The relationship between the MySQL transaction isolation level and the lock

In fact, it's been a long time. MySQL has never been particularly clear about the relationship between the lock and the MySQL transaction. Then search the United States Regiment this article, by the way, combined with the problems you have encountered summary.

The first transaction has four isolation levels:

Reference:

Relationship of transaction isolation level and lock in http://tech.meituan.com/innodb-lock.html InnoDB

The relationship between the MySQL transaction isolation level and the lock