MySQL根据SUM列别名计算2列之间的百分比差异（未更改）

Question

I have 2 table t1 where price and item type is stored with an ENUM value and t2 where the sold quantity is stored. I have a query to calculate the total sale (price * qty) but I need to calculate the difference between the 2 resulted numbers.

The calculation formula for difference between 2 numbers should be:

((N1 - N2) / ((N1 + N2) / 2)) * 100

Using the above formula I need to calculate the difference in percentage between N1 and N2. Both numbers are the result of 2 separate SUM cases based on 2 ENUM values in col_type (please see below)

SELECT
  CONCAT('£ ',SUM(CASE WHEN t1.col_type = 'N1' THEN t2.qty * t1.price ELSE 0 END)) AS 'Total N1',
  CONCAT('£ ',SUM(CASE WHEN t1.col_type = 'N2' THEN t2.qty * t1.price ELSE 0 END)) AS 'Total N2'            
FROM t2
  INNER JOIN t1
    ON t2.col_id = t1.col_id

The resulted output should be like:

+----------+----------+------------------------+
| Total N1 | Total N2 | Diff between N1 and N2 |
+----------+----------+------------------------+
|  3765.50 |  3246.15 | 14.81%                 |
+----------+----------+-----------------------

My question is how do I pass the values of N1 and N2 (already generated) to a 3rd column and calculate the difference between N1 and N2 using above formula?

Note: Please note that is percentage difference between 2 numbers and not the percentage change where (N2-N1) / N1 * 100 applies...

Thank you in advance for any reply...

Answer 1

You have a strange definition of "difference". For this, use a subquery:

SELECT CONCAT('£ ', n1) AS Total_N1,
       CONCAT('£ ', n2) AS Total_N2,
       100 * (N1 - N2) / (N1+N2) / 2 as diff 
FROM (SELECT SUM(CASE WHEN t1.col_type = 'N1' THEN t2.qty * t1.price ELSE 0 END) as n1,
             SUM(CASE WHEN t1.col_type = 'N2' THEN t2.qty * t1.price ELSE 0 END) as n2
      FROM t2 INNER JOIN
           t1
           ON t2.col_id = t1.col_id
      ) t