SQL quick start

Basic query

Query multiple columns

Query statements SELECT Field name FROM Table name ;

answer

select device_id, gender, age, university from user_profile;

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Query all columns

* No. is to select all column names

answer ：

SELECT * FROM user_profile

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Simple processing of query results

Query result de duplication

Go to check again DISTINCT Put it in front of the column name

answer ：

SELECT DISTINCT university FROM user_profile 

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Query results limit the number of rows returned

LIMIT Show the first two lines .

answer ：

SELECT device_id FROM user_profile LIMIT 0,2;

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Rename the queried column

Change column headings ( Take the alias ) grammar ： Name | expression [ AS ] New column names or ： New column names ＝ Name | expression

answer ：

SELECT device_id  as user_infors_example
from user_profile 
LIMIT 2

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Conditions of the query

Basic sorting

adopt order by Sort :
asc : Ascending
desc : Descending

answer ：

select device_id,age
from user_profile
order by age asc;

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Sort multiple columns after searching

answer ：

SELECT device_id, gpa, age 
FROM user_profile 
ORDER BY gpa, age;

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Sort in descending order after finding

answer ：

select device_id,gpa,age
FROM user_profile
ORDER BY gpa desc,age desc 

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Basic operators

Find the student information of the school is Peking University

stay where Add the condition as ‘ Peking University, ’ that will do .

answer ：

select device_id,university
from user_profile
where university=' Peking University, '

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Look for ages greater than 24 Years old user information

answer ：

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

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Find user information for a certain age group

Determine scope use BETWEEN…AND and NOT BETWEEN…AND Is a logical operator , It can be used to find tuples whose attribute values are or are not within the specified range , among BETWEEN The lower limit of the specified range at the back ,AND The upper limit of the range specified later .BETWEEN…AND… The format is ： Name | expression [ NOT ] BETWEEN Lower limit AND Upper limit value

answer ：

select device_id,gender,age 
FROM user_profile
WHERE age>=20 and age<=23

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Find user information except Fudan University

answer ：

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

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use where Filter null exercises

answer ：

SELECT `device_id`,`gender`,`age`,`university`
from user_profile 
where not age is NULL

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