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HomeDatabaseMysql TutorialMysql查询(笔记二)_MySQL

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1.两结构相同的表数据间移植

Inset into 表一

Select 字段1,字段2,....字段n from表二

建立数据库时设置数据库编码

create database 数据库名 charset uft8

模糊查询时

%(任意字符任意个数)

_(单个字符)

 

2.聚合函数使用时必须有group by给定分组条件

 

 

比如有一个表

求平均成绩是用聚合函数avg()时必须制定此函数作用的范围依据

 

3.分组,排序,再次筛选,取几条执行顺序

Where>group by >having>order by >limit

4.如上表要求查询2门及2门以上同学的平均分

方法一:分步查询

先查出2门及2门以上不及格学生是谁

然后再查这些同学的平均分,需要2个select语句

select name from stu where score=2

便得到表

name

张三

李四

 

注意sql的执行顺序

先执行select name from stu where score

会得到表2:

name

张三

张三

李四

李四

王五

 

之后再分组和再筛选

但要注意虽然group by在having前,但group by是作为having的筛选分组条件的

然后在表2中having筛选即得

第二步:

找到这些学生后,在把学生名最为查询条件再次筛选,因为第一步筛选掉了及格的了的成绩,结果破坏了数据的原始性,所以还得进行一次筛选

 

 select name,avg(score) from stu where name in (select name from stu where

 score=2) group by name;

方法二:利用select后可以直接是判断式及本题特点

 

分析:首先运行sql查询

select name ,score

得到下表3:

name

Score

张三

0

张三

1

张三

1

李四

1

李四

1

王五

1

在select后的字段或条件mysql会依次与表中的数据对比,如score

比如字段name没有要比较的值就显示它在表中的值

从表三发现:sum(score

于是

 select name ,avg(score),sum(score

 gk>=2;

聚合函数的执行时同时进行

得到结果表:

显然方法二比一更简洁高效

未完待续....!

 

 

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