[SQL's 18 dragon subduing palms] 01 - Kang long regrets: introductory 10 questions

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?? Author's brief introduction ：, Long distance runner , Determined to persist in writing 10 Blog of the year , Focus on java Back end
?? Column Introduction ：mysql Basics 、 Advanced , Main explanation mysql database sql Brush problem 、 Advanced knowledge , Include index 、 Database tuning 、 Sub warehouse, sub table, etc
?? The article brief introduction ： This article will introduce the recommended collection for standby .
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• Big factory SQL Complete interview questions

List of articles

• 1 Query result de duplication
• 2. Rename the queried column
• 3 Find user information for a certain age group
• 4 Find user information except Fudan University
• 5 use where Filter null exercises
• 6 Where in and Not in
• 7 Operators mix
• 8 View users with Beijing in the school name
• 9 Calculate the number of boys and the average GPA
• 10 Group calculation exercises

1 Query result de duplication

subject ： Now the operation needs to check which schools users come from , Please take the school's de duplication data from the user information table .

Original link ：sql Question bank ：sql3 Query result de duplication

Answer key ：

select distinct university from user_profile 

2. Rename the queried column

Answer key ：

select device_id as user_infos_example from user_profile limit 2 

Original link ：sql Question bank ：SQL5 Rename the queried column

3 Find user information for a certain age group

Answer key ：

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

Original link ：sql Question bank ：SQL8 Find user information for a certain age group

4 Find user information except Fudan University

Answer key ：

select device_id,gender,age,university from user_profile where university != " Fudan University "

Original link ：sql Question bank ：SQL9 Find user information except Fudan University

5 use where Filter null exercises

select device_id,gender,age,university from user_profile where age is not null

Original link ：sql Question bank ：SQL10 use where Filter null exercises

6 Where in and Not in

select device_id,gender,age,university,gpa 
from user_profile 
where university in (" Peking University, "," Fudan University "," Shandong University ")

Original link ：sql Question bank ：SQL13 Where in and Not in

7 Operators mix

Answer key 1( Written in simple , Query efficiency is low )

select device_id,gender,age,university,gpa 
from user_profile 
where gpa > 3.5 and university = " Shandong University "
or gpa > 3.8 and university = " Fudan University "

Answer key 2（ The writing is complicated , High query efficiency ）

 select device_id, gender, age, university, gpa 
 from user_profile 
 where device_id in 
 (select device_id from user_profile where gpa>3.5 and university=' Shandong University ')
 or device_id in 
 (select device_id from user_profile where gpa>3.8 and university=' Fudan University ')

Original link ：sql Question bank ：SQL14 Operators mix

8 View users with Beijing in the school name

?? Knowledge point
The matching string can contain the following four wildcards ：
_： Matches any character ;
%： matching 0 Characters or more ;
[ ]： matching [ ] Any character in ( If the characters to be compared are continuous , You can use hyphens “-” surface reach );
[^ ]： Mismatch [ ] Any character in .

Answer key ：

select device_id, age, university from user_profile where university like "% Beijing %"

Original link ：sql Question bank ：SQL15 View users with Beijing in the school name

9 Calculate the number of boys and the average GPA

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

Original link ：sql Question bank ：SQL17 Calculate the number of boys and the average GPA

10 Group calculation exercises

Answer key ：

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

Original link ：sql Question bank ：SQL18 Group calculation exercises

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