SQL Query between three tables - which joins?

Question

I'm trying to create a query in mySQL to select data from pre-existing tables on a database (Moodle to be specific). I realise that the scheme isn't great, but this has come from the Moodle database with the 'ratings' plugin installed.

The current data structure looks like this:

mdl_ranking_points

CREATE TABLE `mdl_ranking_points` (
  `id` bigint(10) NOT NULL AUTO_INCREMENT,
  `userid` bigint(10) NOT NULL,
  `courseid` bigint(10) NOT NULL,
  `points` decimal(10,1) NOT NULL,
  `timecreated` bigint(10) NOT NULL,
  `timemodified` bigint(10) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8 COMMENT='Points of users'

mdl_ranking_logs

 CREATE TABLE `mdl_ranking_logs` (
      `id` bigint(10) NOT NULL AUTO_INCREMENT,
      `rankingid` bigint(10) NOT NULL,
      `courseid` bigint(10) NOT NULL,
      `course_modules_completion` bigint(10) NOT NULL,
      `points` decimal(10,1) NOT NULL,
      `timecreated` bigint(10) NOT NULL,
      PRIMARY KEY (`id`)
    ) ENGINE=InnoDB AUTO_INCREMENT=23 DEFAULT CHARSET=utf8 COMMENT='Points of users'

mdl_user

 CREATE TABLE `mdl_user` (
      `id` bigint(10) NOT NULL AUTO_INCREMENT,
      `auth` varchar(20) NOT NULL DEFAULT 'manual',
      `confirmed` tinyint(1) NOT NULL DEFAULT '0',
      `username` varchar(100) NOT NULL DEFAULT '',
      `password` varchar(255) NOT NULL DEFAULT '',
      `idnumber` varchar(255) NOT NULL DEFAULT '',
      `firstname` varchar(100) NOT NULL DEFAULT '',
      `lastname` varchar(100) NOT NULL DEFAULT '',
      `email` varchar(100) NOT NULL DEFAULT '',
      PRIMARY KEY (`id`),
      UNIQUE KEY `mdl_user_mneuse_uix` (`mnethostid`,`username`),
      KEY `mdl_user_fir_ix` (`firstname`),
      KEY `mdl_user_las_ix` (`lastname`),
      KEY `mdl_user_idn_ix` (`idnumber`),
    ) ENGINE=InnoDB AUTO_INCREMENT=1045 DEFAULT CHARSET=utf8 COMMENT='One record for each person'

mdl_course

CREATE TABLE `mdl_course` (
  `id` bigint(10) NOT NULL AUTO_INCREMENT,
  `category` bigint(10) NOT NULL DEFAULT '0',
  `sortorder` bigint(10) NOT NULL DEFAULT '0',
  `fullname` varchar(254) NOT NULL DEFAULT '',
  `shortname` varchar(255) NOT NULL DEFAULT '',
  `idnumber` varchar(100) NOT NULL DEFAULT '',
  `summary` longtext,
  `summaryformat` tinyint(2) NOT NULL DEFAULT '0',
  `format` varchar(21) NOT NULL DEFAULT 'topics',
  `showgrades` tinyint(2) NOT NULL DEFAULT '1',
  `newsitems` mediumint(5) NOT NULL DEFAULT '1',
  `startdate` bigint(10) NOT NULL DEFAULT '0',
  `enddate` bigint(10) NOT NULL DEFAULT '0',
  `marker` bigint(10) NOT NULL DEFAULT '0',
  `maxbytes` bigint(10) NOT NULL DEFAULT '0',
  `legacyfiles` smallint(4) NOT NULL DEFAULT '0',
  `showreports` smallint(4) NOT NULL DEFAULT '0',
  `visible` tinyint(1) NOT NULL DEFAULT '1',
  `visibleold` tinyint(1) NOT NULL DEFAULT '1',
  `groupmode` smallint(4) NOT NULL DEFAULT '0',
  `groupmodeforce` smallint(4) NOT NULL DEFAULT '0',
  `defaultgroupingid` bigint(10) NOT NULL DEFAULT '0',
  `lang` varchar(30) NOT NULL DEFAULT '',
  `theme` varchar(50) NOT NULL DEFAULT '',
  `timecreated` bigint(10) NOT NULL DEFAULT '0',
  `timemodified` bigint(10) NOT NULL DEFAULT '0',
  `requested` tinyint(1) NOT NULL DEFAULT '0',
  `enablecompletion` tinyint(1) NOT NULL DEFAULT '0',
  `completionnotify` tinyint(1) NOT NULL DEFAULT '0',
  `cacherev` bigint(10) NOT NULL DEFAULT '0',
  `calendartype` varchar(30) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`),
  KEY `mdl_cour_cat_ix` (`category`),
  KEY `mdl_cour_idn_ix` (`idnumber`),
  KEY `mdl_cour_sho_ix` (`shortname`),
  KEY `mdl_cour_sor_ix` (`sortorder`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8 COMMENT='Central course table';

I need to return a query with the following data:

lastname | points | average

I've tried the following query, but this doesnt return what I need (it seems that it only counts user id's that have been awarded points.

SELECT lastname AS academy,
    SUM(points) AS score,
    COUNT(*) AS users,
    (SUM(points)/COUNT(*)) AS norm
FROM mdl_ranking_points r
LEFT JOIN mdl_user ON r.userid = mdl_user.id
LEFT JOIN mdl_course ON r.courseid = mdl_course.id
GROUP BY lastname
ORDER BY norm DESC

Any help would be much appreciated. I may be approaching this completely the wrong way.

Answer 1

I've tried the following query, but this doesnt return what I need (it seems that it only counts user id's that have been awarded points.

This is because of the order in the joins. This will return all users even if they don't have rewarded some points

FROM mdl_user
    LEFT OUTER JOIN mdl_ranking_points
    LEFT OUTER JOIN mdl_course

Or change your query with a right outer join.