1. 多表联查心德

# 1. 分析需求

# 2. 确定表与表的关联做为判断条件

# 3. 确定每表要获取的数据

# 4. 查找所需的表内容加上条件

2. 查询练习

#1.查看表结构

mysql> desc city;

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

| Field | Type | Null | Key | Default | Extra |

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

| ID | int(11) | NO | PRI | NULL | auto_increment |

| Name | char(35) | NO | | | |

| CountryCode | char(3) | NO | MUL | | |

| District | char(20) | NO | | | |

| Population | int(11) | NO | | 0 | |

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

5 rows in set (0.00 sec)

#2.查看所有数据

mysql> select * from city;

#3.查看指定列的数据

mysql> select Name,Population from city;

#4.查看数据时排序(按照人口数量)

#升序

mysql> select Name,Population from city order by Population;

#降序

mysql> select Name,Population from city order by Population desc;

#5.查询部分数据

#查看前十条数据

mysql> select Name,Population from city order by Population desc limit 10;

#6.按照步长查询数据

mysql> select id,Name,Population from city limit 50,50;

#50起始位置 50步长

3.条件查询

#1.条件查询就是使用where语句，where语句可以使用的符号

条件符号：= < > <= >= != <> or and like

精确匹配：=

范围匹配：< > <= >= != <>

模糊匹配：like

连接语句：or and

#2.查询中国的城市人口

mysql> select name,population from city where CountryCode='CHN';

#3.查询黑龙江人口数量

mysql> select name,population from city where countrycode='CHN' and District='heilongjiang';

#4.查询中国人口数量小于100000的城市

mysql> select name,population from city where countrycode='CHN' and population < 100000;

#5.模糊匹配

#匹配以N结尾的数据

mysql> select name,countrycode from city where countrycode like '%N';

#匹配以N开头的数据

mysql> select name,countrycode from city where countrycode like 'N%';

#匹配包含N的数据

mysql> select name,countrycode from city where countrycode like '%N%';

#6.查询中国或美国的人口数量

#使用or

mysql> select name,population from city where countrycode = 'CHN' or countrycode = 'USA';

#使用in

mysql> select name,population from city where countrycode in ('CHN','USA');

#使用union all

mysql> select name,population from city where countrycode = 'CHN' union all select name,population from city where countrycode = 'USA';

select高级用法

#多表联查，联表查询

1.传统连接

1)集合

#集合

[qiudao,zengdao,qiandao]

[80,90,100]

#数据库

id:[1,2,3]

name:[qiudao,zengdao,qiandao]

id:[1,2,3]

mark:[80,90,100]

2)建表

mysql> create table students(id int,name varchar(10));

Query OK, 0 rows affected (0.08 sec)

mysql> create table score(id int,mark int);

Query OK, 0 rows affected (0.05 sec)

3)插入数据

mysql> insert into students values(1,'qiudao'),(2,'qiandao'),(3,'zengdao');

Query OK, 3 rows affected (0.02 sec)

Records: 3 Duplicates: 0 Warnings: 0

mysql> insert into score values(1,80),(2,90),(3,100);

Query OK, 3 rows affected (0.01 sec)

Records: 3 Duplicates: 0 Warnings: 0

4)数据查询

#查看两个表的数据

mysql> select * from students;

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

| id | name |

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

| 1 | qiudao |

| 2 | qiandao |

| 3 | zengdao |

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

3 rows in set (0.00 sec)

mysql> select * from score;

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

| id | mark |

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

| 1 | 80 |

| 2 | 90 |

| 3 | 100 |

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

3 rows in set (0.00 sec)

#查看邱导的分数

mysql> select students.name,score.mark from students,score where students.id=1 and score.id=1;

mysql> select students.name,score.mark from students,score where students.id=score.id and name='qiudao';

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

| name | mark |

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

| qiudao | 80 |

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

1 row in set (0.01 sec)

#查询所有学生成绩

mysql> select students.name,score.mark from students,score where students.id=score.id

5)练习题一：

连表查询：世界上小于100人的城市在哪个国家？请列出城市名字，国家名字与人口数量

#1.确认我要查哪些内容

国家名字 城市名字 城市人口数量 小于100人

#2.确认在哪个表

country.name city.name city.population

#3.找出两个表相关联的字段

city.countrycode country.code

#4.编写语句

mysql> select country.name,city.name,city.population from country,city where city.countrycode=country.code and city.population < 100;

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

| name | name | population |

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

| Pitcairn | Adamstown | 42 |

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

1 row in set (0.01 sec)

6)练习题二：

连表查询：世界上小于100人的城市在哪个国家，是用什么语言？请列出城市名字，国家名字与人口数量和国家语言

#1.确认我要查哪些内容

国家名字 城市名字 城市人口数量 国家使用的语言 小于100人

#2.确认在哪个表

country.name city.name city.population countrylanguage.language

#3.找出三个表相关联的字段

country.code city.countrycode countrylanguage.countrycode

#4.写sql语句

mysql> select country.name,city.name,city.population,countrylanguage.language from country,city,countrylanguage where country.code=city.countrycode and city.countrycode=countrylanguage.countrycode and city.population < 100;

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

| name | name | population | language |

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

| Pitcairn | Adamstown | 42 | Pitcairnese |

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

1 row in set (0.04 sec)