汇总功能无法正常工作

Question

I was writing a query to find the median from a table. The table had a single column comprising consecutive natural numbers from 1 to 106.

CREATE TABLE med 
AS (SELECT rs 
    FROM ( SELECT rownum rs
           FROM employees
           WHERE employee_id<=106));

I successfully executed the query to find the median of this 106 natural numbers as 53.5

SELECT AVG(median)
FROM (
        SELECT a.rs median
        FROM med a,
             med b
        GROUP BY a.rs
        HAVING  SUM(CASE 
                        WHEN b.rs<=a.rs THEN 1
                        ELSE 0
                    END)>=(COUNT(*)/2)
        AND     SUM(CASE 
                        WHEN b.rs>=a.rs THEN 1
                        ELSE 0
                    END)>=(COUNT(*)/2));

But on using the aggregate function AVG without using subquery like this:

SELECT AVG(a.rs) median
        FROM med a,
             med b
        GROUP BY a.rs
        HAVING  SUM(CASE 
                        WHEN b.rs<=a.rs THEN 1
                        ELSE 0
                    END)>=(COUNT(*)/2)
        AND     SUM(CASE 
                        WHEN b.rs>=a.rs THEN 1
                        ELSE 0
                    END)>=(COUNT(*)/2)

then it doesn't give the expected output as 53.5 rather it gives output as 53, 54. why so?

Answer 1

That's because in your first query with subquery, you are calculating the average between the values 53 and 54.

On your second query without subquery the aggregation function is executed but it is performed on each value separatly, meaning AVG(53) and AVG(54). That's why you are getting 2 rows back.

I hope it's clear to you.