Sets the MySQL transaction isolation level and mysql transaction level.

Sets the MySQL transaction isolation level and mysql transaction level.
Select @ tx_isolation; // view isolation level
Set transaction isolation level read uncommitted; // you can specify the read uncommitted level.
Start transaction; // open the transaction
Rollback; // rollback
Commit; // submit
Set transaction isolation level read committed; // you can specify the read submission level.
Set transaction isolation level repeatable read; // sets the repeatable read (default) to ensure that the results of each read are the same
Set transaction isolation level serializable; // set to serialization
How to isolate MySql transactions

MySQL InnoDB tables have
Transaction-isolation =
READ-UNCOMMITTED
READ-COMMITTED
REPEATABLE-READ
SERIALIZABLE
Four
None for MyIsam,
You can see what type of Enginee is in your table.

Detailed description of database and mysql transaction isolation level repeatable-read

First question:
The new version of mysql solves the phantom read problem through mvcc, so you do not see the phantom read Phenomenon

Second question:
Transaction B is unable to see the changes made by transaction A, unless your transaction B level is lower than repeatable-read