CODEWITHSUNDEEP
mysqldatabase
kcay King
kcay King

Reputation: 77

MYSQL Join 3 Tables and calculate cummulative percentage

I'm experiencing errors trying to calculate a cummulative group score from the table below:

group_details:
id      name
=====================
1       Group 1
2       Group 2
3       Group 3

group_members:
id      group_id  
======================
1       1           
2       1           
3       2       
4       2       
5       3     

answers:
id      member_id   is_correct      
=================================
1       1           1        
2       1           0      
3       2           1          
4       2           1         
5       3           1          
6       3           0
7       4           0          
8       4           1

I am trying to achieve this:

Group Name            Total Members     Total Score (%)
==============================================
Group 1                  2              75.00
----------------------------------------------
Group 2                  2              50.00
---------------------------------------------- 
Group 3                  1              0
----------------------------------------------

I get an empty result returned when I run the query. Kindly see my codes below.

 SELECT 
    ((SUM(a.is_correct) / (2 * SUM(m.id))) * 100)  as cummulative_score,
    SUM(m.id) as total_members,
    g.name
  FROM
    `group_details` AS g 
    LEFT JOIN `group_members` m 
      ON m.group_id = g.id
    LEFT JOIN `answers` a 
      ON a.member_id = m.id 
  WHERE a.is_correct = 1
  GROUP BY g.id;

Upvotes: 1

Views: 60

Answers (2)

DataVader
DataVader

Reputation: 780

Change "LEFT JOIN answer a" to "LEFT JOIN answers a" and it should work.

Also I recommend avoiding aliases because you don't do any good in renaming tables temporary into "a", "g" or something, it looks really confusing to everybody and it makes debugging/analyzing queries very hard. Also you should use a MySQL-client to highlight such errors to you like MySQL Workbench or something similar.

Upvotes: 0

Nick
Nick

Reputation: 147216

This query will give you the results that you want. Note that based on the table names in your question I have used answers as the answer table, if it's actually called answer you will need to change that JOIN.

SELECT 
    g.name,
    COUNT(DISTINCT m.id) AS `Total Members`,
    ROUND(COALESCE(AVG(a.is_correct), 0) * 100, 2) AS `Total Score (%)`
  FROM
    `group_details` AS g 
    LEFT JOIN `group_members` m 
      ON m.group_id = g.id
    LEFT JOIN `answers` a 
      ON a.member_id = m.id 
  GROUP BY g.id

Output:

name        Total Members   Total Score (%)     
Group 1     2               75.00
Group 2     2               50.00
Group 3     1               0.00

Upvotes: 1

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