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Technical Articles

Database

How to troubleshoot MySQL replication issues

CheckreplicationstatususingSHOWSLAVESTATUS\G,verifyingSlave_IO_RunningandSlave_SQL_RunningareYes,inspectingLast_ErrorandSeconds_Behind_Masterforissues.2.ForSlave_IO_Running=No,resolveconnectionproblemsbyconfirmingmasterhost,port,credentials,networkac

Aug 08, 2025 am 08:15 AM

mysql copy

How to use the HAVING clause versus the WHERE clause in MySQL?

UseWHEREtofilterindividualrowsbeforegrouping,asitoperatesonrawdataandimprovesperformancebyexcludingunwantedrowsearly;2.UseHAVINGtofiltergroupsafteraggregation,asitworkswithaggregatefunctionslikeCOUNT()andAVG()thatarecalculatedafterGROUPBY;3.WHEREcann

Aug 08, 2025 am 08:05 AM

How to write a WHILE loop in SQL?

SQL itself does not support WHILE loops in standard queries, but databases such as SQLServer and MySQL provide support in stored procedures or scripts; 1. When using T-SQL's WHILE loop in SQLServer, variables need to be defined and wrapped with BEGIN...END; 2. When using WHILE loops in MySQL, you must use them in stored procedures and in conjunction with DELIMITER and DECLARE; 3. WHILE cannot be used in standard SELECT statements, and collection operations or recursive CTE should be used first; 4. PostgreSQL, SQLite and Oracle do not support standard WHILE, and they must use their respective procedural languages; therefore

Aug 08, 2025 am 07:43 AM

sql while loop

SQL Profiler and Extended Events for Performance Monitoring

SQLProfiler is suitable for short-term debugging, and ExtendedEvents is more suitable for long-term monitoring. SQLProfiler is a graphical tool with simple operation and is suitable for fast positioning and slow querying and debugging stored procedures, but it consumes a lot of resources and has been marked as outdated; ExtendedEvents has low resource utilization and strong flexibility, supports advanced filtering and custom templates, and is suitable for long-term operation in the production environment; the selection tool should be based on the scenario: short-term debugging, and the ExtendedEvents are preferred for long-term monitoring, and the configuration can be simplified by the SSMS wizard; pay attention to filtering data, managing permissions, and cleaning historical logs when using it.

Aug 08, 2025 am 07:37 AM

How to use ENUM type in phpMyAdmin

ENUMinphpMyAdminisusedtorestrictacolumntopredefinedstringvalues.1.OpenphpMyAdmin,selectyourdatabase,andeithercreateanewtableoreditanexistingone.2.Definethecolumnbyenteringthename,selectingENUMfromtheTypedropdown,andinputtingallowedvaluesinsinglequote

Aug 08, 2025 am 07:15 AM

ENUM類型

How to Monitor MySQL Performance and Identify Bottlenecks?

Enable slow query logs to identify queries that have been executed for too long. By setting slow_query_log, long_query_time, log_queries_not_using_indexes parameters, and using pt-query-digest to analyze the logs to optimize key queries; 2. Use PerformanceSchema and INFORMATION_SCHEMA to view the most time-consuming SQL statements, table I/O waiting and currently running queries, focusing on sessions that are in the "Sendingdata" or "Locked" state for a long time; 3. Monitor Threads_running and Innodb_r

Aug 08, 2025 am 06:51 AM

How to Troubleshoot Common MySQL Connection Errors?

Check whether the MySQL service is running, use sudosystemctlstatusmysql to confirm and start; 2. Make sure that bind-address is set to 0.0.0.0 to allow remote connections and restart the service; 3. Verify whether the 3306 port is open, check and configure the firewall rules to allow the port; 4. For the "Accessdenied" error, you need to check the user name, password and host name, and then log in to MySQL and query the mysql.user table to confirm permissions. If necessary, create or update the user and authorize it, such as using 'your_user'@'%'; 5. If authentication is lost due to caching_sha2_password

Aug 08, 2025 am 06:44 AM

mysql connection error

How to Think in SQL: A Mental Model for Beginners

Thinkinsets,notsteps—SQLoperatesonentiredatasetsatonce,sodescribethedesiredoutcomeratherthanprocedurallogic.2.Treattablesascollectionsoffactsthatcanbefiltered,shaped,grouped,andcombinedusingWHERE,SELECT,GROUPBY,andJOIN.3.UnderstandJOINsasmatchingrows

Aug 08, 2025 am 06:38 AM

How to get the last inserted ID in MySQL

To get the last inserted ID in MySQL, use the LAST_INSERT_ID() function. 1. After executing the INSERT statement, you can directly call SELECTLAST_INSERT_ID() to get the last self-increment ID in the current session; 2. This function is session-safe and will not interfere with each other when multiple users concurrent operations; 3. Its value persists in the same connection until the next time the self-increment data is inserted; 4. Returns the valid ID only when inserting the table containing the AUTO_INCREMENT column, otherwise it will return 0; 5. In PHP, you can use the lastInsertId() method of PDO and cursor.lastro in Python.

Aug 08, 2025 am 06:26 AM

How can you upgrade a MongoDB replica set or sharded cluster with minimal downtime?

Upgrading a MongoDB replica set or sharded cluster without causing significant downtime is feasible, but best practices are required. First check version compatibility to ensure that the new version is compatible with the current settings, especially when crossing multiple major versions, you need to check the changes in the release notes and upgrade the components in sequence; secondly, use rolling upgrades for the replica set, upgrade the secondary nodes one by one, and then upgrade the main nodes, maintain at least three nodes to maintain arbitration; again upgrade the shard cluster in stages, first upgrade the configuration server, then upgrade the mongos router, and finally upgrade each shard one by one; finally, in a production-like test environment, comprehensively verify the upgrade path, including backup recovery, load performance and monitoring tool compatibility, to ensure that all steps are executed in sequence and verified correctly before going online.

Aug 08, 2025 am 06:24 AM

最小停機時間

How to view query execution time?

To view the execution time of an SQL query, it can be achieved through the database's own commands, client tools, or application-layer records. 1. Use EXPLAINANALYZE to view the actual execution time of the query in PostgreSQL or MySQL8.0. 2. SHOWPROFILE can be used in the old version of MySQL to analyze each stage time. 3. Most client tools such as DBeaver, MySQLWorkbench and psql display execution time by default. 4. The application layer can also count the query time by recording the start and end timestamps, which is suitable for code debugging and remote call scenarios.

Aug 08, 2025 am 06:05 AM

sql Performance analysis

How to find the size of a database and its tables in SQL?

To view the size of the database and its tables, you need to use the corresponding methods according to different database management systems: 1. In MySQL, query information_schema.TABLES, use data_length and index_length fields to calculate the size; 2. In PostgreSQL, use functions such as pg_database_size() and pg_total_relation_size() to format the output together with pg_size_pretty(); 3. In SQLServer, you can execute sp_spaceused stored procedures or query sys.master_files and sys.all

Aug 08, 2025 am 05:57 AM

How to copy a table to another database in MySQL

Tocopybothstructureanddata,useCREATETABLE...LIKEfollowedbyINSERTINTO...SELECT,whichpreservesindexes,constraints,andAUTO_INCREMENTsettings.2.Tocopyonlythestructure,useCREATETABLE...LIKE,whichcreatesanemptytablewiththesameschema.3.Tocopystructureanddat

Aug 08, 2025 am 05:21 AM

How to prevent SQL injection in MySQL

Usepreparedstatementswithparameterizedqueriestotreatuserinputasdata,notexecutablecode.2.Validateandsanitizeinputbyfilteringdatatypes,limitinglength,andwhitelistingvalues.3.Escapeinputsonlyifpreparedstatementsarenotfeasible,usingfunctionslikereal_esca

Aug 08, 2025 am 05:09 AM

mysql sql injection