SQL 复习笔记1查询

知识久了不用就会忘记，今早起床突然发现自己连最基本的sql语句都不会写了。

复习了半天，好记性不如烂键盘，做个复习笔记。

SQL 命令动词：

       数据定义：CREATE、DROP、ALTER

       数据操作：SELECT、INSERT、UPDATE、DELTE

       数据控制：GRANT、REVOKE 

-- Table "student" DDL
CREATE TABLE `student` (
  `id` int(11) NOT NULL,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- Table "courses" DDL
CREATE TABLE `courses` (
  `id` int(11) NOT NULL,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- Table "student_course" DDL
CREATE TABLE `student_course` (
  `sid` int(11) NOT NULL,
  `cid` int(11) NOT NULL,
  PRIMARY KEY (`cid`,`sid`),
  KEY `FKB0A3729FDBF3B7BF` (`sid`),
  KEY `FKB0A3729FE343F963` (`cid`),
  CONSTRAINT `FKB0A3729FDBF3B7BF` FOREIGN KEY (`sid`) REFERENCES `student` (`id`),
  CONSTRAINT `FKB0A3729FE343F963` FOREIGN KEY (`cid`) REFERENCES `courses` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- Table "department" DDL
CREATE TABLE `department` (
  `id` int(11) NOT NULL,
  `name` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


-- Table "employee" DDL
CREATE TABLE `employee` (
  `id` int(11) NOT NULL,
  `name` varchar(10) NOT NULL,
  `id_department` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `emp_id__dep_id` (`id_department`),
  CONSTRAINT `emp_id__dep_id` FOREIGN KEY (`id_department`) REFERENCES `department` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


#1统计每个部门的员工数
select d.* ,e.num from department d 

	left join (
		select id_department,count(*) num from employee group by id_department			
		)e on e.id_department= d.id
		
#2统计选修了某一门课程的学生人数（结果集中不包含课程名）
select cid,count(sid) from student_course group by cid

#3统计选修了某一门课程的学生人数（结果集中包含课程id 、课程名、选修人数）
select c.id ,c.name , num as "选修人数" from courses c
	left join (
		select cid,count(*) num from student_course group by cid
)sc on c.id=sc.cid

#4统计选修课程数超过三门的学生学号
select sid from student_course  group by sid  having count(*) >3

#5统计选修课超过三门课的学生（学号、姓名）
select id,name,sc.num as "选修课程数量" from student s 
	left join (
		select sid,count(sid) num from student_course  group by sid  having count(*) >3
	)sc  on s.id=sc.sid  
 	 where sc.num>3
	 
		
#6统计选修了某门课程的学生人数
select  count(* ) as "选修了课程5的人数" from student_course where cid=5 

#7求学生总数
select count(*) from 学生

#8求选修了课程的学生人数(distinct 去除重复的sid)
select count(distinct sid) from student_course