学习四：高级查询语句

标签（空格分隔）： mysql

---

一，聚合函数

• count

select count(*) FROM school.student;  //查询总个数select count(distinct name) FROM school.student;  //去除name重复的数据个数

• max

select max(birthday) FROM school.student;

• min

select min(birthday) FROM school.student;

• sum 求和

select sum(sid) FROM school.student;

• avg 求平均值

select avg(sid) FROM school.student;

二，分组查询

• group by

select CountryCode,count(*) FROM world.city group by CountryCode limit 5;

• group by having
  where是在分组前对数据进行分组
  having是在分组后进行分组

select CountryCode,count(*) cnt FROM world.city group by CountryCode having cnt>2 limit 5;

select CountryCode,count(*) cnt FROM world.city group by CountryCode having cnt>2  order by cnt desc limit 5;

三，联合查询 union

• union

select * FROM world.city where id<5 union select * FROM world.city where id>10 and id<15

union前后连接的表的字段类型必须相同，显示的列名和前一个表保持一致。

三，交叉查询

• 交叉连接

select * from world.city t_city,world.country t_country;

• 内连接 inner join on

select count(*) from world.city t_city inner join world.country t_country on t_city.CountryCode=t_country.Code;   //注释：只返回符合条件的table1和table2的列 

• 外连接

select count(*) from world.country t_country left join world.city t_city on t_city.CountryCode=t_country.Code;//包含t_country的所有子句，根据指定条件返回t_country相应的字段，不符合的以null显示 

select count(*) from world.country t_country right join world.city t_city on t_city.CountryCode=t_country.Code; //包含t_country的所有子句，根据指定条件返回t_city相应的字段，不符合的以null显示 

use world;select count(*) from country full join city on Code=city.CountryCode;//完整外部联接返回左表和右表中的所有行。当某行在另一个表中没有匹配行时，则另一个表的选择列表列包含空值。如果表之间有匹配行，则整个结果集行包含基表的数据值

四，子查询

• in

select * from world.city where CountryCode in (select code FROM world.country where Name='Afghanistan');

• exists

select * from world.city where exists (select code FROM world.country where Name='Afghanistan');

EXISTS用于检查子查询是否至少会返回一行数据，该子查询实际上并不返回任何数据，而是返回值True或False。