一、单表查询
1、select
select @@xxx; 获取参数信息。
结尾\G竖直显示
show variables 查看MySQL中所有的参数。
2、select语句的执行顺序（单表）
slect 开始
from
where 条件匹配
group by 分组
select之后的语句
having 在group by 后继续匹配
order by 排序（desc）降序
limit 分页显示
3、多表连接查询
传统连接：
基于where条件，思路为，找表之间的关系（管系列），排列查询条件。
内连接：join on
1.找表之间的关系列
2.将两表放在join左右
3.将关联条件放在on后
4.将所有的查询条件进行罗列
select A.a,B.b
from
A join B
on A.x=B.y
where
group by
order by
limit
group by必须做到唯一
MySQL5.7版本开启了group by 严格，因此当group by 出现不唯一的可能性比如没有主键列时，会报错。
二、习题
1.查询中国城市信息（显示前五行）
mysql> select * from world.city where countrycode='CHN' limit 5;
+------+-----------+-------------+-----------+------------+
| ID | Name | CountryCode | District | Population |
+------+-----------+-------------+-----------+------------+
| 1890 | Shanghai | CHN | Shanghai | 9696300 |
| 1891 | Peking | CHN | Peking | 7472000 |
| 1892 | Chongqing | CHN | Chongqing | 6351600 |
| 1893 | Tianjin | CHN | Tianjin | 5286800 |
| 1894 | Wuhan | CHN | Hubei | 4344600 |
+------+-----------+-------------+-----------+------------+
2.查询美国的城市信息（显示五行）
mysql> select * from world.city where countrycode='USA' limit 5;
+------+--------------+-------------+--------------+------------+
| ID | Name | CountryCode | District | Population |
+------+--------------+-------------+--------------+------------+
| 3793 | New York | USA | New York | 8008278 |
| 3794 | Los Angeles | USA | California | 3694820 |
| 3795 | Chicago | USA | Illinois | 2896016 |
| 3796 | Houston | USA | Texas | 1953631 |
| 3797 | Philadelphia | USA | Pennsylvania | 1517550 |
+------+--------------+-------------+--------------+------------+
3.查询世界上人口小于100的城市
vmysql> select * from world.city where population<100;
+------+-----------+-------------+----------+------------+
| ID | Name | CountryCode | District | Population |
+------+-----------+-------------+----------+------------+
| 2912 | Adamstown | PCN | – | 42 |
+------+-----------+-------------+----------+------------+
4.查询城市人口在1w到2w之间的城市（前五）
mysql> select * from world.city where population>10000 and population<20000 limit 5;
+-----+-------------+-------------+--------------+------------+
| ID | Name | CountryCode | District | Population |
+-----+-------------+-------------+--------------+------------+
| 553 | George Town | CYM | Grand Cayman | 19600 |
| 583 | Avarua | COK | Rarotonga | 11900 |
| 586 | Roseau | DMA | St George | 16243 |
| 901 | Tórshavn | FRO | Streymoyar | 14542 |
| 917 | Nuuk | GRL | Kitaa | 13445 |
+-----+-------------+-------------+--------------+------------+
或
mysql> select * from world.city where population between 10000 and 200000 limit 5;
+----+----------------+-------------+---------------+------------+
| ID | Name | CountryCode | District | Population |
+----+----------------+-------------+---------------+------------+
| 3 | Herat | AFG | Herat | 186800 |
| 4 | Mazar-e-Sharif | AFG | Balkh | 127800 |
| 10 | Tilburg | NLD | Noord-Brabant | 193238 |
| 11 | Groningen | NLD | Groningen | 172701 |
| 12 | Breda | NLD | Noord-Brabant | 160398 |
+----+----------------+-------------+---------------+------------+
5.查询中国或美国的城市信息（前五）
mysql> select * from world.city where CountryCode='CHN' or CountryCode='USA' limit 5;
+------+-----------+-------------+-----------+------------+
| ID | Name | CountryCode | District | Population |
+------+-----------+-------------+-----------+------------+
| 1890 | Shanghai | CHN | Shanghai | 9696300 |
| 1891 | Peking | CHN | Peking | 7472000 |
| 1892 | Chongqing | CHN | Chongqing | 6351600 |
| 1893 | Tianjin | CHN | Tianjin | 5286800 |
| 1894 | Wuhan | CHN | Hubei | 4344600 |
+------+-----------+-------------+-----------+------------+
6.查询city表中所有数据（极少用）
mysql> select * from world.city limit 5;
+----+----------------+-------------+---------------+------------+
| ID | Name | CountryCode | District | Population |
+----+----------------+-------------+---------------+------------+
| 1 | Kabul | AFG | Kabol | 1780000 |
| 2 | Qandahar | AFG | Qandahar | 237500 |
| 3 | Herat | AFG | Herat | 186800 |
| 4 | Mazar-e-Sharif | AFG | Balkh | 127800 |
| 5 | Amsterdam | NLD | Noord-Holland | 731200 |
+----+----------------+-------------+---------------+------------+
7.查询name和population的所有值（前五）
mysql> select name,Population from world.city limit 5;
+----------------+------------+
| name | Population |
+----------------+------------+
| Kabul | 1780000 |
| Qandahar | 237500 |
| Herat | 186800 |
| Mazar-e-Sharif | 127800 |
| Amsterdam | 731200 |
+----------------+------------+
8.统计每个国家的总人数（前五）
mysql> select CountryCode,sum(Population) from world.city group by CountryCode limit 5;
+-------------+-----------------+
| CountryCode | sum(Population) |
+-------------+-----------------+
| ABW | 29034 |
| AFG | 2332100 |
| AGO | 2561600 |
| AIA | 1556 |
| ALB | 270000 |
+-------------+-----------------+
9.统计每个国家的城市个数（前五）
mysql> select countrycode,count(id) from world.city group by countrycode limit 5;
+-------------+-----------+
| countrycode | count(id) |
+-------------+-----------+
| ABW | 1 |
| AFG | 4 |
| AGO | 5 |
| AIA | 2 |
| ALB | 1 |
+-------------+-----------+
10.统计中国每个省的城市列表
mysql> select district,group_concat(name) from world.city where countrycode='CHN'group by district limit 5;
+-----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| district | group_concat(name) |
+-----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Anhui | Hefei,Huainan,Bengbu,Wuhu,Huaibei,Ma´anshan,Anqing,Tongling,Fuyang,Suzhou,Liu´an,Chuzhou,Chaohu,Xuangzhou,Bozhou,Huangshan |
| Chongqing | Chongqing |
| Fujian | Fuzhou,Amoy [Xiamen],Nanping,Quanzhou,Zhangzhou,Sanming,Longyan,Yong´an,Fu´an,Fuqing,Putian,Shaowu |
| Gansu | Lanzhou,Tianshui,Baiyin,Wuwei,Yumen,Jinchang,Pingliang |
| Guangdong | Kanton [Guangzhou],Shenzhen,Shantou,Zhangjiang,Shaoguan,Chaozhou,Dongwan,Foshan,Zhongshan,Jiangmen,Yangjiang,Zhaoqing,Maoming,Zhuhai,Qingyuan,Huizhou,Meixian,Heyuan,Shanwei,Jieyang |
+-----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
11.统计中国每个省的总人口数
mysql> select district,sum(population) from world.city where countrycode='CHN' group by district limit 5;
+-----------+-----------------+
| district | sum(population) |
+-----------+-----------------+
| Anhui | 5141136 |
| Chongqing | 6351600 |
| Fujian | 3575650 |
| Gansu | 2462631 |
| Guangdong | 9510263 |
+-----------+-----------------+
12.统计中国每个省的平均人口
mysql> select district,avg(population) from world.city where countrycode='CHN' grroup by district limit 5;
+-----------+-----------------+
| district | avg(population) |
+-----------+-----------------+
| Anhui | 321321.0000 |
| Chongqing | 6351600.0000 |
| Fujian | 297970.8333 |
| Gansu | 351804.4286 |
| Guangdong | 475513.1500 |
+-----------+-----------------+
13.统计中国每个省大于1000W的省及人口数
mysql> select district,sum(population) from world.city where countrycode='CHN' group by district having sum(population)>10000000;
+--------------+-----------------+
| district | sum(population) |
+--------------+-----------------+
| Heilongjiang | 11628057 |
| Liaoning | 15079174 |
| Shandong | 12114416 |
+--------------+-----------------+
14.降序排列中国每个省大于1000W的省及人口数
mysql> select district,sum(population) from world.city where countrycode='CHN' group by district having sum(population)>10000000 order by sum(population);
+--------------+-----------------+
| district | sum(population) |
+--------------+-----------------+
| Heilongjiang | 11628057 |
| Shandong | 12114416 |
| Liaoning | 15079174 |
+--------------+-----------------+
三、多表查询
1.查询世界上人口数量小于100的国家，城市名，国土面积
mysql> SELECT country.name,city.name,country.surfacearea
-> FROM
-> city JOIN country
-> ON city.countrycode=country.code
-> WHERE city.population<100;
+----------+-----------+-------------+
| name | name | surfacearea |
+----------+-----------+-------------+
| Pitcairn | Adamstown | 49.00 |
+----------+-----------+-------------+
2.查询oldguo老师和他的课程
on course.tno=teacher.tno where teacher.tname='oldguo';
+--------+-------+
| tname | cname |
+--------+-------+
| oldguo | mysql |
+--------+-------+
3.统计没门课程的总成绩
mysql> select course.cname,sum(sc.score)
-> from student
-> join sc
-> on student.sno=sc.sno
-> join course
-> on sc.cno=course.cno
-> group by course.cno;
+--------+---------------+
| cname | sum(sc.score) |
+--------+---------------+
| linux | 484 |
| python | 210 |
| mysql | 614 |
+--------+---------------+
4.查询oldguo老师教的学生姓名列表
mysql> select teacher.tname,group_concat(student.sname)
-> from student
-> join sc
-> on student.sno=sc.sno
-> join course
-> on course.cno=sc.cno
-> join teacher
-> on teacher.tno=course.tno
-> where teacher.tname='oldguo'
-> group by teacher.tno;
+--------+---------------------------------------------+
| tname | group_concat(student.sname) |
+--------+---------------------------------------------+
| oldguo | zhang4,li4,wang5,zh4,zhao4,ma6,oldgirl,oldp |
+--------+---------------------------------------------+
5.查询所有老师教的学生姓名列表
mysql> select teacher.tname,group_concat(student.sname) from student join sc on student.sno=sc.sno join course on course.cno=sc.cno join teacher on teacher.tno=course.tno group by teacher.tno;
+--------+---------------------------------------------+
| tname | group_concat(student.sname) |
+--------+---------------------------------------------+
| oldboy | zhang3,li4,wang5,zhao4,ma6,oldboy |
| hesw | zhang3,zhang4,wang5 |
| oldguo | zhang4,li4,wang5,zh4,zhao4,ma6,oldgirl,oldp |
+--------+---------------------------------------------+
6.查询oldguo老师所教不及格的学生姓名
mysql> select teacher.tname,group_concat(student.sname)
-> from student
-> join sc
-> on sc.sno=student.sno
-> join course
-> on course.cno=sc.cno
-> join teacher
-> on teacher.tno=course.tno
-> where teacher.tname='oldguo' and sc.score<60
-> ;
+--------+-----------------------------+
| tname | group_concat(student.sname) |
+--------+-----------------------------+
| oldguo | li4,zh4 |
+--------+-----------------------------+
7.统计zhang3学了几门课
mysql> select student.sname,count(sc.sno) from student join sc on sc.sno=student.sno join course on course.cno=sc.cno where student.sname='zhang3' group by sc.snno;
+--------+---------------+
| sname | count(sc.sno) |
+--------+---------------+
| zhang3 | 2 |
+--------+---------------+
8.查询zhang3学习的课程有哪些
mysql> select student.sname,group_concat(course.cname) from student join sc on scc.sno=student.sno join course on course.cno=sc.cno where student.sname='zhang3';
+--------+----------------------------+
| sname | group_concat(course.cname) |
+--------+----------------------------+
| zhang3 | linux,python |
+--------+----------------------------+
9.查询oldboy老师教的学生列表
mysql> select teacher.tname,group_concat(student.sname) from student join sc on sc.sno=student.sno join course on course.cno=sc.cno join teacher on teacher.tno=course.tno where teacher.tname='oldboy' group by teacher.tno;
+--------+-----------------------------------+
| tname | group_concat(student.sname) |
+--------+-----------------------------------+
| oldboy | zhang3,li4,wang5,zhao4,ma6,oldboy |
+--------+-----------------------------------+
10.查询oldguo所教课程平均分
mysql> select teacher.tname,avg(sc.score) from student join sc on sc.sno=student..sno join course on course.cno=sc.cno join teacher on teacher.tno=course.tno wherre teacher.tname='oldguo' group by course.cno;
+--------+---------------+
| tname | avg(sc.score) |
+--------+---------------+
| oldguo | 76.7500 |
11.每位老师所教课程的平均分，并降序排列
mysql> select teacher.tname,avg(sc.score) from student join sc on sc.sno=student.sno join course on course.cno=sc.cno join teacher on teacher.tno=course.tno group by teacher.tno order by avg(sc.score) desc;
+--------+---------------+
| tname | avg(sc.score) |
+--------+---------------+
| oldboy | 80.6667 |
| oldguo | 76.7500 |
| hesw | 70.0000 |
+--------+---------------+
12.查询oldguo所教学生不及格的姓名
mysql> select teacher.tname,group_concat(student.sname) from student join sc on sc.sno=student.sno join course on course.cno=sc.cno join teacher on teacher.tno=ccourse.tno where teacher.tname='oldguo' and sc.score<60;
+--------+-----------------------------+
| tname | group_concat(student.sname) |
+--------+-----------------------------+
| oldguo | li4,zh4 |
+--------+-----------------------------+
13.查询所有老师所教学生不及格信息
mysql> select teacher.tname,group_concat(student.sname) from student join sc on sc.sno=student.sno join course on course.cno=sc.cno join teacher on teacher.tno=course.tno where sc.score<60 group by teacher.tno;
+--------+-----------------------------+
| tname | group_concat(student.sname) |
+--------+-----------------------------+
| hesw | zhang3 |
| oldguo | li4,zh4 |
+--------+-----------------------------+
四、总结
1.模糊查询中like语句在MySQL中不要出现%在前面的情况，因为当有大量数据时，效率极低。
2.对于合并表
union 合并两个表，若其中有重复，将自动去重
union all 合并两个表，不去重，因此性能最好
select * from xxx; 适合表数据较少情况下
group by 配合聚合函数应用
常用聚合函数：
avg() 平均
count() 计算个数
sum() 总数
max() 最大
min() 最小
group_concat() 集中显示所有
group by 后接归类条件
select 后接查看条件
where 后接查询条件
having： guoup by 后过滤
显示统计完的某一部分数据
order by 排序
desc 降序
limit： 分页显示