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) unsigned\zerofill; 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……), (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=1\2 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\rollback 触发器 trigger 监视地点:表 监视行为:增 删 改 触发时间:after\before 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: +----------+------------------------------+--------+----------+-----------+--------------+------------+-------------+ | goods_id | goods_name | cat_id | brand_id | goods_sn | goods_number | shop_price | click_count | +----------+------------------------------+--------+----------+-----------+--------------+------------+-------------+ | 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%'; 1.15 一道面试题 有如下表和数据,查出num>=20 and num<=39的数字, 并且,把num值处于[20,29]之间,显示为20 num值处于[30,39]之间的,显示30 mian表 +------+ | num | +------+ | 3 | | 12 | | 15 | | 25 | | 23 | | 29 | | 34 | | 37 | | 32 | | 45 | | 48 | | 52 | +------+ 1.16 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 | | 张三 | 1 | | 李四 | 1 | | 李四 | 1 | | 王五 | 1 | | 赵六 | 0 | | 赵六 | 0 | | 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 | 公益联队 | +------+----------+ 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; +------+----------+------+------+----------+------------+ | hid | hname | mres | gid | gname | matime | +------+----------+------+------+----------+------------+ | 1 | 国安 | 2:0 | 2 | 申花 | 2006-05-21 | | 2 | 申花 | 1:2 | 3 | 公益联队 | 2006-06-21 | | 3 | 公益联队 | 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 | +------+------+ 要求查询出以下效果: +------+----------+ | id | num | +------+----------+ | a | 5 | | b | 15 | | c | 30 | | d | 30 | | e | 99 | +------+----------+ create table a ( id char(1), num int )engine myisam charset utf8; insert into a values ('a',5),('b',10),('c',15),('d',10); create table b ( id char(1), num int )engine myisam charset utf8; insert into b values ('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 | +------+------+ 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 | 30 | | 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; 7.5 用exists型子查询,查出所有有商品的栏目 select * from category where exists (select * from goods where goods.cat_id=category.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 2012-03-25更新,添加了面试案例