Summary of easy-to-use MySQL interview questions (Part 1)

Course introduction ：

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 .

MySQL Introduction to database B Station portal ：

https://www.bilibili.com/video/BV1fx411X7BD

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What is? MySQL?

MySQL It's a relational database , stay Java Very common in enterprise development , because MySQL It's open source and free , And it's easy to expand . Alibaba database system is also widely used MySQL, So its stability is guaranteed .MySQL It's open source , So anyone can be in GPL(General Public License) Download and modify it according to the needs of personalization .MySQL The default port number for is 3306.

Transaction related （ What is business ?）

A transaction is a logical set of operations , Or both , Either not .

The most classic and often cited example of a transaction is a transfer . If Xiaoming wants to transfer money to Xiaohong 1000 element , There are two key operations involved in this transfer ： Reduce Xiao Ming's balance 1000 element , Increase the balance of Xiaohong 1000 element . In case of a sudden error between these two operations, such as the collapse of the banking system , As a result, Xiaoming's balance decreased while Xiaohong's balance did not increase , That's not right . Transaction is to ensure the success of both critical operations , Or they all have to fail .

The four characteristics of things (ACID)：

• Atomicity ： Transactions are the smallest unit of execution , Division is not allowed . The atomicity of the transaction ensures that the action is either complete , Or it doesn't work at all ;
• Uniformity ： Before and after the execution of the transaction , Data consistency , Multiple transactions read the same data with the same result ;
• Isolation, ： When accessing the database concurrently , One user's transaction is not interfered by other transactions , The database between concurrent transactions is independent ;
• persistence ： After a transaction is committed . Its changes to the data in the database are persistent , Even if the database fails, it should not have any impact .

What are the problems of concurrent transactions ?

In a typical application , Multiple transactions run concurrently , Often operate the same data to complete their own tasks （ Multiple users operate on unified data ）. Concurrency is necessary , But it may cause the following problems .

• Dirty reading （Dirty read）: When a transaction is accessing data and modifying the data , This modification has not yet been committed to the database , At this time, another transaction also accesses the data , And then I used this data . Because this data is not submitted yet , So the data read by another transaction is “ Dirty data ”, basis “ Dirty data ” The operation may not be correct .
• Missing changes （Lost to modify）: When a transaction reads a data , Another transaction also accesses the data , After modifying the data in the first transaction , The second transaction also modifies the data . In this way, the modification result in the first transaction will be lost , So it's called lost modification . for example ： Business 1 Read data from a table A=20, Business 2 Also read A=20, Business 1 modify A=A-1, Business 2 Also modify A=A-1, final result A=19, Business 1 The modification of is lost .
• It can't be read repeatedly （Unrepeatableread）: Reading the same data multiple times in a transaction . Before the end of the business , Another transaction also accesses the data . that , Between two reads in the first transaction , Due to the modification of the second transaction, the data read by the first transaction twice may be different . This happens when the data read twice in a transaction is different , So it's called unrepeatable reading .
• Fantasy reading （Phantom read）: Unreal reading is similar to nonrepeatable reading . It happens in a transaction （T1） Read a few lines of data , Then another concurrent transaction （T2） When some data is inserted . In the subsequent query , The first thing （T1） There will be more records that don't exist , It's like an illusion , So it's called Unreal reading .

The difference between nonrepeatability and unreal reading ：

The point of unrepeatable reading is to modify , The point of unreal reading is to add or delete .

example 1（ The same conditions , The data you read , Read it again and find that the value is different ）： Business 1 Medium A Mr. a read his salary as 1000 The operation of is not finished , Business 2 Medium B Mr. a modified A The salary of is 2000, guide Cause A When I read my salary again, it changed to 2000; This is unrepeatable reading .

example 2（ The same conditions , The first 1 Time and number 2 The number of records read is not the same ）： The salary in a payroll is greater than 3000 There are 4 people , Business 1 Read all salaries greater than 3000 People who , A total of 4 Bar record , When the transaction 2 Another one is inserted that the salary is greater than 3000 The record of , Business 1 The record found when reading again becomes 5 strip , This leads to unreal reading .

What are the transaction isolation levels ?MySQL The default isolation level for is ?

SQL The standard defines four isolation levels ：

• READ-UNCOMMITTED( Read uncommitted )： Lowest isolation level , Allow read of uncommitted data changes , Can cause dirty reading 、 Phantom or unrepeatable reading .
• READ-COMMITTED( Read committed )： Allow to read data submitted by concurrent transactions , Can prevent dirty reading , But phantom or unrepeatable reads can still occur .
• REPEATABLE-READ( Repeatable )： Multiple reads of the same field are consistent , Unless the data is modified by the transaction itself , Can prevent dirty and unrepeatable read , But phantom reading can still happen .
• SERIALIZABLE( Serializable )： Highest isolation level , Completely obey ACID Isolation level . All transactions are executed one by one , In this way, there is no interference between transactions , in other words , This level prevents dirty reads 、 Unrepeatable reading and phantom reading .

MySQL InnoDB The default isolation level supported by the storage engine is REPEATABLE-READ（ Can be reread ）. We can go through SELECT @@tx_isolation; Order to see

mysql> SELECT @@tx_isolation;

+-----------------+

| @@tx_isolation |

+-----------------+

| REPEATABLE-READ |

+-----------------+

What needs to be noted here is ： And SQL The difference in standards is InnoDB Storage engine in REPEATABLE-READ（ Can be reread ） The transaction isolation level uses Next-Key Lock Lock algorithm , So we can avoid the production of unreal reading , It works with other database systems ( Such as SQL Server) Is different . So InnoDB The default isolation level supported by the storage engine is REPEATABLE-READ（ Can be reread ） The transaction isolation requirements can be fully guaranteed , That is to say SQL The standard SERIALIZABLE( Serializable ) Isolation level .

Because the lower the isolation level , The fewer locks the transaction requests , So the isolation level of most database systems is READ-COMMITTED( Read submissions ):, But what you need to know is InnoDB The storage engine uses... By default REPEATABLE-READ（ Can be reread ） There will be no loss of performance .

InnoDB Storage engine in Distributed transactions In general, we use SERIALIZABLE( Serializable ) Isolation level .

To be continued ......