当数据库数据量大的时候，删除重复元素是很讨厌的，有时候怎么也写不对，闲下来整理一下删除
         重复数据的sql，数据表是之前看mysql教学视频的素材，但是找不到哪里的了，如果原作者看到请
         留言。

    -- 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 ×××10 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);

 --插入重复数据
 INSERT tdb_goods(goods_name,cate_id,brand_id) 
 SELECT goods_name,cate_id,brand_id,goods_price FROM tdb_goods
 WHERE goods_id IN (19,20)

 --查询重复数据
 SELECT goods_id,goods_name,goods_price FROM tdb_goods 
 GROUP BY goods_name,goods_price HAVING count(goods_name) >= 2

 --将要删除的id查询出来
 select t1.goods_id from  tdb_goods as t1 left  JOIN
 (SELECT goods_id,goods_name,goods_price FROM tdb_goods 
 GROUP BY goods_name,goods_price HAVING count(goods_name) >= 2) t2 
 on t1.goods_name = t2.goods_name where t1.goods_id>t2.goods_id

    --执行删除
delete t1 from  tdb_goods as t1 left  JOIN 
(SELECT goods_id,goods_name,goods_price FROM tdb_goods 
GROUP BY goods_name,goods_price HAVING count(goods_name) >= 2) t2 
on t1.goods_name = t2.goods_name 
where t1.goods_id>t2.goods_id