Java编程基础32——MySQL多表联查

摘要：会创建多表及多表的关系多表之间的关系如何来维护添加外键约束从分类表中删除分类为信息删除失败首先得去表删除所有分类商品建数据库原则通常情况下一个项目应用建一个数据库多表之间的建表原则一对多建表原则在多的一方增加一个外键指向一的一方多对多建表原

1.多表之间的关系如何来维护

添加外键约束: foreign key

alter table product add foreign key(cno) references category(cid);

从分类表中,删除分类为5信息,

delete from category where cid =5; //删除失败

首先得去product表, 删除所有分类ID5 商品

2.建数据库原则:

通常情况下,一个项目/应用建一个数据库

3.多表之间的建表原则

一对多:

建表原则: 在多的一方增加一个外键,指向一的一方

多对多:

建表原则: 将多对多转成一对多的关系,创建一张中间表

一对一: 不常用, 拆表操作

建表原则: 将两张表合并成一张表

将两张表的主键建立起关系

将一对一的关系当作一对多的关系去处理

网上商城表实例的分析: 用户购物流程

用户表 (用户的ID,用户名,密码,手机)

create table user(
    uid int primary key auto_increment,
      username varchar(31),
      password varchar(31),
      phone  varchar(11)
);

insert into user values(1,"zhangsan","123","13811118888");

订单表 (订单编号,总价,订单时间 ,地址,外键用户的ID)

create table orders(
      oid int primary key auto_increment,
    sum int not null,
    otime timestamp,
    address varchar(100),
    uno int,
    foreign key(uno) references user(uid)
);
  
insert into orders values(1,200,null,"黑马前台旁边小黑屋",1);
insert into orders values(2,250,null,"黑马后台旁边1702",1);

商品表 (商品ID, 商品名称,商品价格,外键cno)

create table product(
    pid int primary key auto_increment,
      pname varchar(10),
      price double,
      cno int,
      foreign key(cno) references category(cid)
);

insert into product values(null,"小米mix4",998,1);
insert into product values(null,"锤子",2888,1);
insert into product values(null,"阿迪王",99,2);
insert into product values(null,"老村长",88,3);
insert into product values(null,"劲酒",35,3);
insert into product values(null,"小熊饼干",1,4);
insert into product values(null,"卫龙辣条",1,5);
insert into product values(null,"旺旺大饼",1,5);

订单项: 中间表(订单ID,商品ID,商品数量,订单项总价)

create table orderitem(
    ono int,
      pno int,
      foreign key(ono) references orders(oid),
      foreign key(pno) references product(pid),
      ocount int,
      subsum double
);
--给1号订单添加商品 200块钱的商品
insert into orderitem values(1,7,100,100);
insert into orderitem values(1,8,101,100);

--给2号订单添加商品 250块钱的商品 ()
insert into orderitem values(2,5,1,35);
insert into orderitem values(2,3,3,99);

商品分类表(分类ID,分类名称,分类描述)

create table category(
    cid int primary key auto_increment,
      cname varchar(15),
      cdesc varchar(100)
);

insert into category values(null,"手机数码","电子产品,黑马生产");
insert into category values(null,"鞋靴箱包","江南皮鞋厂倾情打造");
insert into category values(null,"香烟酒水","黄鹤楼,茅台,二锅头");
insert into category values(null,"酸奶饼干","娃哈哈,蒙牛酸酸乳");
insert into category values(null,"馋嘴零食","瓜子花生,八宝粥,辣条");

主键约束: 默认就是不能为空, 唯一

外键都是指向另外一张表的主键

主键一张表只能有一个

唯一约束: 列面的内容, 必须是唯一, 不能出现重复情况, 为空

唯一约束不可以作为其它表的外键

可以有多个唯一约束

-- 笛卡尔积,查出来的是两张表的乘积
select * from product,category;

-- 过滤出有意义的数据
select * from product,category where cno = cid;
select * from product p,category c where p.cno = c.cid;

-- 内连接查询
-- 隐式内连接: 在查询结果上做的where条件过滤
select * from product p,category c where p.cno = c.cid;
-- 显式内连接: 带着条件查结果,执行效率高
select * from product p inner join category c on p.cno = c.cid;

-- 左外连接:会将左表中的所有数据都查询出来,如果右表中没有对应的数据,用null代替
select * from product p left outer join category c on p.cno = c.cid;
-- 右外连接:
select * from product p right outer join category c on p.cno = c.cid;

-- 起始索引: startIndex  = (index-1)*3
-- 第一个参数是索引,第二个参数显示的个数
select * from product limit 0,3;
select * from product limit 3,3;

-- 查询出(商品名称,商品分类名称)信息
    -- 左连接
    select * from product p left outer join category c on p.cno = c.cid;
-- 查询分类名称为手机数码的所有商品
    select * from product where cno = (select cid from category where cname ="手机数码");

按照商品分类的名称统计商品的个数:

-- 查询分类名称为手机数码的所有商品
select * from product where cno = (select cid from category where cname ="手机数码");

-- 查询出(商品名称,商品分类名称)信息
  -- 左连接
    select * from product p left join category c on p.cno = c.cid;
    select p.pname,c.cname from product p left join category c on p.cno = c.cid;
  -- 子查询
    select pname,(select cname from category c where p.cno = c.cid) as 分类名称 from product p;
 

查询1号订单的订单项信息和商品信息

员工信息表

--员工信息表
CREATE TABLE emp(
    empno INT,
    ename VARCHAR(50),
    job VARCHAR(50),
    mgr    INT,
    hiredate DATE,
    sal    DECIMAL(7,2),
    comm DECIMAL(7,2),
    deptno INT
) ;

INSERT INTO emp values(7369,"SMITH","CLERK",7902,"1980-12-17",800,NULL,20);
INSERT INTO emp values(7499,"ALLEN","SALESMAN",7698,"1981-02-20",1600,300,30);
INSERT INTO emp values(7521,"WARD","SALESMAN",7698,"1981-02-22",1250,500,30);
INSERT INTO emp values(7566,"JONES","MANAGER",7839,"1981-04-02",2975,NULL,20);
INSERT INTO emp values(7654,"MARTIN","SALESMAN",7698,"1981-09-28",1250,1400,30);
INSERT INTO emp values(7698,"BLAKE","MANAGER",7839,"1981-05-01",2850,NULL,30);
INSERT INTO emp values(7782,"CLARK","MANAGER",7839,"1981-06-09",2450,NULL,10);
INSERT INTO emp values(7788,"SCOTT","ANALYST",7566,"1987-04-19",3000,NULL,20);
INSERT INTO emp values(7839,"KING","PRESIDENT",NULL,"1981-11-17",5000,NULL,10);
INSERT INTO emp values(7844,"TURNER","SALESMAN",7698,"1981-09-08",1500,0,30);
INSERT INTO emp values(7876,"ADAMS","CLERK",7788,"1987-05-23",1100,NULL,20);
INSERT INTO emp values(7900,"JAMES","CLERK",7698,"1981-12-03",950,NULL,30);
INSERT INTO emp values(7902,"FORD","ANALYST",7566,"1981-12-03",3000,NULL,20);
INSERT INTO emp values(7934,"MILLER","CLERK",7782,"1982-01-23",1300,NULL,10);
INSERT INTO emp values(7981,"MILLER","CLERK",7788,"1992-01-23",2600,500,20);

部门信息表

CREATE TABLE dept(
    deptno        INT,
    dname        varchar(14),
    loc        varchar(13)
);

INSERT INTO dept values(10, "ACCOUNTING", "NEW YORK");
INSERT INTO dept values(20, "RESEARCH", "DALLAS");
INSERT INTO dept values(30, "SALES", "CHICAGO");
INSERT INTO dept values(40, "OPERATIONS", "BOSTON");

基本查询

--所有员工的信息
--薪资大于等于1000并且小于等于2000的员工信息
--从员工表中查询出所有的部门编号
--查询出名字以A开头的员工的信息
--查询出名字第二个字母是L的员工信息
--查询出没有奖金的员工信息
--所有员工的平均工资
--所有员工的工资总和
--所有员工的数量
-- 最高工资
    select max(sal) from emp;
-- 最少工资
    select min(sal) from emp;
-- 最高工资的员工信息
    select * from emp where sal = (select max(sal) from emp);
-- 最低工资的员工信息
    select * from emp where sal = (select min(sal) from emp);

分组查询

--每个部门的平均工资

子查询

-- 单行子查询(> < >= <= = <>)
    -- 查询出高于10号部门的平均工资的员工信息
    
-- 多行子查询(in  not in any all)    >any  >all
    -- 查询出比10号部门任何员工薪资高的员工信息

-- 多列子查询(实际使用较少)   in
    -- 和10号部门同名同工作的员工信息
-- Select接子查询
    -- 获取员工的名字和部门的名字
-- from后面接子查询
    -- 查询emp表中经理信息
-- where 接子查询
    -- 薪资高于10号部门平均工资的所有员工信息
-- having后面接子查询
    -- 有哪些部门的平均工资高于30号部门的平均工资

-- 工资>JONES工资
-- 查询与SCOTT同一个部门的员工
-- 工资高于30号部门所有人的员工信息
-- 查询工作和工资与MARTIN完全相同的员工信息
-- 有两个以上直接下属的员工信息
-- 查询员工编号为7788的员工名称,员工工资,部门名称,部门地址

SQL查询的综合案例

查询出高于本部门平均工资的员工信息

列出达拉斯加工作的人中,比纽约平均工资高的人

查询7369员工编号,姓名,经理编号和经理姓名

查询出各个部门薪水最高的员工所有信息

CREATE TABLE test(
  name CHAR(20),
  kecheng CHAR(20),
  fenshu CHAR(20)
);

INSERT INTO test VALUES("张三","语文",81),
("张三","数学",75),
("李四","语文",76),
("李四","数学",90),
("王五","语文",81),
("王五","数学",82);

--请用一条Sql语句查处分数大于80的学生