There is no unified built-in function to calculate the median of a column in SQL, but it can be implemented by the following methods: 1. Use window functions to calculate manually, which is suitable for PostgreSQL, MySQL 8.0, etc., by sorting, numbering, counting the total number of rows and taking the intermediate value or average; 2. Use user variables to simulate the sorting process in MySQL 5.x to obtain the median; 3. If the database supports such as Oracle or some SQLite versions, you can directly use the MEDIAN() function, but you need to pay attention to performance issues. The selection method depends on the database type and version.
Finding the median number of a column in SQL is actually not a simple matter, because standard SQL does not have a built-in MEDIAN() function. However, when actually analyzing data, the median reflects the centralized trend better than the average, especially when the data distribution is skewed.
The following are several commonly used methods that are suitable for different database systems.
Use window functions to calculate medians manually
This is the most common and common practice, suitable for most databases that support window functions (such as PostgreSQL, MySQL 8.0, SQL Server, etc.).
The basic idea is:
- Sort by value and assign each row a sequence number
- Find the value of the middle position, or the average of the two middle values ??(even-numbered cases)
The example statements are as follows:
WITH ordered_values ??AS ( SELECT value, ROW_NUMBER() OVER (ORDER BY value) AS rn, COUNT(*) OVER () AS total FROM your_table ) SELECT AVG(value) AS median FROM ordered_values WHERE rn IN ((total 1) / 2, (total 2) / 2);
This query does a few things:
- Sort by
valuefirst and number - Total records are counted simultaneously
- Finally, select one or two values ??in the middle to average
Note: Different databases may handle integer division differently, and expressions need to be adjusted according to the specific system.
Using user variables in MySQL (for older versions)
If you are using MySQL 5.x, which does not support window functions, you can use variables to simulate the sorting process.
The general steps are as follows:
- Initialize a variable to count
- Add variables while sorting by column
- Finally, filter out the rows in the middle
example:
SELECT AVG(value) AS median
FROM (
SELECT value,
@rownum := @rownum 1 AS row_number,
@total_rows := @rownum
FROM your_table, (SELECT @rownum := 0) AS r
ORDER BY value
) AS x
WHERE x.row_number IN (FLOOR((@total_rows 1)/2), CEIL((@total_rows 1)/2));Although this method is a bit confusing, it is very practical in the older version of MySQL.
What if the database comes with its own MEDIAN function?
Some databases such as Oracle and certain versions of SQLite support MEDIAN() , and can be written directly:
SELECT MEDIAN(value) FROM your_table;
But be aware that even with this function, there may be performance problems, especially when facing large tables.
In general, there is no unified and shortcut way to find medians in SQL, and in most cases it needs to be implemented manually. The above methods can be used in different scenarios, the key is to see which syntax your database type and version support. Basically that's it.
The above is the detailed content of How to find the median of a column in SQL. For more information, please follow other related articles on the PHP Chinese website!
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