SQL (3) DQL statements (2)

Constraint aggregate functions query packet paging query

Sort query

 select * from stu order by age desc;

 

Aggregate function

Grouping queries

https://www.bilibili.com/video/av55246614/?p=494

select sex 性别,avg(age) 平均年龄,count(id) 人数 from stu group by sex;

Students to identify older than 19 years of age by age Ascending

select * from stu where age >= 19 order by age asc;

having

According to the gender, the number of statistics were older than 19, and then select a number greater than one.

select sex,count(*) as 人数 from stu where age >= 19 group by sex having count(*)>1;

According to the gender, number of people older than 19 are statistics.

select sex,count(*) as 人数 from stu where age >= 19 group by sex;

select name,sex from stu;

select distinct name,sex from stu;

 

ALL here is a database of the default handling of the same row, it may be omitted.

So SELECT ALL * FROM STUDENT

和  SELECT * FROM STUDENT

It is equivalent to

select sex 性别,avg(age) 平均年龄 from stu group by sex;

And the number of statistics

select sex 性别,avg(age) 平均年龄,count(id) from stu group by sex;

Paging query