Question

I’d like to find the sum of a column in a single query given joins between multiple tables.

I have a table of Activities, and a table that maps Users performing an Activity, as well as a table mapping Teams to performed Activities. Both Users and Teams can perform the same activity multiple times.

Each activity is worth a set number of points, and I’d like to know the total number of points for a given user by totalling their activities with their team’s activities.

I’ve tried various combinations of joins between the three tables, but cannot work out the correct query to total the points for a given user.

The following SQL will create a minimal version of this setup:

CREATE TABLE `activity` (
  `id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(255) NOT NULL DEFAULT '',
  `points` INT(10) UNSIGNED NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=INNODB;

CREATE TABLE `team_action` (
  `id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
  `activity_id` INT(11) UNSIGNED NOT NULL,
  `date` DATETIME NOT NULL,
  PRIMARY KEY (`id`),
  CONSTRAINT `team_action_ibfk_1` FOREIGN KEY (`activity_id`) REFERENCES `activity` (`id`)
) ENGINE=INNODB;

CREATE TABLE `user_action` (
  `id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
  `activity_id` INT(11) UNSIGNED NOT NULL,
  `date` DATETIME NOT NULL,
  PRIMARY KEY (`id`),
  CONSTRAINT `user_action_ibfk_1` FOREIGN KEY (`activity_id`) REFERENCES `activity` (`id`)
) ENGINE=INNODB;

INSERT INTO `activity` (`id`, `name`, `points`)
VALUES (1,'Running',10), (2,'Swimming',20), (3,'Hiking',30), (4,'Cycling',40);

INSERT INTO `team_action` (`id`, `activity_id`, `date`)
VALUES (1,2,'2012-05-22 14:32:31'), (2,4,'2012-05-22 14:32:36');

INSERT INTO `user_action` (`id`, `activity_id`, `date`)
VALUES (1,1,'2012-05-22 14:32:08'), (2,1,'2012-05-22 14:32:18'), (3,3,'2012-05-22 14:32:23');

Answer 1

It is not clear from the table definitions how users are related to teams (i.e. for a user, how do you know which is “their” team?) But I think the key to summing the points will be to use SUM on the result of UNION ALL in a subquery.

Something along the lines of:

SELECT SUM(points) AS total
FROM
(SELECT points
 FROM team_action JOIN activity ON(activity.id = team_action.activity_id)
 WHERE team_action.id = my_team
 UNION ALL
 SELECT points
 FROM user_action JOIN activity ON(activity.id = user_action.activity_id)
 WHERE user_action.id = my_user) me_and_team