mysql数据库入门教程之学习笔记

时间：2013-10-17 编辑：简简单单来源：一聚教程网

mysql复习

一:复习前的准备

1:确认你已安装wamp

2:确认你已安装ecshop,并且ecshop的数据库名为shop

二基础知识:

1.数据库的连接

mysql -u -p -h

-u 用户名

-p 密码

-h host主机

2:库级知识

2.1 显示数据库: show databases;

2.2 选择数据库: use dbname;

2.3 创建数据库: create database dbname charset utf8;

2.3 删除数据库: drop database dbname;

3: 表级操作:

3.1 显示库下面的表

show tables;

3.2 查看表的结构:

desc tableName;

3.3 查看表的创建过程:

show create table tableName;

3.4 创建表:

create table tbName (

列名称1　列类型　[列参数]　[not null default ],

....列2...

....

列名称N　列类型　[列参数]　[not null default ]

)engine myisam/innodb charset utf8/gbk

3.4的例子:

create table user (

id int auto_increment,

name varchar(20) not null default "",

age tinyint unsigned not null default 0,

index id (id)

)engine=innodb charset=utf8;

注:innodb是表引擎,也可以是myisam或其他,但最常用的是myisam和innodb,

charset 常用的有utf8,gbk;

3.5 修改表

3.5.1 修改表之增加列:

alter table tbName

add 列名称１　列类型　[列参数]　[not null default ]　#(add之后的旧列名之后的语法和创建表时的列声明一样)

3.5.2 修改表之修改列

alter table tbName

change 旧列名新列名列类型　[列参数]　[not null default ]

(注:旧列名之后的语法和创建表时的列声明一样)

3.5.3 修改表之减少列:

alter table tbName

drop 列名称;

3.5.4 修改表之增加主键

alter table tbName add primary key(主键所在列名);

例:alter table goods add primary key(id)

该例是把主键建立在id列上

3.5.5 修改表之删除主键

alter table tbName　drop primary key;

3.5.6 修改表之增加索引

alter table tbName add [unique|fulltext] index 索引名(列名);

3.5.7 修改表之删除索引

alter table tbName drop index 索引名;

3.5.8 清空表的数据

truncate tableName;

4:列类型讲解

列类型:

整型:tinyint (0~255/-128~127) smallint (0~65535/-32768~32767) mediumint int bigint (参考手册11.2)

参数解释:

unsigned 无符号(不能为负) zerofill 0填充 M 填充后的宽度

举例:tinyint unsigned;

tinyint(6) zerofill;

数值型

浮点型:float double

格式:float(M,D) unsignedzerofill;

字符型

char(m) 定长

varchar(m)变长

text

列实存字符i 实占空间利用率

char(M) 0<=i<=M M i/m<=100%

varchar(M) 0<=i<=M i+1,2 i/i+1/2<100%

year YYYY 范围:1901~2155. 可输入值2位和4位(如98,2012)

日期时间类型 date YYYY-MM-DD 如:2010-03-14

time HH:MM:SS 如:19:26:32

datetime YYYY-MM-DD HH:MM:SS 如:2010-03-14 19:26:32

timestamp YYYY-MM-DD HH:MM:SS 特性:不用赋值,该列会为自己赋当前的具体时间

5:增删改查基本操作

5.1 插入数据

insert into 表名(col1,col2,……) values(val1,val2……); -- 插入指定列

insert into 表名 values (,,,,); -- 插入所有列

insert into 表名 values -- 一次插入多行

(val1,val2……),

(val1,val2……);

5.3修改数据

update tablename

set

col1=newval1,

col2=newval2,

...

colN=newvalN

where 条件;

5.4，删除数据 delete from tablenaeme where 条件;

5.5， select 查询

（1）条件查询 where a. 条件表达式的意义，表达式为真，则该行取出

b. 比较运算符 = ，!=，< > <= >=

c. like , not like ("%"匹配任意多个字符,"_"匹配任意单个字符)

in , not in , between and

d. is null , is not null

（2）分组 group by

一般要配合5个聚合函数使用:max,min,sum,avg,count

（3）筛选 having

（4）排序 order by

（5）限制 limit

6: 连接查询

6.1，左连接

.. left join .. on

table A left join table B on tableA.col1 = tableB.col2 ;

例句:

select 列名 from table A left join table B on tableA.col1 = tableB.col2

2. 右链接: right join

3. 内连接: inner join

左右连接都是以在左边的表的数据为准,沿着左表查右表.

内连接是以两张表都有的共同部分数据为准,也就是左右连接的数据之交集.

7 子查询

where 型子查询:内层sql的返回值在where后作为条件表达式的一部分

例句: select * from tableA where colA = (select colB from tableB where ...);

from 型子查询:内层sql查询结果,作为一张表,供外层的sql语句再次查询

例句:select * from (select * from ...) as tableName where ....

8: 字符集

客服端sql编码 character_set_client

服务器转化后的sql编码 character_set_connection

服务器返回给客户端的结果集编码 character_set_results

快速把以上3个变量设为相同值: set names 字符集

存储引擎 engine=12

1 Myisam 速度快不支持事务回滚

2 Innodb 速度慢支持事务,回滚

①开启事务 start transaction

②运行sql;

③提交,同时生效回滚 commit

ollback

触发器 trigger

监视地点:表

监视行为:增删改

触发时间:afterefore

触发事件:增删改

创建触发器语法

create trigger tgName

after/before insert/delete/update

on tableName

for each row

sql; -- 触发语句

删除触发器:drop trigger tgName;

索引

提高查询速度,但是降低了增删改的速度,所以使用索引时,要综合考虑.

索引不是越多越好,一般我们在常出现于条件表达式中的列加索引.

值越分散的列，索引的效果越好

索引类型

primary key主键索引

index 普通索引

unique index 唯一性索引

fulltext index 全文索引

综合练习:

连接上数据库服务器

创建一个gbk编码的数据库

建立商品表和栏目表,字段如下:

商品表:goods

goods_id　--主键,

goods_name -- 商品名称

cat_id -- 栏目id

brand_id -- 品牌id

goods_sn -- 货号

goods_number -- 库存量

shop_price -- 价格

goods_desc　--商品详细描述

栏目表:category

cat_id --主键

cat_name -- 栏目名称

parent_id -- 栏目的父id

建表完成后,作以下操作:

删除goods表的goods_desc 字段,及货号字段

并增加字段:click_count -- 点击量

在goods_name列上加唯一性索引

在shop_price列上加普通索引

在clcik_count列上加普通索引

删除click_count列上的索引

对goods表插入以下数据:

+----------+------------------------------+--------+----------+-----------+--------------+------------+-------------+

+----------+------------------------------+--------+----------+-----------+--------------+------------+-------------+

| 1 | KD876 | 4 | 8 | ECS000000 | 10 | 1388.00 | 7 |

| 4 | 诺基亚N85原装充电器 | 8 | 1 | ECS000004 | 17 | 58.00 | 0 |

| 3 | 诺基亚原装5800耳机 | 8 | 1 | ECS000002 | 24 | 68.00 | 3 |

| 5 | 索爱原装M2卡读卡器 | 11 | 7 | ECS000005 | 8 | 20.00 | 3 |

| 6 | 胜创KINGMAX内存卡 | 11 | 0 | ECS000006 | 15 | 42.00 | 0 |

| 7 | 诺基亚N85原装立体声耳机HS-82 | 8 | 1 | ECS000007 | 20 | 100.00 | 0 |

| 8 | 飞利浦9@9v | 3 | 4 | ECS000008 | 17 | 399.00 | 9 |

| 9 | 诺基亚E66 | 3 | 1 | ECS000009 | 13 | 2298.00 | 20 |

| 10 | 索爱C702c | 3 | 7 | ECS000010 | 7 | 1328.00 | 11 |

| 11 | 索爱C702c | 3 | 7 | ECS000011 | 1 | 1300.00 | 0 |

| 12 | 摩托罗拉A810 | 3 | 2 | ECS000012 | 8 | 983.00 | 14 |

| 13 | 诺基亚5320 XpressMusic | 3 | 1 | ECS000013 | 8 | 1311.00 | 13 |

| 14 | 诺基亚5800XM | 4 | 1 | ECS000014 | 4 | 2625.00 | 6 |

| 15 | 摩托罗拉A810 | 3 | 2 | ECS000015 | 3 | 788.00 | 8 |

| 16 | 恒基伟业G101 | 2 | 11 | ECS000016 | 0 | 823.33 | 3 |

| 17 | 夏新N7 | 3 | 5 | ECS000017 | 1 | 2300.00 | 2 |

| 18 | 夏新T5 | 4 | 5 | ECS000018 | 1 | 2878.00 | 0 |

| 19 | 三星SGH-F258 | 3 | 6 | ECS000019 | 0 | 858.00 | 7 |

| 20 | 三星BC01 | 3 | 6 | ECS000020 | 13 | 280.00 | 14 |

| 21 | 金立 A30 | 3 | 10 | ECS000021 | 40 | 2000.00 | 4 |

| 22 | 多普达Touch HD | 3 | 3 | ECS000022 | 0 | 5999.00 | 15 |

| 23 | 诺基亚N96 | 5 | 1 | ECS000023 | 8 | 3700.00 | 17 |

| 24 | P806 | 3 | 9 | ECS000024 | 148 | 2000.00 | 36 |

| 25 | 小灵通/固话50元充值卡 | 13 | 0 | ECS000025 | 2 | 48.00 | 0 |

| 26 | 小灵通/固话20元充值卡 | 13 | 0 | ECS000026 | 2 | 19.00 | 0 |

| 27 | 联通100元充值卡 | 15 | 0 | ECS000027 | 2 | 95.00 | 0 |

| 28 | 联通50元充值卡 | 15 | 0 | ECS000028 | 0 | 45.00 | 0 |

| 29 | 移动100元充值卡 | 14 | 0 | ECS000029 | 0 | 90.00 | 0 |

| 30 | 移动20元充值卡 | 14 | 0 | ECS000030 | 9 | 18.00 | 1 |

| 31 | 摩托罗拉E8 | 3 | 2 | ECS000031 | 1 | 1337.00 | 5 |

| 32 | 诺基亚N85 | 3 | 1 | ECS000032 | 1 | 3010.00 | 9 |

+----------+------------------------------+--------+----------+-----------+--------------+------------+-------------+

三查询知识

注:以下查询基于ecshop网站的商品表(ecs_goods)

在练习时可以只取部分列,方便查看.

1: 基础查询 where的练习:

查出满足以下条件的商品

1.1:主键为32的商品

select goods_id,goods_name,shop_price

from ecs_goods

where goods_id=32;

1.2:不属第3栏目的所有商品

select goods_id,cat_id,goods_name,shop_price from ecs_goods

where cat_id!=3;

1.3:本店价格高于3000元的商品

select goods_id,cat_id,goods_name,shop_price from ecs_goods

where shop_price >3000;

1.4:本店价格低于或等于100元的商品

select goods_id,cat_id,goods_name,shop_price from ecs_goods where shop_price <=100;

1.5:取出第4栏目或第11栏目的商品(不许用or)

select goods_id,cat_id,goods_name,shop_price from ecs_goods

where cat_id in (4,11);

1.6:取出100<=价格<=500的商品(不许用and)

select goods_id,cat_id,goods_name,shop_price from ecs_goods

where shop_price between 100 and 500;

1.7:取出不属于第3栏目且不属于第11栏目的商品(and,或not in分别实现)

select goods_id,cat_id,goods_name,shop_price from ecs_goods where cat_id!=3 and cat_id!=11;

select goods_id,cat_id,goods_name,shop_price from ecs_goods where cat_id not in (3,11);

1.8:取出价格大于100且小于300,或者大于4000且小于5000的商品()

select goods_id,cat_id,goods_name,shop_price from ecs_goods where shop_price>100 and shop_price <300 or shop_price >4000 and shop_price <5000;

1.9:取出第3个栏目下面价格<1000或>3000,并且点击量>5的系列商品

select goods_id,cat_id,goods_name,shop_price,click_count from ecs_goods where

cat_id=3 and (shop_price <1000 or shop_price>3000) and click_count>5;

1.10:取出第1个栏目下面的商品(注意:1栏目下面没商品,但其子栏目下有)

select goods_id,cat_id,goods_name,shop_price,click_count from ecs_goods

where cat_id in (2,3,4,5);

1.11:取出名字以"诺基亚"开头的商品

select goods_id,cat_id,goods_name,shop_price from ecs_goods where goods_name like "诺基亚%";

1.12:取出名字为"诺基亚Nxx"的手机

select goods_id,cat_id,goods_name,shop_price from ecs_goods

where goods_name like "诺基亚N__";

1.13:取出名字不以"诺基亚"开头的商品

select goods_id,cat_id,goods_name,shop_price from ecs_goos

where goods_name not like "诺基亚%";

1.14:取出第3个栏目下面价格在1000到3000之间,并且点击量>5 "诺基亚"开头的系列商品

select goods_id,cat_id,goods_name,shop_price from ecs_goods where

cat_id=3 and shop_price>1000 and shop_price <3000 and click_count>5 and goods_name like "诺基亚%";

select goods_id,cat_id,goods_name,shop_price from ecs_goods where

shop_price between 1000 and 3000 and cat_id=3 and click_count>5 and goods_name like "诺基亚%";

一道面试题

有如下表和数组

把num值处于[20,29]之间,改为20

num值处于[30,39]之间的,改为30

mian表

+------+

| num |

+------+

| 3 |

| 12 |

| 15 |

| 25 |

| 23 |

| 29 |

| 34 |

| 37 |

| 32 |

| 45 |

| 48 |

| 52 |

+------+

练习题:

把good表中商品名为"诺基亚xxxx"的商品,改为"HTCxxxx",

提示:大胆的把列看成变量,参与运算,甚至调用函数来处理 .

substring(),concat()

2 分组查询group:

2.1:查出最贵的商品的价格

select max(shop_price) from ecs_goods;

2.2:查出最大(最新)的商品编号

select max(goods_id) from ecs_goods;

2.3:查出最便宜的商品的价格

select min(shop_price) from ecs_goods;

2.4:查出最旧(最小)的商品编号

select min(goods_id) from ecs_goods;

2.5:查询该店所有商品的库存总量

select sum(goods_number) from ecs_goods;

2.6:查询所有商品的平均价

select avg(shop_price) from ecs_goods;

2.7:查询该店一共有多少种商品

select count(*) from ecs_goods;

2.8:查询每个栏目下面

最贵商品价格

最低商品价格

商品平均价格

商品库存量

商品种类

提示:(5个聚合函数,sum,avg,max,min,count与group综合运用)

select cat_id,max(shop_price) from ecs_goods group by cat_id;

3 having与group综合运用查询:

3.1:查询该店的商品比市场价所节省的价格

select goods_id,goods_name,market_price-shop_price as j

from ecs_goods ;

3.2:查询每个商品所积压的货款(提示:库存*单价)

select goods_id,goods_name,goods_number*shop_price from ecs_goods

3.3:查询该店积压的总货款

select sum(goods_number*shop_price) from ecs_goods;

3.4:查询该店每个栏目下面积压的货款.

select cat_id,sum(goods_number*shop_price) as k from ecs_goods group by cat_id;

3.5:查询比市场价省钱200元以上的商品及该商品所省的钱(where和having分别实现)

select goods_id,goods_name,market_price-shop_price as k from ecs_goods

where market_price-shop_price >200;

select goods_id,goods_name,market_price-shop_price as k from ecs_goods

having k >200;

3.6:查询积压货款超过2W元的栏目,以及该栏目积压的货款

select cat_id,sum(goods_number*shop_price) as k from ecs_goods group by cat_id

having k>20000

3.7:where-having-group综合练习题

有如下表及数据

+------+---------+-------+

| name | subject | score |

+------+---------+-------+

| 张三 | 数学 | 90 |

| 张三 | 语文 | 50 |

| 张三 | 地理 | 40 |

| 李四 | 语文 | 55 |

| 李四 | 政治 | 45 |

| 王五 | 政治 | 30 |

+------+---------+-------+

要求:查询出2门及2门以上不及格者的平均成绩

## 一种错误做法

mysql> select name,count(score<60) as k,avg(score) from stu group by name having k>=2;

+------+---+------------+

| name | k | avg(score) |

+------+---+------------+

| 张三 | 3 | 60.0000 |

| 李四 | 2 | 50.0000 |

+------+---+------------+

2 rows in set (0.00 sec)

mysql> select name,count(score<60) as k,avg(score) from stu group by name;

+------+---+------------+

| name | k | avg(score) |

+------+---+------------+

| 张三 | 3 | 60.0000 |

| 李四 | 2 | 50.0000 |

| 王五 | 1 | 30.0000 |

+------+---+------------+

3 rows in set (0.00 sec)

mysql> select name,count(score<60) as k,avg(score) from stu group by name having k>=2;

+------+---+------------+

| name | k | avg(score) |

+------+---+------------+

| 张三 | 3 | 60.0000 |

| 李四 | 2 | 50.0000 |

+------+---+------------+

2 rows in set (0.00 sec)

#加上赵六后错误暴露

mysql> insert into stu

-> values

-> ("赵六","A",100),

-> ("赵六","B",99),

-> ("赵六","C",98);

Query OK, 3 rows affected (0.05 sec)

Records: 3 Duplicates: 0 Warnings: 0

#错误显现

mysql> select name,count(score<60) as k,avg(score) from stu group by name having k>=2;

+------+---+------------+

| name | k | avg(score) |

+------+---+------------+

| 张三 | 3 | 60.0000 |

| 李四 | 2 | 50.0000 |

| 赵六 | 3 | 99.0000 |

+------+---+------------+

3 rows in set (0.00 sec)

#正确思路,先查看每个人的平均成绩

mysql> select name,avg(score) from stu group by name;

+------+------------+

| name | avg(score) |

+------+------------+

| 张三 | 60.0000 |

| 李四 | 50.0000 |

| 王五 | 30.0000 |

| 赵六 | 99.0000 |

+------+------------+

4 rows in set (0.00 sec)

mysql> # 看每个人挂科情况

mysql> select name,score < 60 from stu;

+------+------------+

| name | score < 60 |

+------+------------+

| 张三 | 0 |

| 张三 | 1 |

| 李四 | 1 |

| 王五 | 1 |

| 赵六 | 0 |

+------+------------+

9 rows in set (0.00 sec)

mysql> #计算每个人的挂科科目

mysql> select name,sum(score < 60) from stu group by name;

+------+-----------------+

| name | sum(score < 60) |

+------+-----------------+

| 张三 | 2 |

| 李四 | 2 |

| 王五 | 1 |

| 赵六 | 0 |

+------+-----------------+

4 rows in set (0.00 sec)

#同时计算每人的平均分

mysql> select name,sum(score < 60),avg(score) as pj from stu group by name;

+------+-----------------+---------+

| name | sum(score < 60) | pj |

+------+-----------------+---------+

| 张三 | 2 | 60.0000 |

| 李四 | 2 | 50.0000 |

| 王五 | 1 | 30.0000 |

| 赵六 | 0 | 99.0000 |

+------+-----------------+---------+

4 rows in set (0.00 sec)

#利用having筛选挂科2门以上的.

mysql> select name,sum(score < 60) as gk ,avg(score) as pj from stu group by name having gk >=2;

+------+------+---------+

| name | gk | pj |

+------+------+---------+

| 张三 | 2 | 60.0000 |

| 李四 | 2 | 50.0000 |

+------+------+---------+

2 rows in set (0.00 sec)

4: order by 与 limit查询

4.1:按价格由高到低排序

select goods_id,goods_name,shop_price from ecs_goods order by shop_price desc;

4.2:按发布时间由早到晚排序

select goods_id,goods_name,add_time from ecs_goods order by add_time;

4.3:接栏目由低到高排序,栏目内部按价格由高到低排序

select goods_id,cat_id,goods_name,shop_price from ecs_goods

order by cat_id ,shop_price desc;

4.4:取出价格最高的前三名商品

select goods_id,goods_name,shop_price from ecs_goods order by shop_price desc limit 3;

4.5:取出点击量前三名到前5名的商品

select goods_id,goods_name,click_count from ecs_goods order by click_count desc limit 2,3;

5 连接查询

5.1:取出所有商品的商品名,栏目名,价格

select goods_name,cat_name,shop_price from

ecs_goods left join ecs_category

on ecs_goods.cat_id=ecs_category.cat_id;

5.2:取出第4个栏目下的商品的商品名,栏目名,价格

select goods_name,cat_name,shop_price from

ecs_goods left join ecs_category

on ecs_goods.cat_id=ecs_category.cat_id

where ecs_goods.cat_id = 4;

5.3:取出第4个栏目下的商品的商品名,栏目名,与品牌名

select goods_name,cat_name,brand_name from

ecs_goods left join ecs_category

on ecs_goods.cat_id=ecs_category.cat_id

left join ecs_brand

on ecs_goods.brand_id=ecs_brand.brand_id

where ecs_goods.cat_id = 4;

5.4: 用友面试题

根据给出的表结构按要求写出SQL语句。

Match 赛程表

字段名称字段类型描述

matchID int 主键

hostTeamID int 主队的ID

guestTeamID int 客队的ID

matchResult varchar(20) 比赛结果，如（2:0）

matchTime date 比赛开始时间

Team 参赛队伍表

字段名称字段类型描述

teamID int 主键

teamName varchar(20) 队伍名称

Match的hostTeamID与guestTeamID都与Team中的teamID关联

查出 2006-6-1 到2006-7-1之间举行的所有比赛，并且用以下形式列出：

拜仁 2：0 不来梅 2006-6-21

mysql> select * from m;

+-----+------+------+------+------------+

| mid | hid | gid | mres | matime |

+-----+------+------+------+------------+

| 1 | 1 | 2 | 2:0 | 2006-05-21 |

| 2 | 2 | 3 | 1:2 | 2006-06-21 |

| 3 | 3 | 1 | 2:5 | 2006-06-25 |

| 4 | 2 | 1 | 3:2 | 2006-07-21 |

+-----+------+------+------+------------+

4 rows in set (0.00 sec)

mysql> select * from t;

+------+----------+

| tid | tname |

+------+----------+

| 1 | 国安 |

| 2 | 申花 |

| 3 | 传智联队 |

+------+----------+

3 rows in set (0.00 sec)

mysql> select hid,t1.tname as hname ,mres,gid,t2.tname as gname,matime

-> from

-> m left join t as t1

-> on m.hid = t1.tid

-> left join t as t2

-> on m.gid = t2.tid;

+------+----------+------+------+----------+------------+

+------+----------+------+------+----------+------------+

| 1 | 国安 | 2:0 | 2 | 申花 | 2006-05-21 |

| 2 | 申花 | 1:2 | 3 | 传智联队 | 2006-06-21 |

| 3 | 传智联队 | 2:5 | 1 | 国安 | 2006-06-25 |

| 2 | 申花 | 3:2 | 1 | 国安 | 2006-07-21 |

+------+----------+------+------+----------+------------+

4 rows in set (0.00 sec)

6 union查询

6.1:把ecs_comment,ecs_feedback两个表中的数据,各取出4列,并把结果集union成一个结果集.

6.2:3期学员碰到的一道面试题

A表:

+------+------+

| id | num |

+------+------+

| a | 5 |

| b | 10 |

| c | 15 |

| d | 10 |

+------+------+

B表:

+------+------+

| id | num |

+------+------+

| b | 5 |

| c | 15 |

| d | 20 |

| e | 99 |

+------+------+

mysql> # 合并 ,注意all的作用

mysql> select * from ta

-> union all

-> select * from tb;

+------+------+

| id | num |

+------+------+

| a | 5 |

| b | 10 |

| c | 15 |

| d | 10 |

| b | 5 |

| c | 15 |

| d | 20 |

| e | 99 |

+------+------+

要求查询出以下效果:

+------+----------+

| id | sum(num) |

+------+----------+

| a | 5 |

| b | 15 |

| c | 30 |

| d | 30 |

| e | 99 |

+------+----------+

参考答案:

mysql> # sum,group求和

mysql> select id,sum(num) from (select * from ta union all select * from tb) as tmp group by id;

+------+----------+

| id | sum(num) |

+------+----------+

| a | 5 |

| b | 15 |

| c | 25 |

| d | 30 |

| e | 99 |

+------+----------+

5 rows in set (0.00 sec)

7: 子查询:

7.1:查询出最新一行商品(以商品编号最大为最新,用子查询实现)

select goods_id,goods_name from

ecs_goods where goods_id =(select max(goods_id) from ecs_goods);

7.2:查询出编号为19的商品的栏目名称(用左连接查询和子查询分别)

7.3:用where型子查询把ecs_goods表中的每个栏目下面最新的商品取出来

select goods_id,goods_name,cat_id from ecs_goods where goods_id in (select max(goods_id) from ecs_goods group by cat_id);

7.4:用from型子查询把ecs_goods表中的每个栏目下面最新的商品取出来

select * from (select goods_id,cat_id,goods_name from ecs_goods order by goods_id desc) as t group by cat_id;

创建触发器:

CREATE trigger tg2

after insert on ord

for each row

update goods set goods_number=goods_number-new.num where id=new.gid

CREATE trigger tg3

after delete on ord

for each row

update goods set goods_number=good_number+old.num where id=old.gid

CREATE trigger tg4

after update on ord

for each row

update goods set goods_number=goods_number+old.num-new.num where id=old.gid

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