(PARTITION BY TerritoryName ORDER BY OrderDate) AS FirstOrder
FROM Orders

SELECT TerritoryName, OrderDate, 
       #Specify the previous OrderDate in the window
       LAG(OrderDate) 
       #Over the window, partition by territory & order by order date
       OVER(PARTITION BY TerritoryName ORDER BY OrderDate) AS PreviousOrder,
       #Specify the next OrderDate in the window
       LEAD(OrderDate) 
       #Create the partitions and arrange the rows
       OVER(PARTITION BY TerritoryName ORDER BY OrderDate) AS NextOrder
FROM Orders

You can also use aggregate function.

Use Case;

SELECT TerritoryName, OrderDate, 
       #Create a running total
       SUM(OrderPrice) 
       #Create the partitions and arrange the rows
       OVER(PARTITION BY TerritoryName ORDER BY OrderDate) AS TerritoryTotal      
FROM Orders

SELECT TerritoryName, OrderDate, 
       #Assign a row number
       ROW_NUMBER() 
       #Create the partitions and arrange the rows
       OVER(PARTITION BY TerritoryName ORDER BY OrderDate) AS OrderCount
FROM Orders

SELECT OrderDate, TerritoryName, 
       #Calculate the standard deviation
       STDEV(OrderPrice) 
       OVER(PARTITION BY TerritoryName ORDER BY OrderDate) AS StdDevPrice      
FROM Orders

#Create a CTE Called ModePrice which contains two columns
WITH ModePrice (OrderPrice, UnitPriceFrequency)
AS
(
    SELECT OrderPrice, 
    ROW_NUMBER() 
    OVER(PARTITION BY OrderPrice ORDER BY OrderPrice) AS UnitPriceFrequency
    FROM Orders 
)

#Select everything from the CTE
SELECT * FROM ModePrice

Window Function In T-sql

Window syntax in T-SQL