Mysql学习笔记（二）：子查询与连接

//Join的使用

USe mooc;
Select * from tdb_goods group by goods_id ASC;

ALTER TABLE tb_cates rename to goods_cate;
INSERT goods_cate(cate_name) VALUES('路由器'),('交换机'),('网卡');

INSERT goods_brands(brand_name) VALUES('海尔'),('清华同方'),('神舟');
-- 在tdb_goods数据表写入任意记录

//通过这三列数据可以发现，当ID不同时存在（插入）两张表时，内连接无法查询到记录
INSERT tdb_goods(goods_name,cate_id,brand_id,goods_price) VALUES(' LaserJet Pro P1606dn 黑白激光打印机','12','4','1849');
INSERT goods_brands(brand_name) VALUES('LaserJet Pro P1606dn 黑白激光打印机');
INSERT goods_cate(cate_name) VALUES('黑白激光打印机');

Show columns from tdb_goods;
Show tables from mooc;

describe goods_cate;
describe goods_brands;

//确保上面两张表的各字段数据类型是否一致

**内连接**

Select goods_id, goods_name, cate_name From
tdb_goods INNER JOIN goods_cate On
tdb_goods.cate_id = goods_cate.cate_id;
/*新增的三种产品无法显示在内连接中*/

Select * from goods_brands;

**外连接**

Select goods_id, goods_name, brand_name From
tdb_goods LEFT OUTER JOIN goods_brands On
tdb_goods.cate_id = goods_brands.brand_id;

truncate goods_brands; /*清楚表内数据之后，外连接的参照列全部为NULL*/

**多表连接**

Select goods_id, goods_name, a.cate_id, a.brand_id, goods_price, is_show, is_saleoff
from tdb_goods AS a INNER JOIN
goods_brands as b

-- 无限分类的数据表设计
CREATE TABLE tdb_goods_types(
type_id   SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
type_name VARCHAR(20) NOT NULL,
parent_id SMALLINT UNSIGNED NOT NULL DEFAULT 0
);

---------插入测试数据--------
INSERT tdb_goods_types(type_name,parent_id) VALUES('家用电器',DEFAULT);
INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑、办公',DEFAULT);
INSERT tdb_goods_types(type_name,parent_id) VALUES('大家电',1);
INSERT tdb_goods_types(type_name,parent_id) VALUES('生活电器',1);
INSERT tdb_goods_types(type_name,parent_id) VALUES('平板电视',3);
INSERT tdb_goods_types(type_name,parent_id) VALUES('空调',3);
INSERT tdb_goods_types(type_name,parent_id) VALUES('电风扇',4);
INSERT tdb_goods_types(type_name,parent_id) VALUES('饮水机',4);
INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑整机',2);
INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑配件',2);
INSERT tdb_goods_types(type_name,parent_id) VALUES('笔记本',9);
INSERT tdb_goods_types(type_name,parent_id) VALUES('超级本',9);
INSERT tdb_goods_types(type_name,parent_id) VALUES('游戏本',9);
INSERT tdb_goods_types(type_name,parent_id) VALUES('CPU',10);
INSERT tdb_goods_types(type_name,parent_id) VALUES('主机',10);

-- 查找所有分类及其父类

SELECT s.type_id,s.type_name,p.type_name FROM tdb_goods_types AS s LEFT JOIN tdb_goods_types AS  p ON s.parent_id = p.type_id;

-- 查找所有分类及其子类

SELECT p.type_id,p.type_name,s.type_name FROM tdb_goods_types AS p LEFT JOIN tdb_goods_types AS  s ON s.parent_id = p.type_id;

-- 查找所有分类及其子类的数目

SELECT p.type_id,p.type_name,count(s.type_name) AS children_count FROM tdb_goods_types AS p LEFT JOIN tdb_goods_types AS s ON s.parent_id = p.type_id GROUP BY p.type_name ORDER BY p.type_id;

-- 为tdb_goods_types添加child_count字段

ALTER TABLE tdb_goods_types ADD child_count MEDIUMINT UNSIGNED NOT NULL DEFAULT 0;

-- 将刚才查询到的子类数量更新到tdb_goods_types数据表

UPDATE tdb_goods_types AS t1 INNER JOIN ( SELECT p.type_id,p.type_name,count(s.type_name) AS children_count FROM tdb_goods_types AS p

LEFT JOIN tdb_goods_types AS s ON s.parent_id = p.type_id

GROUP BY p.type_name

ORDER BY p.type_id ) AS t2

ON  t1.type_id = t2.type_id

SET t1.child_count = t2.children_count;

-- 复制编号为12,20的两条记录

SELECT * FROM tdb_goods WHERE goods_id IN (19,20);

-- INSERT ... SELECT实现复制

INSERT tdb_goods(goods_name,cate_id,brand_id) SELECT goods_name,cate_id,brand_id FROM tdb_goods WHERE goods_id IN (19,20);

-- 查找重复记录

SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING count(goods_name) >= 2;

-- 删除重复记录

DELETE t1 FROM tdb_goods AS t1 LEFT JOIN (SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING count(goods_name) >= 2 ) AS t2  ON t1.goods_name = t2.goods_name  WHERE t1.goods_id > t2.goods_id;
a.brand_id = b.brand_id INNER JOIN
goods_cate as c on a.cate_id = c.cate_id;

Drop table goods_brands;