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Introduction to MySQL basics day3 power node [Lao Du] class notes

2022-06-30 08:35:00 【hty2dby】

day_3

The original author of this article is Mr. Du, the teaching director of power node. Mr. Du is in his class Lao Du will take you to learn _mysql Fundamentals of entry （mysql Basic video + Database practice ） Lecture notes written

See the link at the end of the document for this document and related materials

I will take notes of the lecture txt After downloading , To convert into markdown Format , And some of the contents are added , Original investment area , In case of infringement , Must delete

The text description at the bottom of the original video is as follows ：

This set is MySQL The database video tutorial is told by Mr. Du, the teaching director of power node , Which explains in detail MySQL Knowledge about , Include MySQL summary ,MySQL Application environment ,MySQL System features ,MySQL Beginners ,MySQL Management tools , How to install MySQL And MySQL New characteristics , By watching this set Java Video tutorial can master MySQL Full knowledge

1. Query the Department name of each employee ？ Employee name and department name are required

mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
+-------+--------+-----------+------+------------+---------+---------+--------+

mysql> select * from dept;
+--------+------------+----------+
| DEPTNO | DNAME      | LOC      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+

from emp From the table ename, from dept From the table dname, There are no conditions. The final query result is ？

ENAME    DNAME
-------------------------------------------
SMITH    ACCOUNTING    Invalid record 
SMITH    RESEARCH      Effective records 
SMITH    SALES         Invalid record 
SMITH    OPERATIONS    Invalid record 

ALLEN    ACCOUNTING
ALLEN    RESEARCH
ALLEN    SALES
ALLEN    OPERATIONS
.....

56 Bar record . Add a condition to achieve 4 choose 1, Also for the validity of data .

select
e.ename,d.dname
from
emp e
join
dept d
on
e.deptno = d.deptno;

Conditions are only added to avoid Cartesian product phenomenon , Just to find out the valid combination records .
The number of matches is not less , still 56 Time .

2. insert Statement can insert more than one record at a time ？【 master 】

Tolerable ！

mysql> desc t_user;
+-------------+-------------+------+-----+---------+-------+
| Field       | Type        | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| id          | int(11)     | YES  |     | NULL    |       |
| name        | varchar(32) | YES  |     | NULL    |       |
| birth       | date        | YES  |     | NULL    |       |
| create_time | datetime    | YES  |     | NULL    |       |
+-------------+-------------+------+-----+---------+-------+

Multiple records can be inserted at a time ：

insert into t_user(id,name,birth,create_time) values
(1,'zs','1980-10-11',now()),
(2,'lisi','1981-10-11',now()),
(3,'wangwu','1982-10-11',now());

 grammar ：insert into t_user( Field name 1, Field name 2) values(),(),(),();

mysql> select * from t_user;
+------+--------+------------+---------------------+
| id   | name   | birth      | create_time         |
+------+--------+------------+---------------------+
|    1 | zs     | 1980-10-11 | 2020-03-19 09:37:01 |
|    2 | lisi   | 1981-10-11 | 2020-03-19 09:37:01 |
|    3 | wangwu | 1982-10-11 | 2020-03-19 09:37:01 |
+------+--------+------------+---------------------+

3. Create tables quickly ？【 Understand the content 】

mysql> create table emp2 as select * from emp;

principle ：

Create a new query result as a table ！！！！！
This can complete the fast copy of the table ！！！！
Create a table , At the same time, the data in the table also exists ！！！

create table mytable as select empno,ename from emp where job = 'MANAGER';

4. Insert the query results into a table ？insert dependent ！！！【 Understand the content 】

create table dept_bak as select * from dept;
mysql> select * from dept_bak;
+--------+------------+----------+
| DEPTNO | DNAME      | LOC      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+

insert into dept_bak select * from dept; // Rarely used ！

mysql> select * from dept_bak;
+--------+------------+----------+
| DEPTNO | DNAME      | LOC      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+

5. Quickly delete data from a table ？【truncate More important , Must master 】

// Delete  dept_bak  Table data 
delete from dept_bak; // This method of deleting data is slow .

mysql> select * from dept_bak;
Empty set (0.00 sec)

delete Statement to delete data ？（delete Belong to DML sentence ！！！）

The data in the table has been deleted , However, the real storage space of this data on the hard disk will not be released ！！！

The disadvantage of this deletion is ： Deletion efficiency is relatively low . The advantage of this deletion is ： Support rollback , If you regret it, you can restore the data ！！！

truncate Statement to delete data ？

This deletion is more efficient , The table was truncated once , Physical delete .

This deletion has the disadvantage of ： Rollback is not supported . This deletion has the advantage of ： Fast .

usage ：truncate table dept_bak; （ This operation belongs to DDL operation .）

The big watch is very big , Hundreds of millions of records ？？？？

When deleting , Use delete, You may need to perform 1 It takes hours to delete ！ Low efficiency . You can choose to use truncate Delete data from table . Need less than 1 The deletion ends in seconds . More efficient . But use truncate Before , The customer must be asked carefully whether they really want to delete it , And warn that it cannot be recovered after deletion ！

truncate Is to delete the data in the table , The watch is still there ！

Delete table operation ？

drop table  Table name ; //  This is not to delete the data in the table , This is to delete the table .

6. The addition, deletion and modification of the table structure ？

What is the modification of table structure ？

Add a field , Delete a field , Modify a field ！！！

To modify the table structure, you need to use ：alter, Belong to DDL sentence

DDL Include ：

create drop alter

In actual development , Once the requirements are determined , Once the watch is designed , Few changes to the table structure . Because when development is in progress , Modify table structure , The cost is relatively high . Modify the structure of the table , Corresponding java The code needs a lot of modification . The cost is relatively high . This responsibility should be borne by the designer ！
Because there are few operations to modify the table structure , So we don't need to master , If you want to modify the table structure one day , You can use tools ！！！！

The operation of modifying the table structure does not need to be written to java In program . Not really java The category of programmers .

7. constraint （ It's very important , Five stars *****）

7.1. What are constraints ？

Constrain the corresponding English word ：constraint

When creating tables , We can add some constraints to the fields in the table , To ensure the integrity of the data in this table 、 effectiveness ！！！

The function of constraints is to ensure ： The data in the table is valid ！！

7.2. What are the constraints ？

Non empty constraint ：not null
Uniqueness constraint : unique
Primary key constraint : primary key （ abbreviation PK）
Foreign key constraints ：foreign key（ abbreviation FK）
Check constraint ：check（mysql I won't support it ,oracle Support ）

Here we focus on four constraints ：

not null
unique
primary key
foreign key

7.3. Non empty constraint ：not null

Non empty constraint not null Constraint field cannot be NULL

drop table if exists t_vip;
create table t_vip(
id int,
name varchar(255) not null  // not null Only column level constraints , No table level constraints ！
);
insert into t_vip(id,name) values(1,'zhangsan');
insert into t_vip(id,name) values(2,'lisi');

insert into t_vip(id) values(3);
ERROR 1364 (HY000): Field 'name' doesn't have a default value

7.3.1. episode

xxxx.sql Such documents are called sql Script files .sql A large number of... Are written in the script file sql sentence .

We execute sql Script files , All... In this file sql The statement will all execute ！

Batch execution SQL sentence , have access to sql Script files . stay mysql How to execute sql The script ？

mysql> source D:\course\03-MySQL\document\vip.sql

In your actual work , The first day I arrived at the company , The project manager will give you a xxx.sql file , You execute this script file , You have the database data on your computer ！

7.4. Uniqueness constraint : unique

Uniqueness constraint unique Constraint fields cannot be repeated , But it can be for NULL.

drop table if exists t_vip;
create table t_vip(
id int,
name varchar(255) unique,
email varchar(255)
);
insert into t_vip(id,name,email) values(1,'zhangsan','[email protected]');
insert into t_vip(id,name,email) values(2,'lisi','[email protected]');
insert into t_vip(id,name,email) values(3,'wangwu','[email protected]');
select * from t_vip;

insert into t_vip(id,name,email) values(4,'wangwu','[email protected]');
ERROR 1062 (23000): Duplicate entry 'wangwu' for key 'name'

insert into t_vip(id) values(4);
insert into t_vip(id) values(5);
+------+----------+------------------+
| id   | name     | email            |
+------+----------+------------------+
|    1 | zhangsan | zhangsan@123.com |
|    2 | lisi     | lisi@123.com     |
|    3 | wangwu   | wangwu@123.com   |
|    4 | NULL     | NULL             |
|    5 | NULL     | NULL             |
+------+----------+------------------+

name Although the field is unique Constrained , But it can be for NULL.

The new demand ：name and email The two fields are unique when combined ！！！！

drop table if exists t_vip;
create table t_vip(
id int,
name varchar(255) unique,  //  The constraint is added directly to the column , It's called column level constraints .
email varchar(255) unique
);

The creation of this table is not in line with my above “ The new demand ” Of .

This creates a representation ：name Have uniqueness ,email Have uniqueness . Each is unique .

The following data is consistent with “ The new demand ” Of .

insert into t_vip(id,name,email) values(1,'zhangsan','[email protected]');
insert into t_vip(id,name,email) values(2,'zhangsan','[email protected]');

But if you create a table in the above way , Definitely failed to create , because ’zhangsan’ and ’zhangsan’ repeated .

How to create such a table , To meet new needs ？

drop table if exists t_vip;
create table t_vip(
id int,
name varchar(255),
email varchar(255),
unique(name,email) //  The constraint is not added after the column , This constraint is called a table level constraint .
);
insert into t_vip(id,name,email) values(1,'zhangsan','[email protected]');
insert into t_vip(id,name,email) values(2,'zhangsan','[email protected]');
select * from t_vip;

name and email The two fields combine to be unique ！！！

insert into t_vip(id,name,email) values(3,'zhangsan','[email protected]');
ERROR 1062 (23000): Duplicate entry '[email protected]' for key 'name'

When to use table level constraints ？
When you need to combine multiple fields to add a constraint , Need to use table level constraints .

unique and not null Can we unite ？

drop table if exists t_vip;
create table t_vip(
id int,
name varchar(255) not null unique
);

mysql> desc t_vip;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int(11)      | YES  |     | NULL    |       |
| name  | varchar(255) | NO   | PRI | NULL    |       |
+-------+--------------+------+-----+---------+-------+

 stay mysql among , If a field is simultaneously not null and unique Bound words ,
 This field automatically becomes the primary key field .（ Be careful ：oracle It's different ！）

insert into t_vip(id,name) values(1,'zhangsan');

insert into t_vip(id,name) values(2,'zhangsan'); // Wrong. ：name Can't repeat 

insert into t_vip(id) values(2); // Wrong. ：name Not for NULL.

7.5. Primary key constraint （primary key, abbreviation PK） Very important five stars *****

7.5.1. Terms related to primary key constraints ？

Primary key constraint ： It's a constraint .
Primary key field ： A primary key constraint is added to this field , Such fields are called ： Primary key field
Primary key value ： Each value in the primary key field is called ： Primary key value .

7.5.2. What is a primary key ？ What's the use ？

The primary key value is the unique identification of each row of records . The primary key value is the ID number of each row. ！！！

remember ： Any table should have a primary key , There is no primary key , Invalid table ！！

The characteristics of primary keys ：not null + unique（ The primary key value cannot be NULL, At the same time, you can't repeat ！）

7.5.3. How to add a primary key constraint to a table ？

//  Column level constraints 
drop table if exists t_vip;
// 1 A field is used as the primary key , be called ： Single primary key 
create table t_vip(
id int primary key,
name varchar(255)
);
insert into t_vip(id,name) values(1,'zhangsan');
insert into t_vip(id,name) values(2,'lisi');

// error ： Can't repeat 
insert into t_vip(id,name) values(2,'wangwu');
ERROR 1062 (23000): Duplicate entry '2' for key 'PRIMARY'

// error ： Not for NULL
insert into t_vip(name) values('zhaoliu');
ERROR 1364 (HY000): Field 'id' doesn't have a default value

7.5.4. Can I add a primary key like this , Use table level constraints ？

Tolerable

// Table level constraints 
drop table if exists t_vip;
create table t_vip(
id int,
name varchar(255),
primary key(id)
);
insert into t_vip(id,name) values(1,'zhangsan');

// error 
insert into t_vip(id,name) values(1,'lisi');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

7.5.5. Table level constraints are mainly used to add constraints to multiple fields ？

drop table if exists t_vip;
// id and name Join forces to do ： Composite primary key ！！！！
create table t_vip(
id int,
name varchar(255),
email varchar(255),
primary key(id,name)
);
insert into t_vip(id,name,email) values(1,'zhangsan','[email protected]');
insert into t_vip(id,name,email) values(1,'lisi','[email protected]');

// error ： Can't repeat 
insert into t_vip(id,name,email) values(1,'lisi','[email protected]');
ERROR 1062 (23000): Duplicate entry '1-lisi' for key 'PRIMARY'

In actual development, it is not recommended to use ： Composite primary key . A single primary key is recommended ！

Because the significance of the primary key value is the ID number of this row. , As long as the meaning is achieved , A single primary key can do . Composite primary keys are complex , Not recommended ！！！

7.5.6. Can I add two primary key constraints to a table ？

drop table if exists t_vip;
create table t_vip(
id int primary key,
name varchar(255) primary key
);
ERROR 1068 (42000): Multiple primary key defined
 Conclusion ： A watch , Primary key constraints can only be added 1 individual .（ The primary key can only have 1 individual .）

The primary key value is recommended to use ：int, bigint, char Other types .

Not recommended ：varchar To make the primary key . Primary key values are usually numbers , It's usually fixed length ！

7.5.7. Except ： Except single primary key and composite primary key , It can also be classified in this way ？

Naturally ： The primary key value is a natural number , It has nothing to do with business .

Business primary key ： The primary key value is closely related to the business , For example, take the bank card account number as the main key value . This is the business primary key ！

7.5.8. There are many ways to use business primary keys in actual development , Or use more natural primary keys ？

Natural primary keys are used more , Because as long as the primary key is not repeated , It doesn't need to be meaningful .

Bad business primary key , Because once the primary key is linked to the business , So when the business changes , It may affect the primary key value , Therefore, business PK is not recommended . Try to use natural primary keys .

7.5.9. Automatically maintain a primary key value

//auto_increment Said the increase , from 1 Start , With 1 Increasing ！
drop table if exists t_vip;
create table t_vip(
id int primary key auto_increment,
name varchar(255)
);
insert into t_vip(name) values('zhangsan');
insert into t_vip(name) values('zhangsan');
insert into t_vip(name) values('zhangsan');
insert into t_vip(name) values('zhangsan');
insert into t_vip(name) values('zhangsan');
insert into t_vip(name) values('zhangsan');
insert into t_vip(name) values('zhangsan');
insert into t_vip(name) values('zhangsan');
select * from t_vip;

+----+----------+
| id | name     |
+----+----------+
|  1 | zhangsan |
|  2 | zhangsan |
|  3 | zhangsan |
|  4 | zhangsan |
|  5 | zhangsan |
|  6 | zhangsan |
|  7 | zhangsan |
|  8 | zhangsan |
+----+----------+

7.6. Foreign key constraints （foreign key, abbreviation FK） Very important five stars *****

7.6.1. Terms related to foreign key constraints

Foreign key constraints ： A constraint （foreign key）
Foreign key field ： A foreign key constraint is added to this field
Foreign key value ： Every value in the foreign key field .

7.6.2. Business background

Please design the database table , To describe “ Class and students ” Information about ？

First option ： Classes and students are stored in a table ？？？

t_student

no(pk)    name        classno      classname
----------------------------------------------------------------------
1         jack        100           Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
2         lucy        100           Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
3         lilei       100           Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
4         hanmeimei   100           Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
5         zhangsan    101           Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 2 class 
6         lisi        101           Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 2 class 
7         wangwu      101           Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 2 class 
8         zhaoliu     101           Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 2 class

Analyze the disadvantages of the above scheme ：

data redundancy , Space waste ！！！！ This design is a failure ！

Second option ： Class a table 、 Students a table ？？

t_class Class table

classno(pk)         classname
------------------------------------------------------
100                  Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
101                  Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class

t_student Student list

no(pk)    name          cno(FK quote t_class This watch is classno)
-------------------------------------------------------------
1         jack          100
2         lucy          100
3         lilei         100
4         hanmeimei     100
5         zhangsan      101
6         lisi          101
7         wangwu        101
8         zhaoliu       101

When cno When the field has no constraints , May cause invalid data . There could be a 102, however 102 Class does not exist .

So to make sure cno The values in the field are 100 and 101, Need to give cno Field add foreign key constraint , that cno Fields are foreign key fields ,cno Each value in the field is a foreign key value .

7.6.3. Be careful

t_class It's the father's watch ,t_student It's a subtable

The order in which tables are deleted ？ Delete the sub first , Then delete the father .

The order in which tables are created ？ Create the parent first , Create Jianzi again .

The order in which data is deleted ？ Delete the sub first , Then delete the father .

The order in which the data is inserted ？ Insert parent first , Then insert the sub .

reflection ： The foreign key in the child table refers to a field in the parent table , Must the referenced field be a primary key ？

Not necessarily the primary key , But at least it has unique constraint .

7.6.4. Foreign key can be NULL Do you ？

The foreign key value can be NULL.

7.6.5. Create foreign key constraints

// foreign key( Foreign key name ) references  Table name ( Field name )
create table students(
id int auto_increment primary key,
uid int not null,
name varchar(6) not null,
foreign key(uid) references class(xuehao)
);

8. Storage engine （ Understand the content ）

8.1. What is a storage engine , What's the use ？

The storage engine is MySQL A term peculiar to , Not in other databases .（Oracle There is , But not by that name ）

The name of storage engine is high-end and high-grade , In fact, the storage engine is a table storage / How to organize data . Different storage engines , Tables store data differently .

8.2. How to add... To a table / Appoint “ Storage engine ” Well ？

show create table t_student;

You can assign a storage engine to the table when creating the table .

CREATE TABLE `t_student` (
`no` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`cno` int(11) DEFAULT NULL,
PRIMARY KEY (`no`),
KEY `cno` (`cno`),
CONSTRAINT `t_student_ibfk_1` FOREIGN KEY (`cno`) REFERENCES `t_class` (`classno`)
) ENGINE=InnoDB AUTO_INCREMENT=11 DEFAULT CHARSET=utf8

When creating a table, it can be in the last parenthesis ")" Use... On the right of ：

ENGINE To specify the storage engine .
CHARSET To specify the character encoding of this table .

8.2.1. Conclusion

mysql The default storage engine is ：InnoDB, The default character encoding method is ：utf8

Specify the storage engine when creating the table , And character encoding .

create table t_product(
id int primary key,
name varchar(255)
)engine=InnoDB default charset=gbk;

8.3. How to check mysql What storage engines are supported ？

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.5.36    |
+-----------+

command ：

show engines \G

*************************** 1. row ***************************
      Engine: FEDERATED
     Support: NO
     Comment: Federated MySQL storage engine
Transactions: NULL
          XA: NULL
  Savepoints: NULL
*************************** 2. row ***************************
      Engine: MRG_MYISAM
     Support: YES
     Comment: Collection of identical MyISAM tables
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 3. row ***************************
      Engine: MyISAM
     Support: YES
     Comment: MyISAM storage engine
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 4. row ***************************
      Engine: BLACKHOLE
     Support: YES
     Comment: /dev/null storage engine (anything you write to it disappears
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 5. row ***************************
      Engine: CSV
     Support: YES
     Comment: CSV storage engine
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 6. row ***************************
      Engine: MEMORY
     Support: YES
     Comment: Hash based, stored in memory, useful for temporary tables
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 7. row ***************************
      Engine: ARCHIVE
     Support: YES
     Comment: Archive storage engine
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 8. row ***************************
      Engine: InnoDB
     Support: DEFAULT
     Comment: Supports transactions, row-level locking, and foreign keys
Transactions: YES
          XA: YES
  Savepoints: YES
*************************** 9. row ***************************
      Engine: PERFORMANCE_SCHEMA
     Support: YES
     Comment: Performance Schema
Transactions: NO
          XA: NO
  Savepoints: NO

mysql Support nine storage engines , At present 5.5.36 Support 8 individual . Different versions support different situations .

8.4. About mysql Common storage engines , A brief introduction

8.4.1. MyISAM Storage engine

The tables it manages have the following characteristics , Use three files to represent each table ：

Format file — Definition of storage table structure （mytable.frm）
Data files — Store the contents of table rows （mytable.MYD）
Index file — Store indexes on tables （mytable.MYI）： The index is the catalogue of a book , Narrow the scanning range , A mechanism to improve query efficiency . Can be converted to compressed 、 Read only tables to save space

Hint ：

For a table , As long as it's a primary key , Or add unique An index is automatically created on the constrained field .

8.4.2. MyISAM Storage engine

Can be converted to compressed 、 Read only tables to save space , This is the advantage of this storage engine ！！！！

MyISAM Transaction mechanism is not supported , Low security .

8.4.3. InnoDB Storage engine

This is a mysql The default storage engine , It is also a heavyweight storage engine .

InnoDB Support transactions , Support automatic recovery mechanism after database crash .InnoDB The main feature of the storage engine is ： Very safe .

The tables it manages have the following main characteristics ：

Every InnoDB The table is in the database directory as .frm The format file represents
InnoDB Table space tablespace Used to store the contents of a table （ A tablespace is a logical name . Tablespaces store data + Indexes .）
Provides a set of log files used to record transactional activities
use COMMIT( Submit )、SAVEPOINT And ROLLBACK( Roll back ) Support transaction processing
Provide full ACID compatible
stay MySQL Provide automatic recovery after server crash
Many versions （MVCC） And row level locking
Support the integrity of foreign keys and references , Including cascading deletion and update

InnoDB The biggest feature is to support transactions ：

To ensure data security . Not very efficient , And can't compress , Cannot convert to read-only , It can't save storage space well .

8.4.4. MEMORY Storage engine

Use MEMORY The table that stores the engine , Its data is stored in memory , And the length of the line is fixed , These two characteristics make MEMORY The storage engine is very fast .

MEMORY The tables managed by the storage engine have the following characteristics ：

In the database directory , Each table is marked with .frm Format file representation .
Table data and indexes are stored in memory .（ The goal is to , Quick query ！）
Table level locking mechanism .
Can not contain TEXT or BLOB Field .

MEMORY The storage engine was formerly known as HEAP engine .

MEMORY Engine benefits ： The query efficiency is the highest . No need to interact with the hard disk .

MEMORY Engine faults ： unsafe , The data disappears after shutdown . Because the data and index are in memory .

9. Business （ a key ： Five stars `*****`, Must understand , Must master ）

9.1. What is business ？

A transaction is actually a complete business logic . Is the smallest unit of work . Can not be further divided .

What is a complete business logic ？

Hypothetical transfer , from A Account to B Transfer from account 10000, take A The money in the account minus 10000（update sentence ）, take B The money in the account plus 10000（update sentence ）, This is a complete business logic .

The above operation is the smallest unit of work , Or at the same time , Or fail at the same time , Can not be further divided .

these two items. update Statement requires both success and failure , In this way, we can ensure that the money is right .

9.2. Only DML Statement will have the statement of transaction , Other statements have nothing to do with transactions

insert
delete
update

Only the above three statements are related to transactions , Nothing else matters .

Because only the above three statements are added to the data in the database table 、 Delete 、 Changed , As long as your operation involves the increase of data 、 Delete 、 Change , Then we must consider the safety issue .

Data security comes first ！！！

9.3. Suppose all businesses , Just one DML Statement can be completed , Is it necessary to have a transaction mechanism ？

Because when you do something , Multiple lines are required DML Statements can be combined to complete , Therefore, we need the existence of transactions . If anything complicated can be done in one way DML Statement handling , Then the transaction has no value of existence .

What exactly is a business ？

At the end of the day , Speaking of essence , A transaction is actually multiple DML Statements succeed at the same time , Or fail at the same time ！

Business ： It's batch DML Statements succeed at the same time , Or fail at the same time ！

9.4. How to do multiple transactions DML What about statements that succeed and fail at the same time ？

InnoDB Storage engine ： Provides a set of log files used to record transactional activities

The transaction is on

insert
insert
delete
delete
update
update

The business is over ！

During the execution of a transaction , Every one of them DML All operations will be recorded in “ Log files for transactional activities ” in .

During the execution of a transaction , We can commit transactions , Transactions can also be rolled back .

9.4.1. Commit transaction ？

Clear the log file of transactional activities , Completely persist all data into database tables .

Committing a transaction marks , The end of the business . And it's the end of all success .

9.4.2. Roll back the transaction ？

All the previous DML Undo all operations , And clear the log files of transactional activities

Rolling back a transaction marks , The end of the business . And it's the end of all failure .

9.5. How to commit transactions , How to roll back a transaction ？

Commit transaction ：commit; sentence
Roll back the transaction ：rollback; sentence （ Rollback can only be rolled back to the last commit point ！）

The English word for a transaction is ：transaction

Test it , stay mysql What is the default transaction behavior ？
mysql Automatic transaction submission is supported by default .（ Automatic submission ）

9.5.1. What is auto submit ？

Every time you execute one DML sentence , Then submit once ！

This automatic submission is actually not in line with our development habits , Because a business usually needs multiple DML Statements can be executed together , To ensure data security , You must ask for success before submitting , So you can't execute one and submit one .

9.5.2. How to mysql The automatic submission mechanism is turned off ？

start transaction;
...
commit

9.5.3. Roll back the transaction

mysql> use bjpowernode;
Database changed
mysql> select * from dept_bak;
Empty set (0.00 sec)

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into dept_bak values(10,'abc', 'tj');
Query OK, 1 row affected (0.00 sec)

mysql> insert into dept_bak values(10,'abc', 'tj');
Query OK, 1 row affected (0.00 sec)

mysql> select * from dept_bak;
+--------+-------+------+
| DEPTNO | DNAME | LOC  |
+--------+-------+------+
|     10 | abc   | tj   |
|     10 | abc   | tj   |
+--------+-------+------+
2 rows in set (0.00 sec)

mysql> rollback;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from dept_bak;
Empty set (0.00 sec)

9.5.4. Commit transaction

mysql> use bjpowernode;
Database changed
mysql> select * from dept_bak;
+--------+-------+------+
| DEPTNO | DNAME | LOC  |
+--------+-------+------+
|     10 | abc   | bj   |
+--------+-------+------+
1 row in set (0.00 sec)

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into dept_bak values(20,'abc', 'tj');
Query OK, 1 row affected (0.00 sec)

mysql> insert into dept_bak values(20,'abc','tj');
Query OK, 1 row affected (0.00 sec)

mysql> insert into dept_bak values(20,'abc','tj');
Query OK, 1 row affected (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from dept_bak;
+--------+-------+------+
| DEPTNO | DNAME | LOC  |
+--------+-------+------+
|     10 | abc   | bj   |
|     20 | abc   | tj   |
|     20 | abc   | tj   |
|     20 | abc   | tj   |
+--------+-------+------+
4 rows in set (0.00 sec)

// Cannot roll back after commit 
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from dept_bak;
+--------+-------+------+
| DEPTNO | DNAME | LOC  |
+--------+-------+------+
|     10 | abc   | bj   |
|     20 | abc   | tj   |
|     20 | abc   | tj   |
|     20 | abc   | tj   |
+--------+-------+------+
4 rows in set (0.00 sec)

9.6. The business includes 4 A feature ？

9.6.1. A： Atomicity

It shows that transaction is the smallest unit of work . Can not be further divided .

9.6.2. C： Uniformity

All transactions require , In the same transaction , All operations must be successful at the same time , Or fail at the same time , To ensure data consistency .

9.6.3. I： Isolation,

A Business and B There is some isolation between transactions . classroom A And classroom B There is a wall between , This wall is isolation .

A When a transaction operates on a table , Another business B What will happen if you also operate this table ？ See later

9.6.4. D： persistence

One guarantee for the final end of a transaction is the transaction commit , It is equivalent to saving data that has not been saved to the hard disk

Save to hard disk ！

9.7. Focus on the isolation of transactions

A Classroom and B There is a wall in the middle of the classroom , This wall can be very thick , It can also be very thin . This is the isolation level of the transaction .
The thicker the wall , Indicates that the higher the isolation level .

What are the isolation levels between transactions ？4 A level

9.7.1. Read uncommitted ：read uncommitted（ Lowest isolation level ）《 Did not submit to read 》

What is read uncommitted ？ Business A Can read to transaction B Uncommitted data .

The problem with this isolation level is ： Dirty reading ！(Dirty Read), We said we read dirty data .

This level of isolation is generally theoretical , Most database isolation levels are second-class ！

9.7.2. Read submitted ：read committed《 You can't read... Until you submit it 》

What is read committed ？ Business A Only transactions can be read B Data after submission .

What problem does this isolation level solve ？ Solved the phenomenon of dirty reading .

What's wrong with this level of isolation ？ Data cannot be read repeatedly .

What is unrepeatable data ？ After the transaction is opened , The first data read is 3 strip , The current transaction is not over yet , Maybe the second time I read it again , The data read is 4 strip ,3 It's not equal to 4 It is called non repeatable read .

This level of isolation is more real data , The data read every time is absolutely true .

oracle The default isolation level of the database is ：read committed

9.7.3. Repeatable ：repeatable read《 I can't read it after submitting it , Always read the data when the transaction was just started 》

What is a repeatable read ？ Business A After opening , No matter how long , Every time in business A The data read in is consistent , Even if the business B The data has been modified , And submitted , Business A The read data still hasn't changed , This is repeatable reading .

What problem does repeatable reading solve ？ It solves the problem of non repeatable data reading .

What are the problems with repeatable reading ？ There can be phantom reading . Every time the data is read, it is an illusion . Not true enough ！

In the morning 9 The transaction started at , As long as the business doesn't end , until night 9 spot , The data read is still like that ！ What I read is an illusion . Not absolutely true .

mysql This is the default transaction isolation level in ！！！！！！！！！！！

9.7.4. Serialization ：serializable（ Highest isolation level ）

This is the highest isolation level , Minimum efficiency . Solved all the problems .

This isolation level indicates that transactions are queued , Can't be concurrent ！ Efficiency is the lowest .

9.8. Check the isolation level

// 5.x  edition 
SELECT @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+

// 8.x  edition 
SELECT @@transaction_isolation;
+-------------------------+
| @@transaction_isolation |
+-------------------------+
| REPEATABLE-READ         |
+-------------------------+

9.9. Set the isolation level

// set global transaction isolation level +  Isolation level ;
set global transaction isolation level read uncommitted;

9.10. Examples of various isolation levels

9.10.1. Read uncommitted read uncommited

mysql> set global transaction isolation level read uncommitted;
 Business A                            Business B
--------------------------------------------------------------------------------
use bjpowernode;
                                use bjpowernode;
start transaction;
select * from t_user;
                                start transaction;
                                insert into t_user values('zhangsan');
select * from t_user;

9.10.2. Read submitted read commited

mysql> set global transaction isolation level read committed;
 Business A                            Business B
--------------------------------------------------------------------------------
use bjpowernode;
                                use bjpowernode;
start transaction;
                                start transaction;
select * from t_user;
                                insert into t_user values('zhangsan');
select * from t_user;
                                commit;
select * from t_user;

9.10.3. Repeatable repeatable read

mysql> set global transaction isolation level repeatable read;
 Business A                            Business B
--------------------------------------------------------------------------------
use bjpowernode;
                                use bjpowernode;
start transaction;
                                start transaction;
select * from t_user;
                                insert into t_user values('lisi');
                                insert into t_user values('wangwu');
                                commit;
select * from t_user;

9.10.4. Serialization serializable

mysql> set global transaction isolation level serializable;
 Business A                            Business B
--------------------------------------------------------------------------------
use bjpowernode;
                                use bjpowernode;
start transaction;
                                start transaction;
select * from t_user;
insert into t_user values('abc');
                                select * from t_user;

10. download

10.1. This document

https://hty.ink/down/mysql/day_3.zip

10.2. Information involved

https://hty.ink/down/mysql/ziliao.zip

11. Read on

11.1. Last one

https://hty.ink/2293/

11.2. Next

https://hty.ink/2213/

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