MySQL learning record (9)

1. The concept of window function and its basic usage

The window function is also called OLAP function .OLAP yes OnLine AnalyticalProcessing For short , It means real-time analysis and processing of database data .

For the sake of understanding , It is called window function . The conventional SELECT Statements are used to query the whole table , The window function allows us to selectively summarize some data 、 Calculate and sort .

1.1 General form of window function

PARTITON BY Is used to group , That is, select which window you want to see , Be similar to GROUP BY Clause grouping function , however PARTITION BY Clause does not have GROUP BY The summary function of clause , It does not change the number of rows recorded in the original table .

ORDER BY It's used to sort , That is, in the decision window , According to that rule ( Field ) To sort .

< Window function > OVER ([PARTITION BY < Name >]
                     ORDER BY < Column names for sorting >)  
-- Example 
select product_name,product_type,sale_price,
       rank() over(partition by product_type
                   order by sale_price) as ranking
                   from product;

1.2 Types of window functions

One is take SUM、MAX、MIN Aggregate functions such as are used in window functions

Two is RANK、DENSE_RANK And other special window functions for sorting

1.2.1  Special window functions

• RANK function **（ British sort ）**

When calculating the sort , If there are records of the same order , be Will skip Next place .

example ） Yes 3 Record number one 1 When a ：1 position 、1 position 、1 position 、4 position ……

• DENSE_RANK function **（ Chinese sort ）**

It's also sort by calculation , Even if there are records of the same rank , I won't skip Next place .

example ） Yes 3 Record number one 1 When a ：1 position 、1 position 、1 position 、2 position ……

• ROW_NUMBER function

give The only consecutive bit .

example ） Yes 3 Record number one 1 When a ：1 position 、2 position 、3 position 、4 position

SELECT  product_name
       ,product_type
       ,sale_price
       ,rank() over(order by sale_price) as ranking
       ,dense_rank() over(order by sale_price) as dense_ranking
       ,row_number() over(order by sale_price) as row_num
       from product;

1.2.2  The use of aggregate functions on window functions

The use of aggregate function in windowing function is the same as the previous special window function , But the result is Cumulative Aggregate function value of .

SELECT  product_id
       ,product_name
       ,sale_price
       ,sum(sale_price) over(order by product_id) as current_sum
       ,avg(sale_price) over(order by product_id) as current_avg
       from product;

1.3  Application of window function - Calculate the moving average

PRECEDING（“ Before ”）, Specify the frame as “ By the end of n That's ok ”, Add your own line

FOLLOWING（“ after ”）, Specify the frame as “ Until after n That's ok ”, Add your own line

BETWEEN 1 PRECEDING AND 1 FOLLOWING, Specify the frame as “ Before 1 That's ok ” + “ after 1 That's ok ” + “ Oneself ”

< Window function > OVER (ORDER BY < Column names for sorting >
                 ROWS n PRECEDING )  
                 
< Window function > OVER (ORDER BY < Column names for sorting >
                 ROWS BETWEEN n PRECEDING AND n FOLLOWING)
SELECT  product_id
       ,product_name
       ,sale_price
       ,sum(sale_price) over(order by product_id rows 2 preceding ) as move_sum
       ,avg(sale_price) over(order by product_id rows between 1 preceding
       and 1 following) as move_avg
       from product;

1.4  Application scope and precautions of window function

• In principle, , Window functions can only be used in SELECT Used in clauses .
• Window function OVER Medium ORDER BY Clause does not affect the sorting of the final results . It is only used to determine the order in which window functions are evaluated .

 1.5 GROUPING Operator

1.5.1 ROLLUP - Calculate total and subtotal

The conventional GROUP BY You can only get the subtotal of each category , Sometimes you need to calculate Total of categories , It can be used ROLLUP keyword .

SELECT  product_type
       ,regist_date
       ,SUM(sale_price) AS sum_price
  FROM product
 GROUP BY product_type, regist_date WITH ROLLUP 

  Exercises

Please name the... Used in this chapter product（ goods ） The table is executed as follows SELECT The result of the statement .

SELECT  product_id
       ,product_name
       ,sale_price
       ,MAX(sale_price) OVER (ORDER BY product_id) AS Current_max_price
  FROM product

Get the maximum value of the scroll

  Continue to use product surface , Calculate according to Registration date （regist_date） Ascending The sales unit price of each date arranged （sale_price） Of Total . Sorting is required to set the registration date to NULL Of “ motion T T-shirt ” The record is at 1 position （ That is, think of it as earlier than other dates ）

SELECT  product_id
       ,product_name
       ,regist_date
       ,sale_price
       ,sum(sale_price) OVER (ORDER BY regist_date) AS sum_date_price
  FROM product;

Thinking questions

① The window function does not specify PARTITION BY What is the effect of ？

Don't specify , Make statistics from the first row to the current row ;

② Why is it that window functions can only be used in SELECT Used in clauses ？ actually , stay ORDER BY Clause using the system does not report an error .