616 lines
16 KiB
Markdown
616 lines
16 KiB
Markdown
# 查询数据
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## 一、单表查询
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### 1.查询所有字段
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#### （1）在 select 语句中使用 * 查询所有字段
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语法：select * from 表名;
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#### （2）在 select 语句中指定所有字段（字段名可以变换顺序）
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语法：select f_id,s_id,f_name,f_price from fruits;
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### 2.查询指定字段
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#### （1）查询单个字段：
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语法：select f_name from fruits;
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#### （2）查询多个字段：
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语法：select f_id,s_id,f_name,f_price from fruits;
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### 3.查询指定记录（where）
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可以跟 =,>,<,<>,!=,<=,>=,between,and
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查询 f_name 是 xxxx 的字段
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语法：select f_name,f_price from fruits where f_name='xxxx';
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查询 f_price 不等于 10.2 的字段
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语法：select f_name,f_price from fruits where f_price<>10.2;
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查询 f_price 在5和10之间的字段
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语法：select f_name,f_price from fruits where f_price between 5 and 10;**（包含端点值）**
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查询 f_price 不在5和10之间的字段
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语法：select f_name,f_price from fruits where f_price not between 5 and 10;**（不包含端点值）**
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### 4.使用in关键字查询
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使用 in 关键字将检索条件括起来，检索条件之间用逗号隔开，只需要满足条件范围内一个值即为匹配项。
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语法：select s_id,f_name,f_price from fruits where s_id in(101,102);
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**可以使用关键字 not in 检索不在条件范围内的记录**
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语法：select s_id,f_name,f_price from fruits where s_id not in(101,103);
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### 5.带 like 的字符匹配查询（ % _ ）
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#### （1）%匹配任意长度的字符，包括0个字符
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以b开头字符查询
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语法：select f_name,f_price from fruits where f_name like 'b%';
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查询水果名中包含p字符的记录
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语法：select f_name,f_price from fruits where f_name like '%p%';
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查询以b开头以y结尾
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语法：select f_name,f_price from fruits where f_name like 'b%y';
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查询以y结尾且y的前面有且仅有4个字母的记录
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语法：select f_name,f_price from fruits where f_name like '____y';
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### 6.查询空值（is 【not】null）
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查询 f_name 为空
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语法：select f_name,f_price from fruits where f_name is null;
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查询 f_name 不为空
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语法：select f_name,f_price from fruits where f_name is not null;
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### 7.带 and 的多条件查询（相当于‘与’，同时满足多个条件）
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查询id为101同时价格大于等于5的字段
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语法：select f_name,f_price from fruits where s_id=101 and f_price>=5;
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语法：select s_id,f_name,f_price from fruits where s_id=101 and f_price>=5 and f_name like 'a%';
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### 8.带 or 的多条件查询（相当于‘或’，满足一个条件即可）
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查询i为101或者103的字段
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语法：select s_id,f_name,f_price from fruits where s_id=101 or s_id=103;
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语法：select s_id,f_name,f_price from fruits where s_id in(101,103);
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这里可以用**in**操作符实现相同功能
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**and 的优先级高于 or**
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**在实际开发中使用 in 操作符开发语句，更加简单明了，并且 in 执行速度要快于 or ，更重要的是使用 in 操作符，可以执行更加复杂的嵌套查询**
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### 9.查询结果不重复（distinct）
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查询 id 不重复
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语法：select distinct s_id from fruits ;
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### 10.对查询结果排序(默认升序【asc】，降序【desc】)
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#### （1）单列排序
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升序
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语法： select distinct s_id from fruits order by s_id;
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降序
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语法：select distinct s_id from fruits order by s_id desc;
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语法：select s_id,f_name from fruits order by f_name;
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#### （2）多列排序
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升序
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语法：select s_id,f_name from fruits order by s_id,f_name;
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先按照 s_id 排序， s_id 相同时再按照 s_name 排序
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降序
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语法：select s_id,f_name from fruits order by s_id desc,f_name desc;
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### 11.分组查询（group by，通常和集合函数配合使用）
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#### （1）创建分组
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语法：select s_id,count(*) from fruits group by s_id;
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语法：select s_id,count(*) as total from fruits group by s_id;
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as 后面是别名
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若查看每个供应商提供的水果种类名称，可以在group by中使用group_concat()函数，将每个分组中各
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个字段的值列举出来，例如：语法：select s_id,group_concat(f_name) from fruits group by s_id;
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语法：select s_id,group_concat(f_name) as names from fruits group by s_id;
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#### （2）使用having过滤分组
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语法：select s_id,group_concat(f_name) as names from fruits group by s_id having count(f_name)>1;、
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分组后过滤用having，分组前用where进行分组
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select s_id,group_concat(f_name) as names from fruits where s_id>=104 group by s_id having count(f_name)>1;
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#### （3）在group by子句中使用 with rollup
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在查出所☐一条记录，该记录出的所有记录的总和
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语法： select s_id,count(*) from fruits group by s_id with rollup;
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**注意： with rollup和order by不能同时使用互斥**
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#### （4）多字段分组
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语法：select s_id,f_name,count(*) from fruits group by s_id,f_name;
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#### （5）group by 和 order by一起使用
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语法：select s_id,count(*) from fruits group by s_id order by s_id;
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先分组再排序
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### 12.使用limit限制查询结果数量
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语法：select * from fruits limit 4,3;
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语法：select * from fruits limit 3 offset 4;
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4代表是起始位置，3是指三条记录
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语法： select * from fruits limit 3;
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第一个参数（位置偏移量）可以省略不写，表示返回前三条记录
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## 二、使用集合函数查询
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### 1.count()函数
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（1）select count (*) as name from 表名;
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计算表中总行数不管是不是空值
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（2）count(字段名)
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计算指定列下的总行数，忽略空值
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（3）和group by关键字一起使用
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语法：select o_num,count(*) from orderitems group by o_num;
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select sum(quantity) from orderitems where o_num=3005;
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### 2.sum()函数（求和）
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语法：select o_num,sum(quantity) from orderitems group by o_num;
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### 3.avg()函数（平均值）
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语法：select o_num,avg(item_price) from orderitems group by o_num;
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### 4.max()函数（最大值）
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select o_num,max(item_price) from orderitems group by o_num;
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### 5.min()函数（最小值）
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select o_num,min(item_price) from orderitems group by o_num;
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max和min不仅可以对数值类型，也可对字符类型求最大最小
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## 三、连接查询
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语法：
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select 查询的列 from 表名 连接类型 表名2 on 连接条件
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【where 筛选条件】【group by 分组字段（having分组后筛选条件）】【order by 排序字段】
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### 1.内连接查询（inner join=join）
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不满足 on 后连接条件的会直接过滤掉
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select student.name,class.name from student join class on student.class_id=class.id where student.name like '%小%';
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也可以自连接
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### 2.外连接查询
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分为一个主表和一个从表
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外连接的查询结果为主表中的所有记录
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如果从表中有和它匹配的，则显示匹配结果
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如果从表中没有和它匹配的，则显示null
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#### （1）左外连接（left outer join=left join）
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select *from student left join class on student.class_id=class.id;//以左表为主若没有与它匹配的是空
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#### （2）右外连接（right outer join=right join）
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select *from student right join class on student.class_id=class.id;//以右表为主若没有与它匹配的是空
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### 3.符合条件连接查询
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select student.name,class.name from student join class on student.class_id=class_id where student.name like "%小%
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等价于
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select student.name,class.name from student, class where student.class_id=class_id and student.name like %小%
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等价于
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select student.name,class.name from student join class on student.class_id-class_id and student.name like %小%
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seletc * from student as s1 join student as s2 on s1.id=s2.id；//自连接
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## 四、子查询
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## 五、合并查询结果
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## 六、为表和字段取别名
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## 七、使用正则表达式查询
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## 八、综合案例讲解
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## 上课代码
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```mysql
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show database;
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use text2;
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```
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```mysql
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create table fruits(
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f_id char(10) not null primary key,
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s_id int not null,
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f_name char(25) not null,
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f_price decimal(8,2) not null
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);
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desc fruits;
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```
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```mysql
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insert into fruits
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values('a1',101,'apple',10.2),
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('b1',101,'blackberry,',5.2),
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('bs1',102,'orange',11.2),
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('bs2',105,'melon',8.2),
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('t1',102,'banana',10.3),
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('t2',102,'grape',5.3),
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('o2',103,'cocount',9.2),
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('c0',101,'chaerry',3.2),
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('a2',103,'apricot',2.2),
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('i2',104,'lemon',6.2),
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('d2',104,'berry',7.6),
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('m1',106,'mango',15.6),
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('m2',105,'xbabay',2.6),
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('t4',107,'xbababa',3.6),
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('m3',105,'xxtt',11.6),
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('b5',107,'xxx',3.6);
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```
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```mysql
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select * from fruits;
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```
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```mysql
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select f_id,s_id,f_name,f_price from fruits;
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```
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```mysql
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select f_name,f_price from fruits where f_price=10.2;
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select f_name,f_price from fruits where f_price>10.2;
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select f_name,f_price from fruits where f_price<>10.2;
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```
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```mysql
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select f_name,f_price from fruits where f_name='xxtt';
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```
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```mysql
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select f_name,f_price from fruits where f_price between 5 and 10;
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```
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```mysql
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select s_id,f_name,f_price from fruits where s_id in(101,102);
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```
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```mysql
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select s_id,f_name,f_price from fruits where s_id not in(101,103);
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```
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```mysql
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select f_name,f_price from fruits where f_price not between 5 and 10;
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```
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```mysql
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select f_name,f_price from fruits where f_name like 'b%';
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```
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```mysql
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select f_name,f_price from fruits where f_name like 'b%y';
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```
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```mysql
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select f_name,f_price from fruits where f_name like '____y';
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```
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```mysql
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select f_name,f_price from fruits where f_name is not null;
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```
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```mysql
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select f_name,f_price from fruits where s_id=101 and f_price>=5;
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```
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```mysql
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select s_id,f_name,f_price from fruits where s_id=101 and f_price>=5 and f_name like 'a%';
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```
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```mysql
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select s_id,f_name,f_price from fruits where s_id=101 or s_id=103;
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```
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```mysql
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select distinct s_id from fruits ;
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```
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```mysql
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select distinct s_id from fruits order by s_id asc;
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select distinct s_id from fruits order by s_id desc;
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```
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```mysql
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select s_id,f_name from fruits order by f_name;
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```
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```mysql
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select s_id,f_name from fruits order by s_id,f_name;
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```
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```mysql
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select s_id,count(*) from fruits group by s_id;
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```
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```mysql
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select s_id,count(*) as total from fruits group by s_id;
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```
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```mysql
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select s_id,group_concat(f_name) from fruits group by s_id;
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```
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```mysql
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select s_id,group_concat(f_name) as names from fruits group by s_id;
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```
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```mysql
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select s_id,group_concat(f_name) as names from fruits group by s_id having count(f_name)>1;
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```
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```mysql
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select s_id,group_concat(f_name) as names from fruits where s_id>=104 group by s_id having count(f_name)>1;
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```
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```mysql
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select s_id,count(*) from fruits group by s_id with rollup;
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```
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```mysql
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select s_id,f_name,count(*) from fruits group by s_id,f_name;
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```
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```mysql
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select s_id,count(*) from fruits group by s_id order by s_id;
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```
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```mysql
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select * from fruits limit 4,3;
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```
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```mysql
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create table orderitems(
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o_num int not null,
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o_item int not null,
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f_id char(10) not null,
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quantity int not null,
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item_price decimal(8,2) not null,
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primary key(o_num,o_item)
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);
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```
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```mysql
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insert into orderitems
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values(3001,1,'a1',10,5.20),
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(3001,2,'b2',3,7.60),
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(3001,3,'bs1',5,11.20),
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(3001,4,'bs2',15,9.20),
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(3002,1,'b3',2,20.00),
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(3003,1,'c0',100,10.00),
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(3004,1,'o2',50,2.50),
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(3005,1,'c0',5,10.00),
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(3005,2,'b1',10,8.99),
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(3005,3,'a2',10,2.20),
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(3005,4,'m1',5,14.99);
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```
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```mysql
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create table customers(
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c_id int not null auto_increment,
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c_name char(50) not null,
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c_address char(50) null,
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c_city char(50) null,
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c_zip char(50) null,
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c_contact char(50) null,
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c_email char(255) null,
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primary key(c_id)
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);
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```
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```
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insert customers values
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(1001,'Redhook','200 Street','Tianjin','300000','Liming','Lming@163.com'),
|
||
(1002,'Stars','333 Fromage lane','Dalian','116000','Zhangbo','Jerry@hotmail.com'),
|
||
(1003,'Netbhood','1 Sunny place','Qingdao','266000','Luocong',''),
|
||
(1004,'J0t0','829 Riverside drive','Haikou','570000','Yangshan','sam@hotmail.com')
|
||
;
|
||
```
|
||
|
||
|
||
|
||
```mysql
|
||
select o_num,count(*) from orderitems group by o_num;
|
||
```
|
||
|
||
|
||
|
||
```mysql
|
||
select sum(quantity) from orderitems where o_num=3005;
|
||
```
|
||
|
||
|
||
|
||
```mysql
|
||
select o_num,sum(quantity) from orderitems group by o_num;
|
||
```
|
||
|
||
|
||
|
||
```mysql
|
||
select o_num,avg(item_price) from orderitems group by o_num;
|
||
```
|
||
|
||
|
||
|
||
```mysql
|
||
select o_num,max(item_price) from orderitems group by o_num;
|
||
```
|
||
|
||
|
||
|
||
```mysql
|
||
select o_num,min(item_price) from orderitems group by o_num;
|
||
```
|
||
|
||
|
||
|
||
```mysql
|
||
select min(f_name) from fruits;
|
||
```
|
||
|
||
|
||
|
||
```mysql
|
||
在fruits中查询不同供应商供应的价格最低的水果
|
||
select s_id,min(f_price) from fruits group by s_id;
|
||
```
|
||
|
||
|
||
|
||
```mysql
|
||
select s_id,min(f_price) from fruits group by s_id order by s_id desc;
|
||
```
|
||
|
||
|
||
|
||
# 底部 |