Displayed values are not what they should be - mysql

Question

There are 2 tables ost_ticket and ost_ticket_action_history.
create table ost_ticket(
ticket_id int not null PRIMARY KEY,
created timestamp,
staff bool,
status varchar(50),
city_id int
);
create table ost_ticket_action_history(
ticket_id int not null,
action_id int not null PRIMARY KEY,
action_name varchar(50),
started timestamp,
FOREIGN KEY(ticket_id) REFERENCES ost_ticket(ticket_id)
);
In the ost_ticket_action_history table the data is:
INSERT INTO newdb.ost_ticket_action_history (ticket_id, action_id, action_name, started) VALUES (1, 1, 'Consultation', '2022-01-06 18:30:29');
INSERT INTO newdb.ost_ticket_action_history (ticket_id, action_id, action_name, started) VALUES (2, 2, 'Bank Application', '2022-02-06 18:30:45');
INSERT INTO newdb.ost_ticket_action_history (ticket_id, action_id, action_name, started) VALUES (3, 3, 'Consultation', '2022-05-06 18:42:48');
In the ost_ticket table the data is:
INSERT INTO newdb.ost_ticket (ticket_id, created, staff, status, city_id) VALUES (1, '2022-04-04 18:26:41', 1, 'open', 2);
INSERT INTO newdb.ost_ticket (ticket_id, created, staff, status, city_id) VALUES (2, '2022-05-05 18:30:48', 0, 'open', 3);
INSERT INTO newdb.ost_ticket (ticket_id, created, staff, status, city_id) VALUES (3, '2022-04-06 18:42:53', 1, 'open', 4);
My task is to get the conversion from the “Consultation” stage to the “Bank Application” stage broken down by months (based on the start date of the “Bank Application” stage).Conversion is calculated according to the following formula: (number of applications with the “Bank Application” stage / number of applications with the “Consultation” stage) * 100%.
My request is like this:
select SUM(action_name='Bank Application')/SUM(action_name='Consultation') * 2 as 'Conversion' from ost_ticket_action_history JOIN ost_ticket ot on ot.ticket_id = ost_ticket_action_history.ticket_id where status = 'open' and created > '2020 -01-01 00:00:00' group by action_name,started having action_name = 'Bank Application';
As a result I get:
Another query:
SELECT
SUM(CASE
WHEN b.ticket_id IS NOT NULL THEN 1
ELSE 0
END) / COUNT(*) conversion,
YEAR(a.started) AS 'year',
MONTH(a.started) AS 'month'
FROM
ost_ticket_action_history a
LEFT JOIN
ost_ticket_action_history b ON a.ticket_id = b.ticket_id
AND b.action_name = 'Bank Application'
WHERE
a.action_name = 'Consultation'
AND a.status = 'open'
AND a.created > '2020-01-01 00:00:00'
GROUP BY YEAR(a.started) , MONTH(a.started)
I apologize if I didn't write very clearly. Please explain what to do.

Answer 1

Like I explained in my comment, you exclude rows with your having clause.
I will show you in the next how to debug.
First check what the raw result of the select query is.
As you see, when you remove the GROUP BY and see what you actually get is only 1 row with bank application, because the having clause excludes all other rows
SELECT
*
FROM
ost_ticket_action_history
JOIN
ost_ticket ot ON ot.ticket_id = ost_ticket_action_history.ticket_id
WHERE
status = 'open'
AND created > '2020-01-01 00:00:00'
GROUP BY
action_name, started
HAVING
action_name = 'Bank Application';
Output:
ticket_id
action_id
action_name
started
ticket_id
created
staff
status
city_id
2
2
Bank Application
2022-02-06 18:30:45
2
2022-05-05 18:30:48
0
open
3
Second step, see what the result set is without calculating anything.
As you can see you make a division with 0, what you have learned in school, is forbidden, hat is why you have as result set NULL
SELECT
SUM(action_name = 'Bank Application')
#/
,SUM(action_name = 'Consultation') * 2 AS 'Conversion'
FROM
ost_ticket_action_history
JOIN
ost_ticket ot ON ot.ticket_id = ost_ticket_action_history.ticket_id
WHERE
status = 'open'
AND created > '2020-01-01 00:00:00'
GROUP BY action_name , started
HAVING action_name = 'Bank Application';
SUM(action_name = 'Bank Application') | Conversion
------------------------------------: | ---------:
1 | 0
db<>fiddle here
#Third what you can do exclude a division with 0, here i didn't remove all othe rows as this is only for emphasis
SELECT
SUM(action_name = 'Bank Application')
/
SUM(action_name = 'Consultation') * 2 AS 'Conversion'
FROM
ost_ticket_action_history
JOIN
ost_ticket ot ON ot.ticket_id = ost_ticket_action_history.ticket_id
WHERE
status = 'open'
AND created > '2020-01-01 00:00:00'
GROUP BY action_name , started
HAVING SUM(action_name = 'Consultation') > 0;
| Conversion |
| ---------: |
| 0.0000 |
| 0.0000 |
db<>fiddle here
Final words,
If you get a strange result, simply go back remove everything that doesn't matter and try to get all values, so hat you can check your math