The default user name of MySQL is usually 'root', but the password varies according to the installation environment; in some Linux distributions, the root account may be authenticated by auth_socket plug-in and cannot log in with the password; when installing tools such as XAMPP or WAMP under Windows, root users usually have no password or use common passwords such as root, mysql, etc.; if you forget the password, you can reset it by stopping the MySQL service, starting in --skip-grant-tables mode, updating the mysql.user table to set a new password and restarting the service; note that the MySQL8.0 version requires additional authentication plug-ins.

GTID (Global Transaction Identifier) ??solves the complexity of replication and failover in MySQL databases by assigning a unique identity to each transaction. 1. It simplifies replication management, automatically handles log files and locations, allowing slave servers to request transactions based on the last executed GTID. 2. Ensure consistency across servers, ensure that each transaction is applied only once on each server, and avoid data inconsistency. 3. Improve troubleshooting efficiency. GTID includes server UUID and serial number, which is convenient for tracking transaction flow and accurately locate problems. These three core advantages make MySQL replication more robust and easy to manage, significantly improving system reliability and data integrity.

MySQL main library failover mainly includes four steps. 1. Fault detection: Regularly check the main library process, connection status and simple query to determine whether it is downtime, set up a retry mechanism to avoid misjudgment, and can use tools such as MHA, Orchestrator or Keepalived to assist in detection; 2. Select the new main library: select the most suitable slave library to replace it according to the data synchronization progress (Seconds_Behind_Master), binlog data integrity, network delay and load conditions, and perform data compensation or manual intervention if necessary; 3. Switch topology: Point other slave libraries to the new master library, execute RESETMASTER or enable GTID, update the VIP, DNS or proxy configuration to

There are three ways to modify or reset MySQLroot user password: 1. Use the ALTERUSER command to modify existing passwords, and execute the corresponding statement after logging in; 2. If you forget your password, you need to stop the service and start it in --skip-grant-tables mode before modifying; 3. The mysqladmin command can be used to modify it directly by modifying it. Each method is suitable for different scenarios and the operation sequence must not be messed up. After the modification is completed, verification must be made and permission protection must be paid attention to.

The steps to connect to the MySQL database are as follows: 1. Use the basic command format mysql-u username-p-h host address to connect, enter the username and password to log in; 2. If you need to directly enter the specified database, you can add the database name after the command, such as mysql-uroot-pmyproject; 3. If the port is not the default 3306, you need to add the -P parameter to specify the port number, such as mysql-uroot-p-h192.168.1.100-P3307; In addition, if you encounter a password error, you can re-enter it. If the connection fails, check the network, firewall or permission settings. If the client is missing, you can install mysql-client on Linux through the package manager. Master these commands

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InnoDB implements repeatable reads through MVCC and gap lock. MVCC realizes consistent reading through snapshots, and the transaction query results remain unchanged after multiple transactions; gap lock prevents other transactions from inserting data and avoids phantom reading. For example, transaction A first query gets a value of 100, transaction B is modified to 200 and submitted, A is still 100 in query again; and when performing scope query, gap lock prevents other transactions from inserting records. In addition, non-unique index scans may add gap locks by default, and primary key or unique index equivalent queries may not be added, and gap locks can be cancelled by reducing isolation levels or explicit lock control.

MySQL transactions follow ACID characteristics to ensure the reliability and consistency of database transactions. First, atomicity ensures that transactions are executed as an indivisible whole, either all succeed or all fail to roll back. For example, withdrawals and deposits must be completed or not occur at the same time in the transfer operation; second, consistency ensures that transactions transition the database from one valid state to another, and maintains the correct data logic through mechanisms such as constraints and triggers; third, isolation controls the visibility of multiple transactions when concurrent execution, prevents dirty reading, non-repeatable reading and fantasy reading. MySQL supports ReadUncommitted and ReadCommi.