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Technical Articles

Database

How to use the LIKE operator for pattern matching in SQL?

The LIKE operator in SQL is used to search for patterns in string data, and is often used for inaccurate matching. The main usage scenarios include: 1. Find strings starting with specific characters, such as LIKE'J%' matching "John", "Jane", etc.; 2. Match content containing specific substrings, such as LIKE'%book%' matching "Notebook", "Textbook", etc.; 3. Match single characters with underscores, such as LIKE'A_3' matching "AB3", "AC3", etc.; 4. Combine wildcards to implement complex patterns,

Aug 08, 2025 am 10:14 AM

How to show all databases in MySQL

To display all databases in MySQL, you need to use the SHOWDATABASES command; 1. After logging into the MySQL server, you can execute the SHOWDATABASES; command to list all databases that the current user has permission to access; 2. System databases such as information_schema, mysql, performance_schema and sys exist by default, but users with insufficient permissions may not be able to see it; 3. You can also query and filter the database through SELECTSCHEMA_NAMEFROMinformation_schema.SCHEMATA; for example, excluding the system database to only display the database created by users; make sure to use

Aug 08, 2025 am 09:50 AM

mysql database

How to change language in phpMyAdmin

TochangethelanguageinphpMyAdmin,selectyourpreferredlanguagefromthedropdownontheloginscreenbeforeloggingin;2.Ifalreadyloggedin,clickthe"Language"buttoninthelower-leftcornerandchooseyourdesiredlanguage;3.Tomakethechangepermanent,gotothe"

Aug 08, 2025 am 09:39 AM

language settings

How do you write a recursive query to traverse a hierarchical data structure in SQL?

TotraverseahierarchicaldatastructureinSQL,usearecursiveCommonTableExpression(CTE)thatstartswithrootnodesandrecursivelyjoinschildren;1.Definetheanchormember(rootnodes,e.g.,WHEREparent_idISNULL);2.UseUNIONALLtodefinetherecursivememberthatjoinschildrent

Aug 08, 2025 am 09:38 AM

How to change the data type of a column in phpMyAdmin

ChangingthedatatypeofacolumninphpMyAdminisastraightforwardprocess.YoucandothisusingthegraphicalinterfacewithoutwritingSQLcommands,thoughbothmethodsareavailable.Here'showtodoit:1.UsingthephpMyAdminInterface(Recommende

Aug 08, 2025 am 09:16 AM

Data Modeling Patterns in MongoDB

Use embedded mode to process small, fixed data that is always accessed together (such as user orders); 2. Use reference mode to process large or independently changing data (such as comments); 3. Use bucket mode to manage fast-growing time series data (such as logs); 4. Use pre-calculated fields to optimize frequent aggregation queries (such as total user consumption). The model should be selected based on the actual access mode, rather than the static structure, ensuring that the performance and maintainability balance end.

Aug 08, 2025 am 08:29 AM

Why are indexes crucial for query performance in MongoDB?

IndexesimproveMongoDBqueryperformancebyreducingdatascansandresourceusage.Withoutindexes,MongoDBperformsfullcollectionscans,whichareslowandinefficient.IndexesallowdirectaccesstorelevantdocumentsusingB-trees,speedinguplookups,rangequeries,andsorts.Fore

Aug 08, 2025 am 08:27 AM

mongodb index

How to perform a case-insensitive select in MySQL

Bydefault,MySQLperformscase-insensitiveSELECTqueriesduetocase-insensitivecollationslikeutf8mb4_general_ci;1.UseLOWER()orUPPER()functionstoconvertbothcolumnandsearchvaluetothesamecase,thoughthismayimpactindexusagewithoutfunctionalindexes;2.UseLIKE,whi

Aug 08, 2025 am 08:25 AM

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