合并并对来自两个不同表的SQL值使用算术运算

Question

1. I have a table with data of guests to an event. I can retrieve the number of people who have attended ( membersAttended ) with this query:

    SELECT COUNT(DISTINCT g.`name1`) AS `membersAttended` FROM `tmc_doorapp_guests` g LEFT JOIN `tmc_doorapp_events` e ON e.`id` = g.`event_id` WHERE `name1` REGEXP '^[0-9]+$' AND e.`date` > DATE_SUB(NOW(), INTERVAL 30 DAY) AND g.`checkin` = 1; 

2. To retrieve the total number of members ( totalMembers ) I use:

    SELECT COUNT(DISTINCT `id`) AS `totalMembers` FROM `tmc_users`; 

3. To get a percentage value ( totalPercent ) of members who have attended across the total members, I use PHP to make the two queries and then calculate like so:

    $totalPercent = number_format(($membersAttended/$totalMembers*100),2); 

---

I've tried combining the two queries and the percentage calculation in a similar way to below, with no luck:

    SELECT COUNT(DISTINCT g.`name1`) AS `membersAttended`,
        COUNT(DISTINCT m.`id`) AS `totalMembers`,
        (`membersAttended` * 100 / `totalMembers`) AS `totalPercent`
        FROM `tmc_doorapp_guests` g, `tmc_users` m
        LEFT JOIN `tmc_doorapp_events` e
        ON e.`id` = g.`event_id`
        WHERE `name1` REGEXP '^[0-9]+$'
        AND e.`date` > DATE_SUB(NOW(), INTERVAL 30 DAY)
        AND g.`checkin` = 1;

I also tried expanding out the columns:

    SELECT COUNT(DISTINCT g.`name1`) AS `membersAttended`,
        COUNT(DISTINCT m.`id`) AS `totalMembers`,
        (COUNT(DISTINCT g.`name1`) * 100 / COUNT(DISTINCT m.`id`)) AS `totalPercent`
        FROM `tmc_doorapp_guests` g, `tmc_users` m
        LEFT JOIN `tmc_doorapp_events` e
        ON e.`id` = g.`event_id`
        WHERE `name1` REGEXP '^[0-9]+$'
        AND e.`date` > DATE_SUB(NOW(), INTERVAL 30 DAY)
        AND g.`checkin` = 1;

This gives me the error:

Error code: 1054. Unknown column 'g.event_id' in 'on clause'

How can I combine these SQL/PHP queries/calculations into one SQL query?

Answer 1

You could use subquery:

SELECT COUNT(DISTINCT g.`name1`) AS `membersAttended`,
       (SELECT COUNT(DISTINCT `id`) FROM `tmc_users`) AS `totalMembers`,
       COUNT(DISTINCT g.`name1`) * 100 / (SELECT COUNT(DISTINCT `id`)
                                          FROM `tmc_users`) AS `totalPercent`
FROM `tmc_doorapp_guests` g
LEFT JOIN `tmc_doorapp_events` e
ON e.`id` = g.`event_id`
WHERE `name1` REGEXP '^[0-9]+$'
AND e.`date` > DATE_SUB(NOW(), INTERVAL 30 DAY)
AND g.`checkin` = 1;