Record the learning record of the exists keyword once

On that day , Say goodbye to summer vacation , Say goodbye to fun , Say goodbye to you .

 

Stepping on the morning sunshine , Take a brisk step , On the way to the freshman Report Office , Uncontrollable excitement in my heart . Go inside , Through a small grove , The reddish buildings are dormitory buildings , To the east of the dormitory building is the artificial lake of the school , The construction is very big , The layout is very stylish . Because of its existence , Let me go on this part of my life in college , Taste all the sweets and bitters .

 

I hope to be in the bustling place , Find a touch of silence , Boating on the lake , A friend or two , A pot of old wine , Family company , Singing and meeting friends . Come on , In the green field 、 Create beauty in the sun , Cure youth .

 

Maybe that's what youth looks like .

 

But now I want to , It is still so beautiful .

 

" classmate , Excuse me "." Oh , sorry ", It happened to block the north gate of the playground , New student registration office , Just go in from here . Someone patted me on the shoulder ," This student is here to report for duty ?", A handsome old man told me ,

" Um. , Yes ",

" Which department are you from ?",

" Department of Computer Science ",

“ What's your name? ?”,

" My name is Wangxiaofeng ".

" Um. , Xiao Feng , I have a computer here , Come and help me count the relevant data of freshmen ":

 

mysql Version number :

 

CREATE TABLE `t_class` (`id` int unsigned NOT NULL AUTO_INCREMENT,`class_name` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT '' COMMENT ' Class name ',PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci COMMENT=' Class name table ';

CREATE TABLE `t_student` (`id` int unsigned NOT NULL AUTO_INCREMENT,`user_name` varchar(30) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT '' COMMENT ' The student's name ',`age` int NOT NULL COMMENT ' Age ',PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci COMMENT=' Student list ';

 

CREATE TABLE `t_class_student` (`id` int unsigned NOT NULL AUTO_INCREMENT,`student_id` int NOT NULL COMMENT ' Student ID',`class_id` int NOT NULL COMMENT ' Class number ',PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci COMMENT=' Class student association table ';

INSERT INTO stu.t_student (user_name,age) VALUES(' Zhang Yibo ',18),(' Lixiaorui ',19),(' Liulengshuang ',18),(' Zhaoyuzhen ',17),(' Liuyuandie ',19),(' Yingyan ',19),(' Wanqiu ',20),(' Liugaomin ',19),(' Wangkunxiong ',20),(' Zhaoxingpeng ',19);INSERT INTO stu.t_student (user_name,age) VALUES(' Gaokunhui ',18),(' Hao Lei ',18),(' Wangxiaofeng ',19);

INSERT INTO stu.t_class (class_name) VALUES(' Class 1, Department of economics and trade '),(' Class two, Department of medicine '),(' Foreign languages A class '),(' Class 3, journalism department '),(' Class 1, Department of landscape tourism '),(' Class one, computer department ');

INSERT INTO stu.t_class_student (student_id,class_id) VALUES(1,1),(2,1),(3,2),(4,2),(5,2),(6,2),(7,3),(8,3),(9,4),(10,4);INSERT INTO stu.t_class_student (student_id,class_id) VALUES(13,6);

t_student It is the list of all the new students who come to check in this year , Including students who have already checked in and those who have not .

 

A list of students who have registered for the department class :

select	id,	user_name as userName,	agefrom	stu.t_student as swhere	exists (	select		student_id	from		stu.t_class_student	where		student_id = s.id);

 

 

A list of students who did not report to the department class :

select	s.id,	s.user_name as userName,	s.agefrom	stu.t_student as swhere	not exists (	select		student_id	from		stu.t_class_student	where		student_id = s.id);

A list of the registered students of class 2 of the Department of medicine :

select	s.id,	s.user_name as userName,	s.agefrom	stu.t_student as swhere	exists (	select		student_id	from		stu.t_class_student	where		student_id = s.id		and class_id = 2);

Class 1, Department of economics and trade and Foreign languages A A list of students who have registered for the class :

select	s.id,	s.user_name as userName,	s.agefrom	stu.t_student as swhere	exists (	select		student_id	from		stu.t_class_student	where		student_id = s.id		and class_id in (1, 3));

A list of students in all departments and classes that have registered :

select	s.id,	s.user_name as userName,	s.agefrom	stu.t_student as swhere	exists (	select		student_id	from		stu.t_class_student	where		1 = 1);

Class 2, Department of medicine and older than 18 A list of students who have registered at the age of :

select	s.id,	s.user_name as userName,	s.agefrom	stu.t_student as swhere	s.age > 18	and exists (	select		student_id	from		stu.t_class_student	where		student_id = s.id		and class_id = 2);

【exists and in Comparison between 】

Execute the process :

Let's start with keywords in, It requires that the query result meet in The conditions in the following brackets , such as id、status etc. . It's going to put it first in Subquery in the following slogan sql After execution , Then, according to the results obtained, execute the previous master sql.

This is similar to the backflow of river water . Such as heavy rainfall , This leads to an increase in the amount of river water nearby , Because the elevation of the ponding road surface is lower than the elevation of the river water level , The river pours back into the storm sewer , Then the road area water is formed . So I call this method of execution " Backflow execution method ".

and exists Just the opposite , It's sequential execution , After execution exists Ahead sql After getting the results , Take the initiative and exists Subqueries in sql To match , Only successfully matched data can be returned to the console .

EXPLAIN Explanation of columns ：

table： Show which table this row's data is about

type： This is an important column , Shows what type of connection is used . The best to worst connection type is const、eq_reg、ref、range、 indexhe and ALL

possible_keys： Show the indexes that may be applied to this table . If it is empty , There is no possible index . Can be related to the domain from WHERE Choose an appropriate statement

key： Actual index used . If NULL, No index is used . In rare cases ,MYSQL Will choose indexes that are not optimized enough . In this case , Can be in SELECT Use in statement USE INDEX（indexname） To force the use of an index or IGNORE INDEX（indexname） Mandatory MYSQL Ignore the index

key_len： Length of index used . Without losing accuracy , The shorter the length, the better

ref： Shows which column of the index is used , If possible , It's a constant

rows：MYSQL The number of rows that must be checked to return the requested data

Extra： About MYSQL How to parse the extra information of the query . The bad example is Using temporary and Using filesort, Meaning MYSQL Index can't be used at all , The result is that the retrieval will be slow

Index usage :

( One ) Query the students who have registered in all departments and classes :

explain select	s.id,	s.user_name as userName,	s.agefrom	stu.t_student as s where s.id in (select id from stu.t_class_student tcs);

Subqueries use indexes ,in Not used

explain select	id,	user_name as userName,	agefrom	stu.t_student as swhere	exists (	select		student_id	from		stu.t_class_student	where		student_id = s.id);

Subqueries use indexes , And the outer layer sqlexists Index is not used

( Two ) Check the status of students who have not registered in all departments :

explain select	s.id,	s.user_name as userName,	s.agefrom	stu.t_student as s where s.id not in (select id from stu.t_class_student tcs);

Similarly, subqueries use indexes ,not in Keyword is not used in index

explain select	id,	user_name as userName,	agefrom	stu.t_student as swhere	not exists (	select		student_id	from		stu.t_class_student	where		student_id = s.id);

Use not exists The same is true of : Subqueries use indexes ,not in Keyword is not used in index , Full scan of walking

【 Conclusion 】

Crossing mountains and mountains is only for one meeting , Go all out for one change . The answer is in tears , Also condensed in sweat 、 Senses and mind . There's always one thing to live elsewhere , You will strengthen your love .