MySQL使用连接查询的方式查询（连接3个表，并使用COUNT内置函数）

首先创建数据库：

CREATE DATABASE mysql_shiyan;
use mysql_shiyan;

MySQL_create.sql免积分下载
创建表：

CREATE TABLE `project` (
  `proj_num` int(10) NOT NULL,
  `proj_name` char(20) NOT NULL,
  `start_date` date NOT NULL,
  `end_date` date DEFAULT "2015-04-01",
  `of_dpt` char(20) DEFAULT NULL,
  PRIMARY KEY (`proj_num`,`proj_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

CREATE TABLE `employee` (
  `id` int(10) NOT NULL,
  `name` char(20) DEFAULT NULL,
  `age` int(10) DEFAULT NULL,
  `salary` int(10) NOT NULL,
  `phone` int(12) NOT NULL,
  `in_dpt` char(20) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `phone` (`phone`),
  KEY `emp_fk` (`in_dpt`),
  CONSTRAINT `emp_fk` FOREIGN KEY (`in_dpt`) REFERENCES `department` (`dpt_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

CREATE TABLE `department` (
  `dpt_name` char(20) NOT NULL,
  `people_num` int(10) DEFAULT "10",
  PRIMARY KEY (`dpt_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

MySQL_insert.sql免积分下载
使用连接查询的方式，查询出各员工所在部门的人数与工程数，工程数命名为count_project。（连接3个表，并使用COUNT内置函数）

SELECT department.dpt_name AS department_name, COUNT(DISTINCT project.proj_name) AS "count_project", COUNT(DISTINCT employee.name) AS "count_people"
FROM department
LEFT JOIN project ON project.of_dpt=department.dpt_name
LEFT JOIN employee ON employee.in_dpt=department.dpt_name
GROUP BY department.dpt_name;