MySQL transactions

One 、 The basic elements of a business （ACID）

　　1、 Atomicity （Atomicity）： All operations after the transaction begins , Or all of them , Or not at all , It's impossible to stay in the middle . An error occurred during the execution of the transaction , It rolls back to the state it was in before the transaction started , All the operations are as if they didn't happen . That is to say, a business is an indivisible whole , It's like atoms in chemistry , Is the basic unit of material composition .

　　 2、 Uniformity （Consistency）： Before and after the transaction starts , The database integrity constraint is not broken . such as A towards B Transfer accounts , impossible A Buckle the money ,B Has received .

　　 3、 Isolation, （Isolation）： At the same time , Only one transaction is allowed to request the same data , There is no interference between the different transactions . such as A Withdrawing money from a bank card , stay A Before the withdrawal process is over ,B You can't transfer money to this card .

　　 4、 persistence （Durability）： After the transaction completes , All updates to the database by the transaction are saved to the database , Cannot be rolled back .

Two 、 Concurrency of transactions

　　1、 Dirty reading ： Business A Read transaction B Updated data , then B Rollback operation , that A The data read is dirty

　　2、 It can't be read repeatedly ： Business A Read the same data multiple times , Business B In the transaction A During multiple reads , The data has been updated and submitted , Cause transaction A When reading the same data multiple times , result atypism .

　　3、 Fantasy reading ： System administrator A Change the scores of all students in the database from specific scores to ABCDE Grade , But the system administrator B At this time, a specific score record was inserted , When the system administrator A After the change, I found that there is another record that hasn't been changed , It's like an illusion , This is called Unreal reading .

　　 Summary ： It's easy to confuse unreal reading with unreal reading , Unrepeatable reading focuses on revision , Unreal reading focuses on adding or deleting . To solve the problem of non repeatable reading, we only need to lock the line that meets the condition , To solve unreal reading, we need to lock the watch

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