数据库简单多表查询

1.第一步准备数据

1）创建数据库
2）创建分类表
3）创建商品表
4) 分类表和商品表数据初始化

#创建数据库
CREATE DATABASE webdb_3;
USE webdb_3;

##创建分类表
CREATE TABLE category (
  cid VARCHAR(32) PRIMARY KEY ,
  cname VARCHAR(50)
);
##创建商品表
CREATE TABLE products(
  pid VARCHAR(32) PRIMARY KEY ,
  pname VARCHAR(50),
  price INT,
  flag VARCHAR(2),				#是否上架标记为：1表示上架、0表示下架
  category_id VARCHAR(32),
  CONSTRAINT products_fk FOREIGN KEY (category_id) REFERENCES category (cid)
);


### 初始化数据
#分类
INSERT INTO category(cid,cname) VALUES('c001','家电');
INSERT INTO category(cid,cname) VALUES('c002','服饰');
INSERT INTO category(cid,cname) VALUES('c003','化妆品');
#商品
INSERT INTO products(pid, pname,price,flag,category_id) VALUES('p001','联想',5000,'1','c001');
INSERT INTO products(pid, pname,price,flag,category_id) VALUES('p002','海尔',3000,'1','c001');
INSERT INTO products(pid, pname,price,flag,category_id) VALUES('p003','雷神',5000,'1','c001');
INSERT INTO products (pid, pname,price,flag,category_id) 
VALUES('p004','JACK JONES',800,'1','c002');
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('p005','真维斯',200,'1','c002');
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('p006','花花公子',440,'1','c002');
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('p007','劲霸',2000,'1','c002');
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('p008','香奈儿',800,'1','c003');
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('p009','相宜本草',200,'1','c003');

内联查询

select * from A inner join B on 条件;   (显式内连接)

查询哪些分类的商品已经上架:
SELECT cname FROM category,products WHERE cid = category_id AND flag = '1';

建议多表查询时，使用别名
SELECT DISTINCT c.cname FROM category c INNER JOIN products p ON c.cid = p.category_id WHERE p.flag = '1';

子查询

一条select语句的结果作为另一条select语法一部分（查询条件，查询结果，表等）

SELECT * FROM products p WHERE p.category_id = (SELECT cid FROM category WHERE cname = '化妆品') AND p.flag = '1';

举例:
查询“化妆品”或“家电”两个分类上架商品详情
SELECT p.* FROM products p , (SELECT * FROM category WHERE cname='化妆品' OR cname='家电') c WHERE c.cid = p.category_id AND p.flag = '1';