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  • How to read the output of the EXPLAIN command and which columns are important?
    How to read the output of the EXPLAIN command and which columns are important?
    When running the EXPLAIN command, you should first pay attention to four core contents: connection type, index usage, number of scanned lines and additional information. 1. The connection type (such as eq_ref, const, ref is efficient, and ALL is inefficient) reflects the table connection efficiency; 2. Index-related fields (key, key_len, ref) show whether the index is used correctly; 3. The rows column estimates the number of rows scanned by the query, and a large value indicates potential performance problems; 4. Extra information (such as Usingfilesort, Usingtemporary needs to be avoided, Usingindex is an ideal state) provides optimization direction. Optimization strategies include: Prioritize the use of efficient connection types, add or adjust indexes to improve query efficiency
    Mysql Tutorial . Database 825 2025-06-14 00:02:21
  • How to back up and restore a database using mysqldump?
    How to back up and restore a database using mysqldump?
    The key commands for backing up and restoring the database using mysqldump are as follows: 1. Use mysqldump-u[username]-p[database name]>[output file path] to backup the database, such as mysqldump-uroot-pmydb>/backup/mydb_backup.sql; 2. Use mysql-u[username]-p[target database name] to restore the database; 2. Use mysql-u[username]-p[target database name] to restore the database;
    Mysql Tutorial . Database 525 2025-06-13 00:35:11
  • What is the default username and password for MySQL?
    What is the default username and password for MySQL?
    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.
    Mysql Tutorial . Database 669 2025-06-13 00:34:51
  • How to change or reset the MySQL root user password?
    How to change or reset the MySQL root user password?
    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.
    Mysql Tutorial . Database 995 2025-06-13 00:33:31
  • What is the difference between VARCHAR and CHAR data types in MySQL?
    What is the difference between VARCHAR and CHAR data types in MySQL?
    Choosing CHAR or VARCHAR depends on data characteristics and performance requirements. CHAR is suitable for data with fixed lengths such as country codes or gender identification, with fixed storage space and high query efficiency; VARCHAR is suitable for data with large lengths such as names or addresses, saving storage space but may sacrifice part of performance; CHAR is up to 255 characters, VARCHAR can reach 65535 characters; CHAR will automatically fill in spaces while VARCHAR ignores tail spaces; small items are not much different, but selection in large-scale data tables will affect performance and storage efficiency.
    Mysql Tutorial . Database 587 2025-06-13 00:32:00
  • How to count the total number of rows in a table?
    How to count the total number of rows in a table?
    The clear answer to counting the total number of rows in the table is to use the database counting function. The most direct method is to execute the SQL COUNT() function, for example: SELECTCOUNT()AStotal_rowsFROMyour_table_name; secondly, for big data tables, you can view the system table or information schema to get the estimated value, such as PostgreSQL uses SELECTreltuplesFROMpg_classWHERErelname='your_table_name'; MySQL uses SELECTTABLE_ROWSFROMinformation_schema.TABLESWHERETA
    Mysql Tutorial . Database 936 2025-06-13 00:30:01
  • How to use a JOIN in an UPDATE statement?
    How to use a JOIN in an UPDATE statement?
    The key to updating data with JOIN is the syntax differences between different databases. 1. SQLServer needs to connect tables in the FROM clause, such as: UPDATEt1SETt1.column=t2.valueFROMTable1t1INNERJOINTable2t2ONt1.id=t2.ref_id; 2.MySQL needs to JOIN directly after UPDATE, such as: UPDATETable1t1JOINTable2t2ONt1.id=t2.ref_idSETt1.column=t2.value; 3.PostgreSQL combines FROM and WHERE, such as: UPDA
    Mysql Tutorial . Database 668 2025-06-13 00:27:11
  • In which MySQL version did the CHECK constraint actually start working?
    In which MySQL version did the CHECK constraint actually start working?
    MySQL has only truly supported and enforced CHECK constraints since version 8.0.16, and was previously parsed but not actually executed. 1. Before 8.0.16, although CHECK constraints were syntactically supported, storage engines such as MyISAM and InnoDB did not implement their data verification functions; 2. Developers cannot rely on this function to ensure data integrity, and insertion or update operations will not trigger verification; 3. Since 8.0.16, CHECK constraints have been enforced by the server, supporting column-level and table-level constraints, complex expressions, and are applicable to all storage engines; 4. Users can use ENFORCED or NOTENFORCED keywords to control their enabled status; 5. After upgrading to this version, please note that the old data may not meet the requirements.
    Mysql Tutorial . Database 460 2025-06-13 00:24:50
  • When do I need to run the FLUSH PRIVILEGES command?
    When do I need to run the FLUSH PRIVILEGES command?
    In MySQL or MariaDB, you need to run the FLUSHPRIVILEGES command after manually modifying the permission table. 1. When you directly execute INSERT, UPDATE or DELETE operations on permission tables such as mysql.user, mysql.db, etc., you must run this command to make the changes effective immediately; 2. When using standard permission management commands such as GRANT, REVOKE or CREATEUSER, you do not need to execute FLUSHPRIVILEGES, because these commands will automatically reload permissions; 3. After modifying the permission table through scripts or external tools, the command should be manually executed, otherwise the changes will not take effect; 4. It is not recommended to directly edit the system permission table, and it is recommended to use standard SQL missions.
    Mysql Tutorial . Database 405 2025-06-13 00:23:21
  • How to restore a master from a slave's data?
    How to restore a master from a slave's data?
    Recovering the master database usually does not obtain data from the slave database, but when the master database goes down and no backup is available, you can follow the following steps: 1. Check whether the slave database has the latest data, run SHOWSLAVESTATUS\G to confirm that Seconds_Behind_Master is 0 and Last_Error is empty; 2. Stop the copy thread of the slave database and reset the copy information, use the STOPSLAVE and RESETSLAVEALL commands; 3. After configuring the original slave database as a new master database, update the application connection settings and reconfigure the new slave database to point to the master database, use CHANGEMASTERTO to specify the correct binary log file and location; 4. Create a new copy user and grant corresponding permissions; 5. Avoid self-directed
    Mysql Tutorial . Database 787 2025-06-13 00:22:40
  • How to perform a wildcard search, and what is the difference between % and _?
    How to perform a wildcard search, and what is the difference between % and _?
    % matches any number of characters suitable for broad searches, and \_ matches a single character suitable for precise positioning. For example: Li% matches all contents starting with Li, Li\_ only matches three letter names such as Liu or Lia; use LIKE to trigger wildcard characters, which contain special characters and need to be escaped; there are differences in the rules of wildcard characters in different environments.
    Mysql Tutorial . Database 785 2025-06-13 00:20:50
  • What is a Phantom Read and how can it be solved?
    What is a Phantom Read and how can it be solved?
    Phantom reading refers to the phenomenon of executing the same query twice in a transaction but obtaining different row sets, which are usually caused by inserting or deleting data by another transaction. 1. Use serialized isolation levels to lock the entire data range to prevent phantom reading but may affect performance; 2. Use range locks or key range locks to avoid full table locks and prevent new rows from inserting; 3. Use optimistic concurrency control to detect and deal with phantom reading problems during submission. This problem is particularly important when multiple queries are required to maintain consistency, such as financial reporting, inventory management and other scenarios.
    Mysql Tutorial . Database 630 2025-06-12 10:40:30
  • How to calculate the difference between two dates in days or seconds?
    How to calculate the difference between two dates in days or seconds?
    To calculate the number of days or seconds between two dates, the core method is to subtract the time unit into a unified unit. The specific methods include: 1. Use programming languages ??(such as Python's datetime module to create a date object, and then subtract the result through days and total_seconds()); 2. Use subtraction formulas to calculate the difference in the day and multiply by 86400 to obtain the difference in the second; 3. Use online tools or manual conversion to calculate the difference in the day first and then multiply by 86400 seconds per day. Different methods are suitable for different scenarios, logically consistent and simple and easy to implement.
    Mysql Tutorial . Database 554 2025-06-12 10:38:30
  • How to enable and view the MySQL slow query log?
    How to enable and view the MySQL slow query log?
    To enable MySQL slow query logs, you need to modify the configuration file and set relevant parameters. 1. Add slow_query_log=1 to enable logging in the [mysqld] part of my.cnf or my.ini; 2. Set slow_query_log_file to specify the log path, such as /var/log/mysql/mysql-slow.log; 3. Define the slow query threshold through long_query_time, the default unit is seconds. If set to 1, it means to record queries that exceed 1 second; 4. After modification, restart MySQL or use the SETGLOBAL command to take effect dynamically; 5. The log is in text format by default, and you can use tail, cat and other commands to view it.
    Mysql Tutorial . Database 422 2025-06-12 10:38:11

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