MySQL tutorial: MySQL database learning classic (from getting started to mastering)

All systems in the field of software engineering , We should start with these two questions

1“ What is it? ？“

2“ Why have this ？

What is a database ？

database , Used to store data . It's made up of tables , There can be 0 To n A watch ,

It's like a watch Excel, It's suitable for storing some simple text messages ——

A person's ID number

Phone number

full name

Excel What information is not suitable for storage ？

Photo

The movie

A novel

These single large file objects , Not in the database .

for instance , All the goods on Taobao , The commodity information behind it 、 Store information 、 Sales and other data , Lying in a table in the database , But the promotional video on the store 、 picture 、 music , Not in the database .

There are all kinds of databases in the world , But fortunately , You just need to learn MySQL Basic usage , You can deal with most scenarios .

MySQL What is it? ？

MySQL It's a database software , Database and MySQL The relationship between , It's like chat software and QQ、 Wechat relationship .

There are all kinds of databases in the world , such as Oracle、MySQL、SQL Server wait , Most Internet companies use MySQL, Because it's free 、 The most widely used .

General interview , I will only ask MySQL Relevant knowledge .

SQL What is it? ？

Is a programming language , It's like Java、C++、Python, But it's very simple .

SQL It's a language for manipulating databases .

Just a hundred English words ,SQL You can do dazzling operations on the data , Very easy to get started .

To sum up , A database is a data storage system , The most representative of them is MySQL, It's the most widely used database , We use SQL Language operates on him .

The second question is ——

Why have databases ？

because Excel Not for a large system , Its performance is not good enough . We need a place where multiple people can visit at the same time 、 manipulation 、 Secure data storage system .

Generally speaking , The database is a shared hard disk , Multiple people can visit at the same time 、 Change data . There's a lot of data （0～300G） When , The performance of the database will be very good .

（ Imagine a few dozen G Of Excel, It's going to be very difficult to operate ）

How to learn database ？

I'm not going to introduce the concept , Let's copy the code and run . Don't be surprised , The starting point for almost all programmers is to copy code , To run the .

Generally speaking , We need to download free MySQL Installation , But it's too expensive for ordinary people , You may encounter various installation errors , I strongly don't recommend wasting time on such things . Fortunately, there is a website , You can use it online

We will start from the following N Let's operate the database step by step .

1, Create a form . There's some data in there .

2, Query data .

3, Fancy query data .

First step , Create tables and insert data

The goal is ： Build a simple table ：

We open the website

Put the following SQL sentence , Copy to the left border of the page ：

Be careful , Don't copy and paste with the mouse , Use the keyboard to operate . Or you might throw it wrong .

If you keep throwing it wrong , Just copy the code to txt In file , And then paste it on the website .

CREATE TABLE `student_info` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT COMMENT 'id, Primary key ',
  `student_name` varchar(50) COMMENT ' The student's name ',
  `student_age` int COMMENT ' Student age ',
  `student_sex` varchar(10) COMMENT ' Student gender ',
  PRIMARY KEY (`id`)
);

INSERT INTO `student_info`
(`student_name`, `student_age`,`student_sex`)
VALUES
(' Ge Yao ', 17 , ' male ')
;
INSERT INTO `student_info`
(`student_name`, `student_age`,`student_sex`)
VALUES
('XH', 23 , ' male ')
;
INSERT INTO `student_info`
(`student_name`, `student_age`,`student_sex`)
VALUES
('QHR', 24 , ' male ')
;

INSERT INTO `student_info`
(`student_name`, `student_age`,`student_sex`)
VALUES
('WR', 22 , ' Woman ')
;

In the left input box , Copy the above SQL, Click on build Schema

This completes the creation of the table , And the operation of inserting data . I feel confused ？ No problem , Let's look up the data .

The second step , Query data

Copy the following code to the input field on the right side of the page , Click on Run SQL

select
id
,student_name
,student_age
from student_info
;
#！ Copy to the right side of the page , Click on Run SQL

With 4 It's data ！

If you're tired of listing , You can also write

select 
  *
from `student_info`;

All columns are pulled out by default , as follows ：
If you still feel confused , No matter what it means , Just know that we already have the following 4 Data

altogether 4 Bar record , Represents the 4 A student , Their names are on the list 、 Age 、 Gender .

The third step , Do some fancy queries

Some commonly used scenes , such as ——

List all the boys ： Copy the following code to the right border , Click on Run SQL

select 
  *
from `student_info`
where `student_sex`=' male '
;

List all older than 20 Year old student

select 
  *
from `student_info`
where `student_age` > 20
;

List all older than 20 A 12-year-old schoolboy

select 
  *
from `student_info`
where `student_age` > 20
  and `student_sex` = ' male '
;

List all the boys , In ascending order of age (1,2,3…)

select 
  *
from `student_info`
where `student_sex` = ' male '
order by `student_age` ASC
;

Sort the boys by age , List the youngest 2 personal ：

select 
  *
from `student_info`
where `student_sex` = ' male '
order by `student_age` ASC
limit 2
;

The last common keyword ,group by

Group the students by gender , Find out the biggest age difference between men and women

This requirement , actually , It's about taking data out of the table first , And then it's divided by gender 2 Group , And then in each group , Find the oldest number .

select 
  `student_sex`
  , max( `student_age` )
from `student_info`
group by `student_sex`
;

This is a complicated operation , We can talk about it later .

Finally, thank everyone who reads my article carefully , The following online link is also a very comprehensive one that I spent a few days sorting out , I hope it can also help you in need ！

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