set names gbk ;客户端以GBK的编码显示数据（只影响客户端显示的数据，并不影响真实的显示数据）
数据准备：
-- 创建数据表
CREATE TABLE IF NOT EXISTS tdb_goods( goods_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT, goods_name VARCHAR(150) NOT NULL, goods_cate VARCHAR(40) NOT NULL, brand_name VARCHAR(40) NOT NULL, goods_price DECIMAL(15,3) UNSIGNED NOT NULL DEFAULT 0, is_show BOOLEAN NOT NULL DEFAULT 1, is_saleoff BOOLEAN NOT NULL DEFAULT 0 ); -- 写入记录 INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('R510VC 15.6英寸笔记本','笔记本','华硕','3399',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Y400N 14.0英寸笔记本电脑','笔记本','联想','4899',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('G150TH 15.6英寸游戏本','游戏本','雷神','8499',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X550CC 15.6英寸笔记本','笔记本','华硕','2799',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X240(20ALA0EYCD) 12.5英寸超极本','超级本','联想','4999',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('U330P 13.3英寸超极本','超级本','联想','4299',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('SVP13226SCB 13.3英寸触控超极本','超级本','索尼','7999',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iPad mini MD531CH/A 7.9英寸平板电脑','平板电脑','苹果','1998',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iPad Air MD788CH/A 9.7英寸平板电脑 （16G WiFi版）','平板电脑','苹果','3388',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES(' iPad mini ME279CH/A 配备 Retina 显示屏 7.9英寸平板电脑 （16G WiFi版）','平板电脑','苹果','2788',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('IdeaCentre C340 20英寸一体电脑 ','台式机','联想','3499',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Vostro 3800-R1206 台式电脑','台式机','戴尔','2899',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iMac ME086CH/A 21.5英寸一体电脑','台式机','苹果','9188',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('AT7-7414LP 台式电脑 （i5-3450四核 4G 500G 2G独显 DVD 键鼠 Linux ）','台式机','宏碁','3699',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Z220SFF F4F06PA工作站','服务器/工作站','惠普','4288',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('PowerEdge T110 II服务器','服务器/工作站','戴尔','5388',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Mac Pro MD878CH/A 专业级台式电脑','服务器/工作站','苹果','28888',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES(' HMZ-T3W 头戴显示设备','笔记本配件','索尼','6999',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('商务双肩背包','笔记本配件','索尼','99',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X3250 M4机架式服务器 2583i14','服务器/工作站','IBM','6888',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('玄龙精英版 笔记本散热器','笔记本配件','九州风神','',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES(' HMZ-T3W 头戴显示设备','笔记本配件','索尼','6999',DEFAULT,DEFAULT); INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('商务双肩背包','笔记本配件','索尼','99',DEFAULT,DEFAULT);
1，比较运算引发的子查询（出现在其他sql语句内的select子句）
SELECT avg(goods_price) from tdb_goods; //计算tdb_goods表中字段goods_price的平均值
SELECT ROUND(avg(goods_price),2) from tdb_goods;//计算tdb_goods表中字段goods_price的平均值，并且四舍五入到小数点后面2位
SELECT * from tdb_goods where goods_price >= (SELECT ROUND(avg(goods_price),2) from tdb_goods); //查询表中goods_price 大于平均值的记录
2，any，some，all
//查询价格大于或等于"超级本"价格的商品，并且按价格降序排列 (因为子查询查出的超基本有几条数据， 因此any的意思是，只要价格大于或等于其中一条都可以)
SELECT goods_id,goods_name,goods_price FROM tdb_goods
WHERE goods_price = ANY(SELECT goods_price FROM tdb_goods WHERE goods_cate = '超级本') ORDER BY goods_price DESC;3， = ANY 或 = SOME 等价于 IN （效果和上面的那条SQL一样） SELECT SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price IN (SELECT goods_price FROM tdb_goods WHERE goods_cate = '超级本') ORDER BY goods_price DESC; 4， 使用insert 。。。select 插入记录 （将查询的结果写入到数据表） select goods_cate from tdb_goods GROUP BY goods_cate //查出表中有多少分类 insert tdb_goods_cates(cate_name) SELECT goods_cate from tdb_goods GROUP BY goods_cate; //将查询出来的分类，插入数据分类表tdb_goods_cates中的字段cate_name
5，多表更新（参照tdb_goods_cates 表更新tdb_goods 表）
//将表tdb_goods中的goods_cate（原先是中文的描述，现在映射成为tdb_goods_cates 表中的id，该id对应的cate_name 就是中文类别描述） 修改成tdb_goods_cates 表中的cate_id
UPDATE tdb_goods INNER JOIN tdb_goods_cates on goods_cate = cate_name SET
goods_cate = cate_id;6，多表更新一步到位（创建数据表的同时，将输入插入到数据表）
//创建表tdb_goods_brands ，并且将tdb_goods 表中的brand_name 分组，然后插入到tdb_goods_brands 中
CREATE TABLE tdb_goods_brands ( brand_id SMALLINT UNSIGNED PRIMARY KEY auto_increment, brand_name VARCHAR (40) NOT NULL ) SELECT brand_name FROM tdb_goods GROUP BY brand_name;//tdb_goods 表中的brand_name 参照tdb_goods_brands 表中的brand_name更新，AS可以给表取别名
UPDATE tdb_goods as g
INNER JOIN tdb_goods_brands as b ON g.brand_name = b.brand_name SET g.brand_name = b.brand_id;7，内连接inner join（多张表查询，为用户呈现可读字段）,使用on来设定连接条件
内连接：只显示A表和B表中的数据的交集部分
//连接两张表，将表tdb_goods 中goods_cate 在表tdb_goods_cates 对应的cate_name 查询出来
SELECT goods_id, goods_name,cate_name from tdb_goods INNER JOIN tdb_goods_cates
on tdb_goods.goods_cate = tdb_goods_cates.cate_id;8，外连接（分为左外连接和右外连接）
a，左外连接
SELECT goods_id, goods_name,cate_name from tdb_goods left JOIN tdb_goods_cateson tdb_goods.goods_cate = tdb_goods_cates.cate_id;
SELECT goods_id, goods_name,cate_name from tdb_goods right JOIN tdb_goods_cates on tdb_goods.goods_cate = tdb_goods_cates.cate_id;
9，多表连接（）
// 查询所有商品的详细信息(通过内连接实现)
SELECT goods_id,goods_name,cate_name,brand_name,goods_price FROM tdb_goods AS g INNER JOIN tdb_goods_cates AS c ON g.cate_id = c.cate_id INNER JOIN tdb_goods_brands AS b ON g.brand_id = b.brand_id;
10,无限级分类的设计（比如：大家电属于家用电器）
//数据准备
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 ); 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); 
//查出上面这张表的子类以及子类对应的父类（没有父类的为null）
-- 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;