TCL statements in SQL (transaction control statements)

Catalog

Business

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

Commit transaction

Roll back the transaction

The business includes 4 A feature

Isolation,

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TCL Statement represents the transaction control language

• commit
  • Transaction submission
• rollback
  • Transaction rollback

Although there are only two statements , But it's connected MySQL A very important point in ： Business .

Business

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

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 ！！！

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

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

Input commit Command means commit transaction

mysql Automatic transaction submission is supported by default . And by default , Every time you execute one DML sentence , Then submit once ！

Input start transaction; The auto commit mechanism can be turned off

Roll back the transaction

All the previous DML Undo all operations , And clearing the log files of transactional activities and rolling back transactions marks , The end of the business . And it's the end of all failure .

Input rollback Command means rollback transaction （ Rollback can only be rolled back to the last commit point ！）

The business includes 4 A feature

abbreviation ACIB

• Atomicity
  • It shows that transaction is the smallest unit of work . Can not be further divided .
• 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 .
• 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 ？？？
• persistence
  • A guarantee of the end of the transaction . Transaction submission , It is equivalent to saving data not saved to the hard disk to the hard disk ！

Isolation,

Isolation has come again 4 A level

• Read uncommitted ：read uncommitted（ Lowest isolation level ）
  • It's a business A Can read to transaction B Uncommitted data . To put it simply, I didn't submit my study
  • 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
  • It's a business A Only transactions can be read B Data after submission . Simply put, you can only read after submitting
  • Solved the phenomenon of dirty reading . However, there is a problem that data cannot be read repeatedly
    • 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
• Repeatable ：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 . Simply put, you can't read it after submitting , Always read the data when the transaction was just started
  • It solves the problem of non repeatable data reading . But phantom reading may occur . Every time the data is read, it is an illusion . Not true enough ！
  • mysql This is the default transaction isolation level in ！！！！！！！！！！！
• serialize ：serializable（ Highest isolation level ）
  • Also called serialization
    • This is the highest isolation level , Minimum efficiency . Solved all the problems .
    • This isolation level indicates that transactions are queued , Can't be concurrent ！
    • The data read every time is the most real , And the efficiency is the lowest .