Single table query of SQL brush questions

basic exercises

query all columns

sql:

select 
    *
from
    user_profile;

query multiple columns

sql:

select
  device_id,
  gender,
  age,
  university
from
  user_profile

Deduplication of query results

sql:

select
  distinct university
from
  user_profile

Method Two:

SELECT university 
FROM user_profile
GROUP BY university 

Query results limit the number of rows returned

sql:

select
  device_id
from
  user_profile
limit
  2

Rename the column after the query

sql:

select
  device_id as 'user_infos_example'
from
  user_profile
where
  id between 1 and 2

Find information about students whose school is Peking University

sql:

select
  device_id,
  university
from
  user_profile
where
  university = '北京大学'

Method Two:

select
  device_id,
  university
from
  user_profile
where
  university = '北京大学'
and device_id = user_profile.device_id; # 索引覆盖

Find and sort

sql:

select
  device_id,
  age
from
  user_profile
order by
  age

Sort by multiple columns after find

sql:

select
  device_id,
  gpa,
  age
from
  user_profile
order by
  gpa,
  age

Sort in descending order after search

sql:

select
  device_id,
  gpa,
  age
from
  user_profile
order by
  gpa desc,
  age desc

Find information about users older than 24

sql:

select
  device_id,
  gender,
  age,
  university
from
  user_profile
where
  age > 24

Find information about users of a certain age group

sql:

select
  device_id,
  gender,
  age
from
  user_profile
where
  age between 20 and 23

Find user information other than Fudan University

sql:

select
  device_id,
  gender,
  age,
  university
from
  user_profile
where
  university not in ('复旦大学')

or:

select
  device_id,
  gender,
  age,
  university
from
  user_profile
where
  university != ('复旦大学')

Use where to filter empty values ​​exercise

sql:

select
  device_id,
  gender,
  age,
  university
from
  user_profile
where
  age is NOT NULL

Advanced Operator Exercise (1)

sql:

select
  device_id,
  gender,
  age,
  university,
  gpa
from
  user_profile
where
  gpa > 3.5
  and gender = 'male'

Advanced Operator Exercise (2)

sql:

select
  device_id,
  gender,
  age,
  university,
  gpa
from
  user_profile
where
  university = '北京大学'
  or gpa > 3.7

Where in 和Not in

sql:

select
  device_id,
  gender,
  age,
  university,
  gpa
from
  user_profile
where
  university in ('北京大学', '复旦大学', '山东大学')

operator mix

sql:

select
  device_id,
  gender,
  age,
  university,
  gpa
from
  user_profile
where
  gpa > 3.5
  and university = '山东大学'
  or gpa > 3.8
  and university = '复旦大学'

View users with Beijing in their school name

sql:

select
  device_id,
  age,
  university
from
  user_profile
where
  university like '北京%'

Advanced Search

Find the highest GPA

sql:

select
  gpa
from
  user_profile
where
  university = '复旦大学'
order by
  gpa desc # 降序
limit
  1 # 限制输出

or:

select
  max(gpa)
from
  user_profile
where
  university = '复旦大学'

Calculate the number of boys and the average GPA

sql:

select
  count(gender) as male_num,
  avg(gpa) as avg_gpa
from
  user_profile
where
  gender = 'male'

group calculation exercises

sql:

select
  gender,
  university,
  count(device_id) as user_num,
  avg(active_days_within_30) as avg_active_days,
  avg(question_cnt) as avg_question_cnt
from
  user_profile
group by
  gender,
  university

Group Filtering Exercises

Topic: Now the operation wants to check the average posts and replies of each school user, and find low-activity schools for key operations. Please select schools with an average number of posts below 5 or schools with an average number of replies below 20.

sql:

#聚合函数结果作为筛选条件时，不能用where，而是用having语法，配合重命名即可；
select
    university,
    avg(question_cnt) as avg_question_cnt,
    avg(answer_cnt) as avg_answer_cnt
from user_profile
group by university
having avg_question_cnt<5 or avg_answer_cnt<20

Group Sorting Exercises

sql:

select
  university,
  avg(question_cnt) as avg_question_cnt
from
  user_profile
group by
  university
order by
  avg_question_cnt

Congratulations

You already have the basic knowledge of single-form query, congratulations on getting started! The questions in this article are very simple. If you can complete each question in one minute on average, you will basically master it.