SQL Yiwen get window function

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  Window function

  What is a window function  

  Syntax of window function

Basic usage of grammar —— Use RANK function

Types of special window functions

Calculate the moving average

Two ORDER BY 

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  Window function

  What is a window function  

•   The window function is also called OLAP function . In order to let everyone quickly form an intuitive impression , Just got such an easy to understand name ,OLAP yes OnLine Analytical Processing For short , It means real-time analysis and processing of database data .

  Syntax of window function

<  Window function  > OVER ([ PARTITION BY <  Make a list  >]
ORDER BY <  Sort by listing  >)
①  Aggregate function that can be used as window function （ SUM 、 AVG 、 COUNT 、 MAX 、 MIN ）
② RANK 、 DENSE _ RANK 、 ROW _ NUMBER  And so on 

Basic usage of grammar —— Use RANK function

  

about Product In the table 8 Commodity , According to different kinds of goods （ product_type ）, According to the sales unit price （ sale_price ） Sort from low to high

SELECT product_name, product_type, sale_price,
RANK () OVER (PARTITION BY product_type
ORDER BY sale_price) AS ranking
FROM Product;

PARTITION BY Can set the range of sorting objects ,ORDER BY Can specify which column to follow 、 In what order . PARTITION BY Group tables horizontally , and ORDER BY Determines the vertical sorting rules .

The key to using window functions is PARTITION BY and GROUP BY . among , PARTITION BY It's not necessary , Even if it is not specified, the window function can be used normally .

SELECT product_name, product_type, sale_price,
RANK () OVER (ORDER BY sale_price) AS ranking
FROM Product;

  

Types of special window functions

RANK function
When calculating the sort , If there are records of the same order , Will skip the next position .
example ） Yes 3 Record number one 1 When a ：1 position 、1 position 、1 position 、4 position ……

 DENSE_RANK function
It's also sort by calculation , Even if there are records of the same rank , And I won't skip the next place .
example ） Yes 3 Record number one 1 When a ：1 position 、1 position 、1 position 、2 position ……

 ROW_NUMBER function
Give a unique continuous position .
example ） Yes 3 Record number one 1 When a ：1 position 、2 position 、3 position 、4 position ……

ps:( Special window functions do not require parameters , Therefore, the brackets are usually empty , In principle, window functions can only be used in SELECT Used in clauses , stay SELECT Out of clause “ Using window functions is meaningless ”)

Calculate the moving average

Window function is to divide the table into windows , And sort in it . In fact, it also includes the alternative function of specifying a more detailed summary range in the window , The summary scope in this alternative function is called the framework

SELECT product_id, product_name, sale_price,
AVG (sale_price) OVER (ORDER BY product_id
ROWS 2 PRECEDING) AS moving_avg
FROM Product;

    ROWS （“ That's ok ”） and PRECEDING （“ Before ”） Two keywords , Specify the frame as “ By the end of ~ That's ok ”,“ ROWS 2 PRECEDING ” Is to specify the frame as “ By the end of 2 That's ok ”, That is, the records as the summary object are limited to the following “ The closest 3 That's ok ”, That is, oneself 、 Before 1 That's ok 、 The first two lines

keyword FOLLOWING （“ after ”） Replace PRECEDING , You can specify “ Until after ~ That's ok ” As a framework

If you want to use the front and back lines of the current record as summary objects , It needs to be used at the same time PRECEDING （“ Before ”） and FOLLOWING （“ after ”） Keyword to implement .

​​​​​​​SELECT product_id, product_name, sale_price,
AVG (sale_price) OVER (ORDER BY product_id
ROWS BETWEEN 1 PRECEDING AND 
1 FOLLOWING) AS moving_avg
FROM Product;

Two ORDER BY 

• When using window functions, you must be in OVER Used in clauses ORDER BY , OVER In Clause ORDER BY It is only used to determine the order in which window functions are calculated , The order of the results does not affect the use of the aggregate function as a window function , The record of the summary object will be determined based on the current record .