外键：
准备：创建两张表，一个班级表和一个学生表#建库CREATE DATABASE chapter05;
#建表USE chapter05;
CREATE TABLE grade(
id int(4) NOT NULL PRIMARY KEY,
name varchar(36)
);
CREATE TABLE student(
sid int(4) NOT NULL PRIMARY KEY,
sname varchar(36),
gid int(4) NOT NULL
);
（1）为表添加外键约束
#student添加外键约束
alter table student add constraint FK_ID foreign key(gid) REFERENCES grade (id);
#查看学生表和班级表
desc grade;
desc student;
#查看表的详细结构
show create table student;
（2）删除外键约束
#student中的外键约束删除
alter table student drop foreign key FK_ID;
2.操作关联表
（1）添加数据#为表student和表grade中添加外键约束来建立两个表的关联关系
alter table student add constraint FK_ID foreign key(gid) REFERENCES grade (id);
#添加数据
INSERT INTO grade(id,name)VALUES(1,'软件一班');
INSERT INTO grade(id,name)VALUES(2,'软件二班');
INSERT INTO student(sid,sname,gid)VALUES(1,'王红',1);
INSERT INTO student(sid,sname,gid)VALUES(2,'李强',1);
INSERT INTO student(sid,sname,gid)VALUES(3,'赵四',2);
INSERT INTO student(sid,sname,gid)VALUES(4,'郝娟',2);
（2）在grade表中查询出班级名称为“软件一班”的id
SELECT id FROM grade WHERE name='软件一班';
（3）在student表中，查询gid=1的学生，即为软件一班的学生，
SELECT sname FROM student WHERE gid=1;
（4）将软件一班的所有学生全部删除
delete from student where sname='王红';
delete from student where sname='李强';
（5）在grade表中，将软件一班删除
delete from grade where id=1;
3.连接查询
准备：
#建表
USE chapter05;
CREATE TABLE department(
did int(4) NOT NULL PRIMARY KEY,
dname varchar(36)
);
CREATE TABLE employee (
id int(4) NOT NULL PRIMARY KEY,
name varchar(36),
age int(2),
did int(4) NOT NULL
);
#添加数据
INSERT INTO department(did,dname)VALUES(1,'网络部');
INSERT INTO department(did,dname)VALUES(2,'媒体部');
INSERT INTO department(did,dname)VALUES(3,'研发部');
INSERT INTO department(did,dname)VALUES(5,'人事部');
INSERT INTO employee(id,name,age,did)VALUES(1,'王红',20,1);
INSERT INTO employee(id,name,age,did)VALUES(2,'李强',22,1);
INSERT INTO employee(id,name,age,did)VALUES(3,'赵四',20,2);
INSERT INTO employee(id,name,age,did)VALUES(4,'郝娟',20,4);
（1）使用交叉连接查询部门表和员工表中所有的数据
SELECT * FROM department CROSS JOIN employee;
（2）在department和employee 表之间使用内连接查询
SELECT employee.name, department.dname FROM department JOIN employee
ON department.did=employee.did;
（3）在department和employee 表之间使用WHRER
SELECT employee.name, department.dname FROM department,employee
WHERE department.did=employee.did;
（4）在department和employee 表之间使用自连接查询
SELECT p1.* FROM employee p1 JOIN employee p2 ON p1.did=p2.did WHERE p2.name='王红';
（5）在department和employee 表之间使用左连接查询
SELECT department.did,department.dname,employee.name FROM department
LEFT JOIN employee on department.did=employee.did;
（6）在department和employee 表之间使用右连接查询
SELECT department.did,department.dname,employee.name FROM department
RIGHT JOIN employee ON department.did=employee.did;
（7）在department和employee 表之间使用内连接查询，并将查询结果按照年龄从大到小进行排序
SELECT employee.name, employee.age, department.dname FROM department JOIN employee
ON department.did=employee.did order by age;
（8）查询存在年龄为20岁的员工部门
SELECT * FROM department WHERE did IN(SELECT did FROM employee WHERE age=20);
（9）查询不存在年龄为20岁的员工部门
SELECT * FROM department WHERE did NOT IN(SELECT did FROM employee WHERE age=20);
（10）查询employee表中是否存在年龄大于21岁的员工，如果存在，则查询department表中的所有记录
SELECT * FROM department WHERE EXISTS(select did from employee where age > 21);
（11）使用带ANY关键字的子查询，查询满足条件的部门
SELECT * FROM department WHERE did>any(select did from employee);
（12）使用带ALL关键字的子查询，查询满足条件的部门
SELECT * FROM department WHERE did>all(select did from employee);
（13）使用带比较运算符的子查询，查询赵四是哪个部门的员工
SELECT * FROM department WHERE did=(select did from employee where name='赵四');