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;