一、介绍
本节主题：
- 多表连接查询
- 复合条件连接查询
- 子查询
准备表：
#建表 create table department( id int, name varchar(20) ); create table employee( id int primary key auto_increment, name varchar(20), sex enum('male','female') not null default 'male', age int, dep_id int ); #插入数据 insert into department values (200,'技术'), (201,'人力资源'), (202,'销售'), (203,'运营'); insert into employee(name,sex,age,dep_id) values ('egon','male',18,200), ('alex','female',48,201), ('wupeiqi','male',38,201), ('yuanhao','female',28,202), ('liwenzhou','male',18,200), ('jingliyang','female',18,204) ; #查看表结构和数据 mysql> desc department; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ mysql> desc employee; +--------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | sex | enum('male','female') | NO | | male | | | age | int(11) | YES | | NULL | | | dep_id | int(11) | YES | | NULL | | +--------+-----------------------+------+-----+---------+----------------+ mysql> select * from department; +------+--------------+ | id | name | +------+--------------+ | 200 | 技术 | | 201 | 人力资源 | | 202 | 销售 | | 203 | 运营 | +------+--------------+ mysql> select * from employee; +----+------------+--------+------+--------+ | id | name | sex | age | dep_id | +----+------------+--------+------+--------+ | 1 | egon | male | 18 | 200 | | 2 | alex | female | 48 | 201 | | 3 | wupeiqi | male | 38 | 201 | | 4 | yuanhao | female | 28 | 202 | | 5 | liwenzhou | male | 18 | 200 | | 6 | jingliyang | female | 18 | 204 | +----+------------+--------+------+--------+
需要注意的是在employee表有一个dep_id=204是department表没有的。同时，department表有一个id=203在employee表没有对应的记录。
二、多表连接查询
#重点：外链接语法 SELECT 字段列表 FROM 表1 INNER|LEFT|RIGHT JOIN 表2 ON 表1.字段 = 表2.字段;
1、交叉连接：不适用任何匹配条件，生成笛卡尔积
mysql> select * from employee, department; +----+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+------------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 1 | egon | male | 18 | 200 | 201 | 人力资源 | | 1 | egon | male | 18 | 200 | 202 | 销售 | | 1 | egon | male | 18 | 200 | 203 | 运营 | | 2 | alex | female | 48 | 201 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 2 | alex | female | 48 | 201 | 202 | 销售 | | 2 | alex | female | 48 | 201 | 203 | 运营 | | 3 | wupeiqi | male | 38 | 201 | 200 | 技术 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 202 | 销售 | | 3 | wupeiqi | male | 38 | 201 | 203 | 运营 | | 4 | yuanhao | female | 28 | 202 | 200 | 技术 | | 4 | yuanhao | female | 28 | 202 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 4 | yuanhao | female | 28 | 202 | 203 | 运营 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | | 5 | liwenzhou | male | 18 | 200 | 201 | 人力资源 | | 5 | liwenzhou | male | 18 | 200 | 202 | 销售 | | 5 | liwenzhou | male | 18 | 200 | 203 | 运营 | | 6 | jingliyang | female | 18 | 204 | 200 | 技术 | | 6 | jingliyang | female | 18 | 204 | 201 | 人力资源 | | 6 | jingliyang | female | 18 | 204 | 202 | 销售 | | 6 | jingliyang | female | 18 | 204 | 203 | 运营 | +----+------------+--------+------+--------+------+--------------+ 24 rows in set (0.01 sec)
每个员工和四个部门做一次对应关系。但这四次匹配仅有一次是需要的：即dep_id和id相同的那个。
mysql> select * from employee, department where employee.dep_id = department.id; +----+-----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | +----+-----------+--------+------+--------+------+--------------+ 5 rows in set (0.00 sec)
通过对笛卡尔积的再处理，得出了两张表有对应关系的部分。但是两张表上，对方没有对上的记录并没有显示出来。因此需要使用mysql给的专用处理工具来处理。
2、内连接：只连接匹配的行
即取两张表共同部分，相当于上面利用条件从笛卡尔积结果中筛选出正确结果。
#department没有204这个部门，因而employee表中关于204这条员工信息没有匹配出来 mysql> select * from employee inner join department on employee.dep_id = department.id; +----+-----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | +----+-----------+--------+------+--------+------+--------------+ 5 rows in set (0.00 sec) # 与上面笛卡尔where筛选的执行结果完全一样
3、外链接之左连接：在内连接的基础上保留左表的记录
在内连接的基础上保留左表有但右边没有的记录——以左表为准，找出所有员工信息，包括没有部门的员工。
mysql> select * from employee left join department on employee.dep_id = department.id; +----+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+------------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 6 | jingliyang | female | 18 | 204 | NULL | NULL | +----+------------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec)
没有对应的记录也显示出来了，并用NULL显示。
4、外链接之右连接：在内连接的基础上保留右表的记录
在内连接的基础上增加右边有左边没有的结果——以右表为准，找出所有部门信息，包括没有员工的部门。
mysql> select * from employee right join department on employee.dep_id = department.id; +------+-----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+-----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | | NULL | NULL | NULL | NULL | NULL | 203 | 运营 | +------+-----------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec)
由于employee表没有运营部门员工信息，以NULL显示。
5、全外连接：在内连接的基础上保留，左右两个表没有对应关系的记录
在内连接的基础上增加左边有右边没有和右边有左边没有的结果——显示全部记录
注意：mysql不支持full join
mysql> select * from employee full join department on employee.dep_id = department.id; ERROR 1054 (42S22): Unknown column 'employee.dep_id' in 'on clause'
mysql使用union来间接实现全外连接
mysql> select * from employee left join department on employee.dep_id = department.id -> union -> select * from employee right join department on employee.dep_id = department.id; +------+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+------------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 6 | jingliyang | female | 18 | 204 | NULL | NULL | | NULL | NULL | NULL | NULL | NULL | 203 | 运营 | +------+------------+--------+------+--------+------+--------------+ 7 rows in set (0.01 sec)
注意union与union all的区别：union会去掉相同的记录。
三、符合条件连接查询
要理解连接查询，可以以下面这个案例为例来理解：
查询出员工平均年龄大于30的部门？
1、（判断是否要连表）通过分析要求可以得出与员工表、部门表都有关系，因此需要做多表连接查询。
2、（判断使用哪种连接方式）进一步分析得出需要查询的就是员工和部门有关系的部门，因此应该使用内连接。
mysql> select * from employee inner join department on employee.dep_id = department.id; +----+-----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | +----+-----------+--------+------+--------+------+--------------+ 5 rows in set (0.00 sec)
上面就得到了一张虚拟表。
虚拟表——就是不在硬盘上，存放在内存中的一张临时表。但可以在虚拟表的基础上进行查询筛选等操作。
mysql> select department.name, avg(age) from employee inner join department on employee.dep_id = department.id -> group by department.name # 按名称分组，方便取部门名，由于name有两个，因此需要指名道姓 -> having avg(age) > 30; +--------------+----------+ | name | avg(age) | +--------------+----------+ | 人力资源 | 43.0000 | +--------------+----------+ 1 row in set (0.01 sec)
多表查询的概念就是把有关系的表通过连接的方式拼为一个整体，进而来进行相应的关联查询。
#示例1：以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,即找出年龄大于25岁的员工以及员工所在的部门 select employee.name,department.name from employee inner join department on employee.dep_id = department.id where age > 25; #示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显示 select employee.id,employee.name,employee.age,department.name from employee,department where employee.dep_id = department.id and age > 25 order by age asc;
四、子查询
子查询概念：子查询是将一个查询语句嵌套在另一个查询语句中。
内层查询语句的查询结果，可以为外层查询语句提供查询条件。
子查询关键字：子查询可以包含 IN 、NOT 、IN 、ANY 、ALL EXISTS 和 NOT EXISTS 等关键字。
子查询还可以包含比较运算符：= 、!= 、> 、<等
1、带IN关键字的子查询
#1、查询平均年龄在25岁以上的部门名 # 分拆问题，先找到平均年龄在25岁以上的人的部门id mysql> select dep_id from employee -> group by dep_id -> having avg(age) > 25; +--------+ | dep_id | +--------+ | 201 | | 202 | +--------+ 2 rows in set (0.01 sec) mysql> select name from department where id in -> (select dep_id from employee -> group by dep_id -> having avg(age) > 25); +--------------+ | name | +--------------+ | 人力资源 | | 销售 | +--------------+ 2 rows in set (0.00 sec) #2、查看技术部门员工姓名 mysql> select id from department where name='技术'; +------+ | id | +------+ | 200 | +------+ 1 row in set (0.00 sec) mysql> select * from employee where dep_id=( -> select id from department where name='技术' -> ); +----+-----------+------+------+--------+ | id | name | sex | age | dep_id | +----+-----------+------+------+--------+ | 1 | egon | male | 18 | 200 | | 5 | liwenzhou | male | 18 | 200 | +----+-----------+------+------+--------+ 2 rows in set (0.00 sec) #3、查看不足1人的部门名：也就是没有人的部门 # 先找到有人的部门id，然后在部门表中取反 mysql> select distinct dep_id from employee; +--------+ | dep_id | +--------+ | 200 | | 201 | | 202 | | 204 | +--------+ 4 rows in set (0.00 sec) mysql> select name from department where id not in ( -> select distinct dep_id from employee -> ); +--------+ | name | +--------+ | 运营 | +--------+ 1 row in set (0.00 sec) mysql> select * from department where id not in ( -> select distinct dep_id from employee -> ); +------+--------+ | id | name | +------+--------+ | 203 | 运营 | +------+--------+ 1 row in set (0.01 sec)
2、带比较运算符的子查询
比较运算符：=、!=、>、>=、<、<=、<>
# 查询大于所有人平均年龄的员工名与年龄 mysql> select name,age from employee where age > (select avg(age) from employee); +---------+------+ | name | age | +---------+------+ | alex | 48 | | wupeiqi | 38 | +---------+------+ 2 rows in set (0.00 sec) #查询大于部门内平均年龄的员工名、年龄 mysql> select t1.name, t1.age from employee t1 -> inner join -> (select dep_id, avg(age) avg_age from employee group by dep_id) t2 -> on t1.dep_id = t2.dep_id -> where t1.age > t2.avg_age; +------+------+ | name | age | +------+------+ | alex | 48 | +------+------+ 1 row in set (0.00 sec)
3、带EXISTS关键字的子查询
EXISTS关字键字表示存在。在使用EXISTS关键字时，内层查询语句不返回查询的记录。而是返回一个真假值：True或False
当返回True时，外层查询语句将进行查询；当返回值为False时，外层查询语句不进行查询
mysql> select * from employee # 子查询能查到结果，执行主查询语句 -> where EXISTS # EXISTS判断子查询是否有结果；与where连用将子查询结果作为条件 -> (select id from department where name = '技术'); # 子查询为条件 +----+------------+--------+------+--------+ | id | name | sex | age | dep_id | +----+------------+--------+------+--------+ | 1 | egon | male | 18 | 200 | | 2 | alex | female | 48 | 201 | | 3 | wupeiqi | male | 38 | 201 | | 4 | yuanhao | female | 28 | 202 | | 5 | liwenzhou | male | 18 | 200 | | 6 | jingliyang | female | 18 | 204 | +----+------------+--------+------+--------+ 6 rows in set (0.00 sec) mysql> select * from employee -> where not exists # 取反 -> (select id from department where id=204); # dep表没有id=204的记录 +----+------------+--------+------+--------+ | id | name | sex | age | dep_id | +----+------------+--------+------+--------+ | 1 | egon | male | 18 | 200 | | 2 | alex | female | 48 | 201 | | 3 | wupeiqi | male | 38 | 201 | | 4 | yuanhao | female | 28 | 202 | | 5 | liwenzhou | male | 18 | 200 | | 6 | jingliyang | female | 18 | 204 | +----+------------+--------+------+--------+ 6 rows in set (0.00 sec)
由此我们看到我们其实是可以把一个sql语句放在括号内作为条件给主sql语句执行的。可以用括号把一个sql语句括起来，起一个别名，当做一张表来操作，与其他表进行连接。以下面的练习为例。
练习：查询每个部门最新入职的那个员工
准备表：运用上一节的employee和department表
# 查询语句当做一张表 mysql> select * from -> (select id,name,sex from employee) as t1; +----+------------+--------+ | id | name | sex | +----+------------+--------+ | 1 | egon | male | | 2 | alex | female | | 3 | wupeiqi | male | | 4 | yuanhao | female | | 5 | liwenzhou | male | | 6 | jingliyang | female | +----+------------+--------+ 6 rows in set (0.01 sec) # 每个部门最新入职的那名员工 select * from employee as t1 inner join (select post, max(hire_date) as max_hire_date from employee group by post) as t2 on t1.post = t2.post where t1.hire_date=t2.max_hire_date; Database changed mysql> show tables; +---------------+ | Tables_in_db2 | +---------------+ | class | | course | | employee | | score | | student | | teacher | +---------------+ 6 rows in set (0.00 sec) mysql> select post, max(hire_date) from employee -> group by post; +-----------------------------------------+----------------+ | post | max(hire_date) | +-----------------------------------------+----------------+ | operation | 2016-03-11 | | sale | 2017-01-27 | | teacher | 2015-03-02 | | 老男孩驻沙河办事处外交大使 | 2017-03-01 | +-----------------------------------------+----------------+ 4 rows in set (0.01 sec) mysql> select * from employee as t1 -> inner join -> (select post, max(hire_date) as max_hire_date from employee group by post) as t2 -> on t1.post = t2.post -> where t1.hire_date=t2.max_hire_date; +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+-----------------------------------------+---------------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | post | max_hire_date | +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+-----------------------------------------+---------------+ | 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 | 老男孩驻沙河办事处外交大使 | 2017-03-01 | | 2 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 | teacher | 2015-03-02 | | 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 | sale | 2017-01-27 | | 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 | operation | 2016-03-11 | +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+-----------------------------------------+---------------+ 4 rows in set (0.00 sec)
mysql> SELECT -> * -> FROM -> employee AS t1 -> INNER JOIN ( -> SELECT -> post, -> max(hire_date) max_date -> FROM -> employee -> GROUP BY -> post -> ) AS t2 ON t1.post = t2.post -> WHERE -> t1.hire_date = t2.max_date; +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+-----------------------------------------+------------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | post | max_date | +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+-----------------------------------------+------------+ | 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 | 老男孩驻沙河办事处外交大使 | 2017-03-01 | | 2 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 | teacher | 2015-03-02 | | 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 | sale | 2017-01-27 | | 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 | operation | 2016-03-11 | +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+-----------------------------------------+------------+ 4 rows in set (0.01 sec)
五、综合练习
| 五、综合练习 |
/* 数据导入： Navicat Premium Data Transfer Source Server : localhost Source Server Type : MySQL Source Server Version : 50624 Source Host : localhost Source Database : sqlexam Target Server Type : MySQL Target Server Version : 50624 File Encoding : utf-8 Date: 10/21/2016 06:46:46 AM */ SET NAMES utf8; SET FOREIGN_KEY_CHECKS = 0; -- ---------------------------- -- Table structure for `class` -- ---------------------------- DROP TABLE IF EXISTS `class`; CREATE TABLE `class` ( `cid` int(11) NOT NULL AUTO_INCREMENT, `caption` varchar(32) NOT NULL, PRIMARY KEY (`cid`) ) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of `class` -- ---------------------------- BEGIN; INSERT INTO `class` VALUES ('1', '三年二班'), ('2', '三年三班'), ('3', '一年二班'), ('4', '二年九班'); COMMIT; -- ---------------------------- -- Table structure for `course` -- ---------------------------- DROP TABLE IF EXISTS `course`; CREATE TABLE `course` ( `cid` int(11) NOT NULL AUTO_INCREMENT, `cname` varchar(32) NOT NULL, `teacher_id` int(11) NOT NULL, PRIMARY KEY (`cid`), KEY `fk_course_teacher` (`teacher_id`), CONSTRAINT `fk_course_teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`tid`) ) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of `course` -- ---------------------------- BEGIN; INSERT INTO `course` VALUES ('1', '生物', '1'), ('2', '物理', '2'), ('3', '体育', '3'), ('4', '美术', '2'); COMMIT; -- ---------------------------- -- Table structure for `score` -- ---------------------------- DROP TABLE IF EXISTS `score`; CREATE TABLE `score` ( `sid` int(11) NOT NULL AUTO_INCREMENT, `student_id` int(11) NOT NULL, `course_id` int(11) NOT NULL, `num` int(11) NOT NULL, PRIMARY KEY (`sid`), KEY `fk_score_student` (`student_id`), KEY `fk_score_course` (`course_id`), CONSTRAINT `fk_score_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`cid`), CONSTRAINT `fk_score_student` FOREIGN KEY (`student_id`) REFERENCES `student` (`sid`) ) ENGINE=InnoDB AUTO_INCREMENT=53 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of `score` -- ---------------------------- BEGIN; INSERT INTO `score` VALUES ('1', '1', '1', '10'), ('2', '1', '2', '9'), ('5', '1', '4', '66'), ('6', '2', '1', '8'), ('8', '2', '3', '68'), ('9', '2', '4', '99'), ('10', '3', '1', '77'), ('11', '3', '2', '66'), ('12', '3', '3', '87'), ('13', '3', '4', '99'), ('14', '4', '1', '79'), ('15', '4', '2', '11'), ('16', '4', '3', '67'), ('17', '4', '4', '100'), ('18', '5', '1', '79'), ('19', '5', '2', '11'), ('20', '5', '3', '67'), ('21', '5', '4', '100'), ('22', '6', '1', '9'), ('23', '6', '2', '100'), ('24', '6', '3', '67'), ('25', '6', '4', '100'), ('26', '7', '1', '9'), ('27', '7', '2', '100'), ('28', '7', '3', '67'), ('29', '7', '4', '88'), ('30', '8', '1', '9'), ('31', '8', '2', '100'), ('32', '8', '3', '67'), ('33', '8', '4', '88'), ('34', '9', '1', '91'), ('35', '9', '2', '88'), ('36', '9', '3', '67'), ('37', '9', '4', '22'), ('38', '10', '1', '90'), ('39', '10', '2', '77'), ('40', '10', '3', '43'), ('41', '10', '4', '87'), ('42', '11', '1', '90'), ('43', '11', '2', '77'), ('44', '11', '3', '43'), ('45', '11', '4', '87'), ('46', '12', '1', '90'), ('47', '12', '2', '77'), ('48', '12', '3', '43'), ('49', '12', '4', '87'), ('52', '13', '3', '87'); COMMIT; -- ---------------------------- -- Table structure for `student` -- ---------------------------- DROP TABLE IF EXISTS `student`; CREATE TABLE `student` ( `sid` int(11) NOT NULL AUTO_INCREMENT, `gender` char(1) NOT NULL, `class_id` int(11) NOT NULL, `sname` varchar(32) NOT NULL, PRIMARY KEY (`sid`), KEY `fk_class` (`class_id`), CONSTRAINT `fk_class` FOREIGN KEY (`class_id`) REFERENCES `class` (`cid`) ) ENGINE=InnoDB AUTO_INCREMENT=17 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of `student` -- ---------------------------- BEGIN; INSERT INTO `student` VALUES ('1', '男', '1', '理解'), ('2', '女', '1', '钢蛋'), ('3', '男', '1', '张三'), ('4', '男', '1', '张一'), ('5', '女', '1', '张二'), ('6', '男', '1', '张四'), ('7', '女', '2', '铁锤'), ('8', '男', '2', '李三'), ('9', '男', '2', '李一'), ('10', '女', '2', '李二'), ('11', '男', '2', '李四'), ('12', '女', '3', '如花'), ('13', '男', '3', '刘三'), ('14', '男', '3', '刘一'), ('15', '女', '3', '刘二'), ('16', '男', '3', '刘四'); COMMIT; -- ---------------------------- -- Table structure for `teacher` -- ---------------------------- DROP TABLE IF EXISTS `teacher`; CREATE TABLE `teacher` ( `tid` int(11) NOT NULL AUTO_INCREMENT, `tname` varchar(32) NOT NULL, PRIMARY KEY (`tid`) ) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of `teacher` -- ---------------------------- BEGIN; INSERT INTO `teacher` VALUES ('1', '张磊老师'), ('2', '李平老师'), ('3', '刘海燕老师'), ('4', '朱云海老师'), ('5', '李杰老师'); COMMIT; SET FOREIGN_KEY_CHECKS = 1;
# 1、查询所有的课程的名称以及对应的任课老师姓名 select cid, cname, teacher.tname from course inner join teacher on course.teacher_id=teacher.tid; +-----+--------+-----------------+ | cid | cname | tname | +-----+--------+-----------------+ | 1 | 生物 | 张磊老师 | | 2 | 物理 | 李平老师 | | 3 | 体育 | 刘海燕老师 | | 4 | 美术 | 李平老师 | +-----+--------+-----------------+ # 2、查询学生表中男女生各有多少人 select gender, count(sid) from student group by gender; +--------+------------+ | gender | count(sid) | +--------+------------+ | 女 | 6 | | 男 | 10 | +--------+------------+ # 3、查询物理成绩等于100的学生的姓名 select student.sname, course.cname, num from score inner join student on score.student_id=student.sid inner join course on score.course_id=course.cid where num =100 and cname='物理'; +--------+--------+-----+ | sname | cname | num | +--------+--------+-----+ | 张四 | 物理 | 100 | | 铁锤 | 物理 | 100 | | 李三 | 物理 | 100 | +--------+--------+-----+ # 4、查询平均成绩大于八十分的同学的姓名和平均成绩 select sname,avg(num) from score inner join student on score.student_id=student.sid group by sname having avg(num)>80; +--------+----------+ | sname | avg(num) | +--------+----------+ | 刘三 | 87.0000 | | 张三 | 82.2500 | +--------+----------+ # 5、查询所有学生的学号，姓名，选课数，总成绩 select student_id,concat(student.sname),count(student_id),sum(num) from score inner join student on score.student_id=student.sid group by student_id; +------------+-----------------------+-------------------+----------+ | student_id | concat(student.sname) | count(student_id) | sum(num) | +------------+-----------------------+-------------------+----------+ | 1 | 理解 | 3 | 85 | | 2 | 钢蛋 | 3 | 175 | | 3 | 张三 | 4 | 329 | | 4 | 张一 | 4 | 257 | | 5 | 张二 | 4 | 257 | | 6 | 张四 | 4 | 276 | | 7 | 铁锤 | 4 | 264 | | 8 | 李三 | 4 | 264 | | 9 | 李一 | 4 | 268 | | 10 | 李二 | 4 | 297 | | 11 | 李四 | 4 | 297 | | 12 | 如花 | 4 | 297 | | 13 | 刘三 | 1 | 87 | +------------+-----------------------+-------------------+----------+ # 6、 查询姓李老师的个数 select count(tname) as li_teacher_num from teacher where tname like '李%'; +----------------+ | li_teacher_num | +----------------+ | 2 | +----------------+ # 7、 查询没有报李平老师课的学生姓名 select student.sname from student where sid not in ( select distinct student_id from score # 找到学习过这两门课的学生ID where course_id in (select cid from course inner join teacher on course.teacher_id=teacher.tid where tname='李平老师') # 查看到李平老师对应的课程 ); +--------+ | sname | +--------+ | 刘三 | | 刘一 | | 刘二 | | 刘四 | +--------+ # 8、 查询物理课程比生物课程高的学生的学号 select t1.student_id from ( select * from score inner join course on score.course_id=course.cid having cname='物理') as t1 inner join ( select * from score inner join course on score.course_id=course.cid having cname='生物') as t2 on t1.student_id=t2.student_id where t1.num > t2.num; +------------+ | student_id | +------------+ | 6 | | 7 | | 8 | +------------+ # 9、 查询没有同时选修物理课程和体育课程的学生姓名 select student.sname from student where sid in ( SELECT student_id FROM score WHERE course_id IN ( # 2、找到成绩表中，只选修过物理或体育中一门的学生id, SELECT cid FROM course WHERE cname = '物理' OR cname = '体育' # 1、得到两门课程的cid ) group by student_id having count(course_id) = 1); +--------+ | sname | +--------+ | 理解 | | 钢蛋 | | 刘三 | +--------+ # 10、查询挂科超过两门(包括两门)的学生姓名和班级 select student.sname, class.caption from student inner join( select student_id from score where num<60 # 1、所有不及格人的科目 group by student_id having count(student_id)>=2) as t1 # 2、过滤出所有挂科超过两门的人 inner join class ON student.sid=t1.student_id # 3、t1再和class表做内连 and student.class_id = class.cid; # 4、student和class做内连 +--------+--------------+ | sname | caption | +--------+--------------+ | 理解 | 三年二班 | +--------+--------------+ # 11、查询选修了所有课程的学生姓名 select student.sname from student where sid in ( # 注意student的sid可以 对 student_id使用in子查询 select student_id from score # 按学生分组，找到学了四门的学生id group by student_id having count(student_id)=(select count(cid) from course) # 引用所有的课程个数 ); +--------+ | sname | +--------+ | 张三 | | 张一 | | 张二 | | 张四 | | 铁锤 | | 李三 | | 李一 | | 李二 | | 李四 | | 如花 | +--------+ # 12、查询李平老师教的课程的所有成绩记录 SELECT * FROM score WHERE course_id IN ( SELECT cid FROM course # 查看所有李平老师课程的cid INNER JOIN teacher ON course.teacher_id = teacher.tid WHERE teacher.tname = '李平老师' ); # 13、查询全部学生都选修了的课程号和课程名 # studentcoursescore 和这三张表关联 select cid,cname from course where cid in ( select course_id # 按课程分组，找出count()后等于学生总数的课程 from score group by course_id having count(student_id)=( SELECT count(sid) # 学生的总数 from student ) ); # 14、查询每门课程被选修的次数 select course_id, count(student_id) as student_num from score group by course_id; +-----------+-------------+ | course_id | student_num | +-----------+-------------+ | 1 | 12 | | 2 | 11 | | 3 | 12 | | 4 | 12 | +-----------+-------------+ # 15、查询只选修了一门课程的学生姓名和学号 -- score——》student select sid,sname from student where sid in ( select student_id from score group by student_id having count(student_id)=1 ); +-----+--------+ | sid | sname | +-----+--------+ | 13 | 刘三 | +-----+--------+ # 16、查询所有学生考出的成绩并按从高到低排序（成绩去重） SELECT DISTINCT num FROM score ORDER BY num DESC; # 17、查询平均成绩大于85的学生姓名和平均成绩 select student.sname, t1.avg_num # 必须要起别名才能打印出来 from student inner join( select student_id, avg(num) as avg_num from score group by student_id having avg_num>85) as t1 on student.sid=t1.student_id; +--------+---------+ | sname | avg_num | +--------+---------+ | 刘三 | 87.0000 | +--------+---------+ # 18、查询生物成绩不及格的学生姓名和对应生物分数 select sname,num from student inner join ( select student_id, num from score where course_id in ( select cid # 生物课程id from course where cname='生物' and num<60 ) ) as t1 on student.sid=t1.student_id; +--------+-----+ | sname | num | +--------+-----+ | 理解 | 10 | | 钢蛋 | 8 | | 张四 | 9 | | 铁锤 | 9 | | 李三 | 9 | +--------+-----+ # 19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)平均成绩最高的学生姓名 -- teacher——》course——》score找到选修李平老师的学生id——》student找到学生的姓名 select sname from student where sid=( # 不支持Limit在子查询里 select student_id from score where course_id in ( select cid # 拿到了课程id from course where teacher_id =( select tid from teacher where tname='李平老师' ) ) group by student_id order by avg(num) desc limit 1 ); # 20、查询每门课程成绩最好的前两名学生姓名 -- score+course——》分组选出前两名学生sid——》student的sname #查看每门课程按照分数排序的信息，为下列查找正确与否提供依据 SELECT * FROM score ORDER BY course_id, num DESC; # 表1：求出每门课程的课程course_id,与最高分数first_num SELECT course_id, max(num) first_num FROM score GROUP BY course_id; # 表2：去掉最高分，再按照课程分组，取得的最高分，就是第二高的分数second_num SELECT score.course_id, max(num) second_num FROM score INNER JOIN ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t ON score.course_id = t.course_id WHERE score.num < t.first_num GROUP BY course_id; # 将表1和表2联合到一起，得到一张表t3，包含课程course_id与该们课程的first_num与second_num SELECT t1.course_id, t1.first_num, t2.second_num FROM ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t1 INNER JOIN ( SELECT score.course_id, max(num) second_num FROM score INNER JOIN ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t ON score.course_id = t.course_id WHERE score.num < t.first_num GROUP BY course_id ) AS t2 ON t1.course_id = t2.course_id; #查询前两名的学生（有可能出现并列第一或者并列第二的情况） SELECT score.student_id, t3.course_id, t3.first_num, t3.second_num FROM score INNER JOIN ( SELECT t1.course_id, t1.first_num, t2.second_num FROM ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t1 INNER JOIN ( SELECT score.course_id, max(num) second_num FROM score INNER JOIN ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t ON score.course_id = t.course_id WHERE score.num < t.first_num GROUP BY course_id ) AS t2 ON t1.course_id = t2.course_id ) AS t3 ON score.course_id = t3.course_id WHERE score.num >= t3.second_num AND score.num <= t3.first_num; # 排序后可以看的明显点 SELECT score.student_id, t3.course_id, t3.first_num, t3.second_num FROM score INNER JOIN ( SELECT t1.course_id, t1.first_num, t2.second_num FROM ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t1 INNER JOIN ( SELECT score.course_id, max(num) second_num FROM score INNER JOIN ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t ON score.course_id = t.course_id WHERE score.num < t.first_num GROUP BY course_id ) AS t2 ON t1.course_id = t2.course_id ) AS t3 ON score.course_id = t3.course_id WHERE score.num >= t3.second_num AND score.num <= t3.first_num ORDER BY course_id; +------------+-----------+-----------+------------+ | student_id | course_id | first_num | second_num | +------------+-----------+-----------+------------+ | 9 | 1 | 91 | 90 | | 10 | 1 | 91 | 90 | | 11 | 1 | 91 | 90 | | 12 | 1 | 91 | 90 | | 9 | 2 | 100 | 88 | | 6 | 2 | 100 | 88 | | 7 | 2 | 100 | 88 | | 8 | 2 | 100 | 88 | | 2 | 3 | 87 | 68 | | 3 | 3 | 87 | 68 | | 13 | 3 | 87 | 68 | | 2 | 4 | 100 | 99 | | 3 | 4 | 100 | 99 | | 4 | 4 | 100 | 99 | | 5 | 4 | 100 | 99 | | 6 | 4 | 100 | 99 | +------------+-----------+-----------+------------+ #可以用以下命令验证上述查询的正确性 SELECT * FROM score ORDER BY course_id, num DESC; # 21、查询不同课程但成绩相同的学号，课程号，成绩 select DISTINCT s1.course_id,s2.course_id,s1.num,s2.num from score as s1, score as s2 where s1.num = s2.num and s1.course_id != s2.course_id; +-----------+-----------+-----+-----+ | course_id | course_id | num | num | +-----------+-----------+-----+-----+ | 1 | 2 | 9 | 9 | | 2 | 4 | 66 | 66 | | 2 | 1 | 77 | 77 | | 4 | 2 | 66 | 66 | | 4 | 3 | 87 | 87 | | 2 | 4 | 100 | 100 | | 2 | 1 | 9 | 9 | | 4 | 2 | 100 | 100 | | 2 | 4 | 88 | 88 | | 4 | 2 | 88 | 88 | | 1 | 2 | 77 | 77 | | 3 | 4 | 87 | 87 | +-----------+-----------+-----+-----+ # 22、查询没学过“李平”老师课程的学生姓名以及选修的课程名称； select student_id,student.sname from score left join student on score.student_id = student.sid where score.course_id not in ( select cid from course left join teacher on course.teacher_id = teacher.tid where tname = '张磊老师' ) group by student_id; +------------+--------+ | student_id | sname | +------------+--------+ | 1 | 理解 | | 2 | 钢蛋 | | 3 | 张三 | | 4 | 张一 | | 5 | 张二 | | 6 | 张四 | | 7 | 铁锤 | | 8 | 李三 | | 9 | 李一 | | 10 | 李二 | | 11 | 李四 | | 12 | 如花 | | 13 | 刘三 | +------------+--------+ # 23、查询所有选修了学号为1的同学选修过的一门或者多门课程的同学学号和姓名； # 24、任课最多的老师中学生单科成绩最高的学生姓名