MySQL day03 class notes

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

 First of all ： In actual development , Once the requirements are determined , Once the watch is designed , Very few 
 Modify 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 ！

 second ： Because there are few operations to modify the table structure , So we don't need to master , If one day 
 Really want to change the table structure , 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 that the data in this table 
 integrity 、 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

 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 | [email protected] |
|    2 | lisi     | [email protected]     |
|    3 | wangwu   | [email protected]   |
|    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 in line with my “ The new demand ” Of .
	 But if you create a table in the above way , Definitely failed to create , because 'zhangsan' and 'zhangsan' repeated .
	insert into t_vip(id,name,email) values(1,'zhangsan','[email protected]');
	insert into t_vip(id,name,email) values(2,'zhangsan','[email protected]');

	 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 *****

 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 .

 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 ！）

 How to add a primary key constraint to a table ？
	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,  // Column level constraints 
		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

 Can I add a primary key like this , Use table level constraints ？
	drop table if exists t_vip;
	create table t_vip(
		id int,
		name varchar(255),
		primary key(id)  //  Table level constraints 
	);
	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'

 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 ！！！

 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 ！

 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 ！

	 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 .

 stay mysql among , There is a mechanism , It can help us maintain a primary key value automatically ？
	drop table if exists t_vip;
	create table t_vip(
		id int primary key auto_increment, //auto_increment Said the increase , from 1 Start , With 1 Increasing ！
		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 *****

 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 .


 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 2 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 .

	 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 .

	drop table if exists t_student;
	drop table if exists t_class;

	create table t_class(
		classno int primary key,
		classname varchar(255)
	);
	create table t_student(
		no int primary key auto_increment,
		name varchar(255),
		cno int,
		foreign key(cno) references t_class(classno)  // Add a foreign key constraint 
	);

	insert into t_class(classno, classname) values(100, ' Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class ');
	insert into t_class(classno, classname) values(101, ' Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 2 class ');

	insert into t_student(name,cno) values('jack', 100);
	insert into t_student(name,cno) values('lucy', 100);
	insert into t_student(name,cno) values('lilei', 100);
	insert into t_student(name,cno) values('hanmeimei', 100);
	insert into t_student(name,cno) values('zhangsan', 101);
	insert into t_student(name,cno) values('lisi', 101);
	insert into t_student(name,cno) values('wangwu', 101);
	insert into t_student(name,cno) values('zhaoliu', 101);

	select * from t_student;
	select * from t_class;

	 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 .

	 test ： Foreign key can be NULL Do you ？
		 The foreign key value can be NULL.

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 .

	 Conclusion ：
		mysql The default storage engine is ：InnoDB
		mysql 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 Introduce the common storage engines

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 .

	MyISAM Storage engine features ：
		 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 .

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 .

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 add 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
insert
delete
update
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 .

 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 .

 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 ）
	 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 of a business 
	 It usually requires more than one DML Statements can be executed together , In order to guarantee the data 
	 The safety of the , You must ask for success before submitting , So you can't execute one 
	 Just submit one .

 How to mysql The automatic submission mechanism is turned off ？
	 Execute this order first ：start transaction;

 Demonstrate transaction ：
	--------------------------------- 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)


	--------------------------------- 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
	Query OK, 1 row affected (0.00 sec)

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

	mysql> insert into dept_bak values(20,'abc
	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)

	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 ？

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

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 .

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 If you also operate this table, it will be like that ？？？

D： persistence 
	 A guarantee of the end of the transaction . Transaction submission , 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 

	 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 ！

	 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 has not yet 
				 end , 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

	 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 Data read from 
			 It's all 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 ！！！！！！！！！！！

	 serialize / 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 ！
		synchronized, Thread synchronization （ Transaction synchronization ）
		 The data read every time is the most real , And the efficiency is the lowest .

9.8、 Verify various isolation levels

Check the isolation level ：SELECT @@tx_isolation
±----------------+
| @@tx_isolation |
±----------------+
| REPEATABLE-READ |
±----------------+
mysql Default isolation level

The table being tested t_user
verification ：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;

verification ：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;

verification ：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;

verification ：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;