MySQL 8.0 introduces window functions to improve SQL analysis capabilities. 1. RANK and ROW_NUMBER are used for rankings. The former handles the sorting rankings, and the latter forces unique numbers; 2. SUM and AVG support rolling calculations to achieve cumulative and moving averages; 3. FIRST_VALUE and LAST_VALUE extract window head and tail data, and the default range needs to be adjusted to obtain complete results. These functions retain the original row structure and simplify complex queries.
MySQL 8.0 introduces window functions, which is a big leap in SQL query capabilities. If you could only use GROUP BY for aggregation before, now you have a more flexible way to complete complex analysis without disrupting the original row structure.
The following common scenarios and specific functions can help you quickly get started with this function.
RANK and ROW_NUMBER: A weapon for ranking issues
This type of function is suitable for rankings, sales rankings, and grade rankings.
The difference is that RANK() will skip the rank of the lane (such as two firsts followed by the third), while ROW_NUMBER() is the mandatory number unique, regardless of whether it is parallel or not.
For example:
SELECT name, score,
RANK() OVER (ORDER BY score DESC) AS rank_score,
ROW_NUMBER() OVER (ORDER BY score DESC) AS row_num
FROM students; This way of writing clearly shows who is the first place and who is squeezed out of the position by the side. If you want to count the rankings by class, just add a PARTITION BY class_id .
SUM and AVG as rolling calculation tools
You may have encountered a demand for displaying sales per month and also displaying the cumulative total as of the current month. At this time, ordinary SUM is not enough, but window functions can be easily implemented.
SELECT month, sales,
SUM(sales) OVER (ORDER BY month ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS cumulative_sales
FROM monthly_sales; The above paragraph means that each line counts the sum from the beginning to the month. Similarly, you can use AVG() to see the moving average.
This trick is especially useful for time series data, such as viewing trend changes.
FIRST_VALUE and LAST_VALUE: Extract key node data
These two functions are used to get the first or last value within the window range. For example, you want to know the highest sales product in each quarter, and the performance comparison of the first month of the quarter.
Example:
SELECT product_id, month, sales,
FIRST_VALUE(sales) OVER (PARTITION BY quarter ORDER BY month) AS first_month_sales,
LAST_VALUE(sales) OVER (PARTITION BY quarter ORDER BY month RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS last_month_sales
FROM quarterly_sales; Note that the default window range is RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW , so to get the last value, you must explicitly specify the range.
Basically that's it. Window functions are not a show-off, but a good tool to solve practical problems. When you first come across it, you may feel that the syntax is a bit confusing, but once you master it, you will find that many originally complex subqueries or temporary tables can be simplified.
The above is the detailed content of Exploring Window Functions Available in MySQL 8. For more information, please follow other related articles on the PHP Chinese website!
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