[leetcode] [SQL] notes

code Format specification :
All keywords are capitalized , Align keywords to the right , Clause indent
The column name should be the same as the column name in the table （ Case case ）

SELECT  nameFROM  customer CWHERE C.id NOT IN ( SELECT C1.id                     FROM customer C1                    WHERE C1.referee_id = 2 );

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Reference material ：
Official documents ：【 Reference resources ：MySQL ：： MySQL 5.7 Reference manual ：： 13 SQL sentence 】
【 Reference resources ：MySQL Chinese document | MySQL Chinese net 】
【 Reference resources ：SQL The predicate in - You know 】
Topics are categorized by type , Record common and error prone types

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IS NULL

【 Reference resources ：584. Looking for user references 【 Finer than official , A thousand words of dry goods ！】（ Ternary operation ,NULL） - Looking for user references - Power button （LeetCode）】
【 Reference resources ：584. Looking for user references - Simple - Power button （LeetCode）】

SELECT name FROM customer WHERE referee_id != 2 OR referee_id IS NULL;

Multi-table query

【 Reference resources ：183. Customers who never order - Power button （LeetCode）】

• Subquery

select customers.name as 'Customers' # as  Alias from customerswhere customers.id not in(    select customerid from orders);#  First from orders To find out in the customerid

• Link query
  【 Reference resources ： The illustration SQL Interview questions ： Find data that is not in the table - Customers who never order - Power button （LeetCode）】
  

SELECT c.Name as CustomersFROM Customers as cleft join Orders as o on c.Id=o.CustomerIdwhere o.Id IS NULL

IF expression

IF( expr1 , expr2 , expr3 )

【 Reference resources ：1873. Calculate special bonuses - Power button （LeetCode）】
【 Reference resources ：MySQL, 7 A solution - Calculate special bonuses - Power button （LeetCode）】

SELECT     employee_id,    IF(         employee_id%2=1 and name not like 'M%',        salary,        0    ) as bonus FROM EmployeesORDER by employee_id 

【 Reference resources ：627. Changing gender - Power button （LeetCode）】

UPDATE Salary set sex=IF(sex='f','m','f')

CASE WHEN END

【 Reference resources ：627. Changing gender - Power button （LeetCode）】

UPDATE salarySET    sex = CASE sex        WHEN 'm' THEN 'f'        ELSE 'm'    END

Self join

【 Reference resources ：196. Delete duplicate email - Power button （LeetCode）】

【 Reference resources ： Yes 「 official 」 In the solution “delete” and “>” The explanation of , recommend ！ - Delete duplicate email - Power button （LeetCode）】
2、p1.Id > p2.Id
Before continuing , First, let's take a brief look at the connection process of the table , I understand this , understand WHERE The conditions are simple
a. From the drive table （ The left table ） Take out N Bar record ;
b. Take this. N Bar record , In turn, go to the driven table （ Right table ） Find satisfaction WHERE Record of conditions ;

DELETE p1 FROM Person p1,     Person p2WHERE    p1.Email = p2.Email AND p1.Id > p2.Id

Regular

【 Reference resources ：1527. A patient with a disease - Power button （LeetCode）】

^DIAB1 Said to DIAB1 start  | Express or  . Indicates that there must be any character  * It means repetition 0 To an infinite number of previous characters   first \ Indicates the escape character  \s It's blank , Including Spaces 、 Line break 、Tab  Indent, all the blanks   therefore .*\sDIAB1 Express DIAB1 There is a space before and 0 To an infinite number of arbitrary characters 

# Write your MySQL query statement belowselect patient_id,       patient_name,       conditionsfrom Patientswhere conditions rlike '^DIAB1|.*\\sDIAB1'

function

upper lower

【 Reference resources ：1667. Fix the name in the table - Power button （LeetCode）】
【 Reference resources ：【JMao】 Simple function solution + Share the experience of problem brushing - Fix the name in the table - Power button （LeetCode）】
【 Reference resources ：12.3. String function _MySQL Chinese document 】

# Write your MySQL query statement belowSELECT user_id,       CONCAT(Upper(Left(name,1)),Lower(substring(name,2))) as name from Users order by user_id

concat

【 Reference resources ：1484. Sell products by date - Power button （LeetCode）】
【 Reference resources ：mysql Group splicing function group_concat - Sell products by date - Power button （LeetCode）】

SELECT sell_date,       count(distinct product) as 'num_sold',       #  Intra group splicing        group_concat(distinct product    #  duplicate removal                     order by product asc #  grouping , In ascending order according to the dictionary                     separator ',') #  interval                     as 'products'from Activitiesgroup by sell_dateorder by sell_date

union all

union and union all Can play the role of associating result sets , The difference lies in :

• union It will automatically remove the duplicate data in the associated two result sets ,

• union all Will not actively remove duplicate data in the two result sets , All the data will be displayed ;

Column turned

【 Reference resources ：1795. The price of each product in different stores - Power button （LeetCode）】
【 Reference resources ： form - Column turned - The price of each product in different stores - Power button （LeetCode）】

# Write your MySQL query statement belowselect product_id, 'store1' as store, store1 as pricefrom Products where store1 is not nullunion allselect product_id, 'store2' as store, store2 as pricefrom Products where store2 is not nullunion allselect product_id, 'store3' as store, store3 as pricefrom Products where store3 is not null;

author ：myaijarvis

Game programming , A game development favorite ~

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