mysql - SQL - 获取地球上人们的认证总数(3 个单独的表)

Question

老实说，我不知道如何解决这个问题。我已经SELECT name了，然后我就迷路了(有点尴尬)。

问题:找出按星球分组的人们持有的认证数量。这应该有两列，第一列，“名称”将是至少具有一个认证的行星的名称。第二列应该是“CertCount”，并且将是来自该星球的人持有的认证数量，例如，如果 Lee 获得了“Viper”和“Mechanic”认证，而 Kara 获得了“Viper”认证，并且他们都来自 Caprica，那么 caprica 的“CertCount”应该是 3:

CREATE TABLE `bsg_cert` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB

CREATE TABLE `bsg_cert_people` (
  `cid` int(11) NOT NULL DEFAULT '0',
  `pid` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`cid`,`pid`),
  KEY `pid` (`pid`),
  CONSTRAINT `bsg_cert_people_ibfk_1` FOREIGN KEY (`cid`) REFERENCES `bsg_cert` (`id`),
  CONSTRAINT `bsg_cert_people_ibfk_2` FOREIGN KEY (`pid`) REFERENCES `bsg_people` (`id`)
) ENGINE=InnoDB

CREATE TABLE `bsg_people` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `fname` varchar(255) NOT NULL,
  `lname` varchar(255) DEFAULT NULL,
  `homeworld` int(11) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `homeworld` (`homeworld`),
  CONSTRAINT `bsg_people_ibfk_1` FOREIGN KEY (`homeworld`) REFERENCES `bsg_planets` (`id`) ON DELETE SET NULL ON UPDATE CASCADE
) ENGINE=InnoDB

CREATE TABLE `bsg_planets` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  `population` bigint(20) DEFAULT NULL,
  `language` varchar(255) DEFAULT NULL,
  `capital` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB

Answer 1

将它们全部加入并使用 group by 进行计数应该可以解决问题:

SELECT  planet.name ,
    COUNT(*) AS cert_count
FROM    bsg_cert_people people_cert
    JOIN bsg_people people ON people.id = people_cert.pid
    JOIN bsg_planet planet ON people.homeworld = planet.id
GROUP BY planet.name