一、子查询
查询所有任课教师的tname 和 depart(任课：说明在课程表中有课程)
---先查询课程表中课程对应教师编号
select tno from course;
---再查询教师编号对应的老师
select tname,depart from teacher where tno in(select tno from course);
+--------+-----------------+
| tname | depart |
+--------+-----------------+
| 李诚 | 计算机系 |
| 王萍 | 计算机系 |
| 刘冰 | 电子工程系 |
| 张旭 | 电子工程系 |
+--------+-----------------+
查询"男"教师 及其所上的课
---先找男老师的编号
select tno from teacher where tsex='男';
---根据编号找到课程
select * from course where tno in(select tno from teacher where tsex='男');
+-------+--------------+-----+
| cno | cname | tno |
+-------+--------------+-----+
| 3-245 | 操作系统 | 804 |
| 6-166 | 数字电路 | 856 |
+-------+--------------+-----+
查询最高分同学的sno cno 和 degree;
---先查询最高分
select max(degree) from score;
---再查最高分的相关信息
select * from score where degree=(select max(degree) from score);
+-----+-------+--------+
| sno | cno | degree |
+-----+-------+--------+
| 103 | 3-105 | 92 |
+-----+-------+--------+
查询和"李军"同性别的所有同学的sname
---先查询李军的性别
select ssex from student where sname='李军';
---再查询同性别的sname
select sname from student where ssex=(select ssex from student where sname='李军');
+-----------+
| sname |
+-----------+
| 曾华 |
| 匡明 |
| 李军 |
| 陆军 |
| 王尼玛 |
| 张全蛋 |
| 赵铁柱 |
| 张飞 |
+-----------+
查询和"李军"同性别并且同班的所有同学的sname
---先查询李军的性别
select ssex from student where sname='李军';
---再查询李军的班级
select class from student where sname='李军');
---加以合并
select sname from student where ssex=(select ssex from student where sname='李军') and class=(select class from student where sname='李军');
+-----------+
| sname |
+-----------+
| 曾华 |
| 李军 |
| 王尼玛 |
+-----------+
查询所有选修’计算机导论’课程的’男’同学的成绩表
---先找男同学
select * from student where ssex='男';
---再找计算机导论
select * from course where cname='计算机导论';
---成绩表中根据sno和cno进行筛选
select * from score
where cno=(select cno from course where cname='计算机导论')
and sno in(select sno from student where ssex='男');
+-----+-------+--------+
| sno | cno | degree |
+-----+-------+--------+
| 101 | 3-105 | 90 |
| 102 | 3-105 | 91 |
| 104 | 3-105 | 89 |
| 109 | 3-105 | 76 |
+-----+-------+--------+
二、条件加分组筛选
查出至少有2名男生的班号
select class from student where ssex='男' group by class having count(*)>1;
+-------+
| class |
+-------+
| 95033 |
| 95031 |
+-------+
三、not like模糊查询取反
查询student 表中 **不姓"王"**的同学的记录
select * from student where sname not like '王%';
+-----+-----------+------+---------------------+-------+
| sno | sname | ssex | sbirthday | class |
+-----+-----------+------+---------------------+-------+
| 101 | 曾华 | 男 | 1977-09-01 00:00:00 | 95033 |
| 102 | 匡明 | 男 | 1975-10-02 00:00:00 | 95031 |
| 104 | 李军 | 男 | 1976-02-20 00:00:00 | 95033 |
| 106 | 陆军 | 男 | 1974-06-03 00:00:00 | 95031 |
| 108 | 张全蛋 | 男 | 1975-02-10 00:00:00 | 95031 |
| 109 | 赵铁柱 | 男 | 1974-06-03 00:00:00 | 95031 |
| 110 | 张飞 | 男 | 1974-06-03 00:00:00 | 95038 |
+-----+-----------+------+---------------------+-------+
四、year函数与now函数
查询student 中每个学生的姓名和年龄(当前时间 - 出生年份)
---当前年份
select year(now());
+-------------+
| year(now()) |
+-------------+
| 2020 |
+-------------+
---出生年份
select year(sbirthday) from student;
+-----------------+
| year(sbirthday) |
+-----------------+
| 1977 |
| 1975 |
| 1976 |
| 1976 |
| 1975 |
| 1974 |
| 1976 |
| 1975 |
| 1974 |
| 1974 |
+-----------------+
---当前年份-出生年份
select sname,year(now())-year(sbirthday) as '年龄' from student;
+-----------+--------+
| sname | 年龄 |
+-----------+--------+
| 曾华 | 43 |
| 匡明 | 45 |
| 王丽 | 44 |
| 李军 | 44 |
| 王芳 | 45 |
| 陆军 | 46 |
| 王尼玛 | 44 |
| 张全蛋 | 45 |
| 赵铁柱 | 46 |
| 张飞 | 46 |
+-----------+--------+
五、max与min函数
查询student中最大和最小的 sbirthday的值
select max(sbirthday) as '最大',min(sbirthday) as '最小' from student;
+---------------------+---------------------+
| 最大 | 最小 |
+---------------------+---------------------+
| 1977-09-01 00:00:00 | 1974-06-03 00:00:00 |
+---------------------+---------------------+
六、多字段排序
以班级号和年龄从大到小的顺序查询student表中的全部记录
注意，年龄和出生日期成反比，所以班级降序，出生日期升序。
select * from student order by class desc,sbirthday;
+-----+-----------+------+---------------------+-------+
| sno | sname | ssex | sbirthday | class |
+-----+-----------+------+---------------------+-------+
| 110 | 张飞 | 男 | 1974-06-03 00:00:00 | 95038 |
| 103 | 王丽 | 女 | 1976-01-23 00:00:00 | 95033 |
| 104 | 李军 | 男 | 1976-02-20 00:00:00 | 95033 |
| 107 | 王尼玛 | 男 | 1976-02-20 00:00:00 | 95033 |
| 101 | 曾华 | 男 | 1977-09-01 00:00:00 | 95033 |
| 106 | 陆军 | 男 | 1974-06-03 00:00:00 | 95031 |
| 109 | 赵铁柱 | 男 | 1974-06-03 00:00:00 | 95031 |
| 105 | 王芳 | 女 | 1975-02-10 00:00:00 | 95031 |
| 108 | 张全蛋 | 男 | 1975-02-10 00:00:00 | 95031 |
| 102 | 匡明 | 男 | 1975-10-02 00:00:00 | 95031 |
+-----+-----------+------+---------------------+-------+
七、按等级查询
假设使用了以下命令建立了一个grade表
CREATE TABLE grade(
low INT(3),
upp INT(3),
grade CHAR(1)
);
INSERT INTO grade VALUES(90,100,'A');
INSERT INTO grade VALUES(80,89,'B');
INSERT INTO grade VALUES(70,79,'c');
INSERT INTO grade VALUES(60,69,'D');
INSERT INTO grade VALUES(0,59,'E');
查询所有同学的sno , cno 和grade列
select sno,cno,grade from score,grade where degree between low and upp;
+-----+-------+-------+
| sno | cno | grade |
+-----+-------+-------+
| 101 | 3-105 | A |
| 102 | 3-105 | A |
| 103 | 3-105 | A |
| 103 | 3-245 | B |
| 103 | 6-166 | B |
| 104 | 3-105 | B |
| 105 | 3-105 | B |
| 105 | 3-245 | c |
| 105 | 6-166 | c |
| 109 | 3-105 | c |
| 109 | 3-245 | D |
| 109 | 6-166 | B |
+-----+-------+-------+
八、连接查询
创建要使用的表：
create database testJoin;
create table person( id int, name varchar(20), cardId int);
create table card( id int,name varchar(20));
添加数据
insert into card values(1,'饭卡');
insert into card values(2,'建行卡');
insert into card values(3,'农行卡');
insert into card values(4,'工商卡');
insert into card values(5,'邮政卡');
---并没有创建外键
insert into person values(1,'张三',1);
insert into person values(2,'李四',3);
insert into person values(3,'王五',6);
（一）内连接 inner join 或者 join
内联查询就是两张表中的数据，通过某个字段相等，查询出相关记录。
select * from person inner join card on person.cardId=card.id;
+------+--------+--------+------+-----------+
| id | name | cardId | id | name |
+------+--------+--------+------+-----------+
| 1 | 张三 | 1 | 1 | 饭卡 |
| 2 | 李四 | 3 | 3 | 农行卡 |
+------+--------+--------+------+-----------+
（二）外连接
1.左连接 left join 或者 left outer join
把左边表所有数据取出来，而右边表中的数据如果有相等的显示，没有则显示为null。
select * from person left join card on person.cardId=card.id;
+------+--------+--------+------+-----------+
| id | name | cardId | id | name |
+------+--------+--------+------+-----------+
| 1 | 张三 | 1 | 1 | 饭卡 |
| 2 | 李四 | 3 | 3 | 农行卡 |
| 3 | 王五 | 6 | NULL | NULL |
+------+--------+--------+------+-----------+
2.右连接 right join 或者 right outer join
把右边表所有数据取出来，而左边表中的数据如果有相等的显示，没有则显示为null。
select * from person right join card on person.cardId=card.id;
+------+--------+--------+------+-----------+
| id | name | cardId | id | name |
+------+--------+--------+------+-----------+
| 1 | 张三 | 1 | 1 | 饭卡 |
| 2 | 李四 | 3 | 3 | 农行卡 |
| NULL | NULL | NULL | 2 | 建行卡 |
| NULL | NULL | NULL | 4 | 工商卡 |
| NULL | NULL | NULL | 5 | 邮政卡 |
+------+--------+--------+------+-----------+
3.完全外连接 full join 或者 full outer join
MySQL不支持全外连接，要实现完全外连接，取左连接和右连接的并集。
select * from person left join card on person.cardId=card.id
union
select * from person right join card on person.cardId=card.id;
+------+--------+--------+------+-----------+
| id | name | cardId | id | name |
+------+--------+--------+------+-----------+
| 1 | 张三 | 1 | 1 | 饭卡 |
| 2 | 李四 | 3 | 3 | 农行卡 |
| 3 | 王五 | 6 | NULL | NULL |
| NULL | NULL | NULL | 2 | 建行卡 |
| NULL | NULL | NULL | 4 | 工商卡 |
| NULL | NULL | NULL | 5 | 邮政卡 |
+------+--------+--------+------+-----------+
