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目标:
多表查询到现在,掌握的不是很深,今晚就在写一遍。(小感慨)
多表查询分为:交叉连接查询,内连接查询,外连接查询,子查询。
首先数据准备:
# 分类表
CREATE TABLE category (
cid VARCHAR(32) PRIMARY KEY ,
cname VARCHAR(50)
);
#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)
);
#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');
INSERT INTO category(cid,cname) VALUES('c004','奢侈品');
#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');
多表查询:
交叉连接查询:
注意:交叉连接查询,会出现笛卡尔积这种情况。
语法:select * from A,B;
问题:
SELECT * FROM products,category;
内连接查询:
内连接查询分为左外连接,右外连接。
隐式内连接:`select * from A,B where 条件;`
显示内连接:`select * from A inner join B on 条件;
问题:查询哪些分类的商品已经上架?
代码:
#隐式内连接
SELECT * FROM products p,category c WHERE c.cid=p.category_id AND p.flag='1';
#内连接
SELECT * FROM products p INNER JOIN category c ON c.cid=p.category_id WHERE p.flag='1';
外连接查询:
外连接分为左外连接查询和右外连接查询
左外连接:left outer join
右外连接:right outer join
问题:查询每个分类所有商品的个数?
代码:
#左外连接
SELECT c.cname,COUNT(p.category_id) FROM category c LEFT OUTER JOIN products p ON c.cid=p.category_id GROUP BY c.cname;
#右外连接
SELECT c.cname,COUNT(p.category_id) FROM products p RIGHT OUTER JOIN category c ON c.cid=p.category_id GROUP BY c.cname;
子查询:
一条select语句结果作为另一条select语法一部分(查询条件,查询结果,表等)。
语法:
select ....查询字段 ... from ... 表.. where ... 查询条件
问题:查询“化妆品”分类上架商品详情
代码:
#隐式内连接
SELECT * FROM products p,category c WHERE c.cid=p.category_id AND cname='化妆品';
##作为查询条件
SELECT *FROM products p WHERE p.category_id=(SELECT cid FROM category WHERE cname='化妆品');
##作为另一张表
SELECT *FROM products p ,(SELECT cid FROM category WHERE cname='化妆品') cc WHERE p.category_id=cc.cid;