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Technical Articles

Database

Mysql Tutorial

How to analyze and repair tables in MySQL

Use ANALYZETABLE to update index statistics to optimize query execution plan, which is suitable for large amounts of data changes or as regular maintenance; 2. Use CHECKTABLE to detect whether the table is damaged. MyISAM table has significant effect. InnoDB usually handles automatically but can also be checked. EXTENDED can be selected for deep scan; 3. If damage is found, use REPAIRTABLE to repair MyISAM table. InnoDB recommends that it be restored through innodb_force_recovery or backup, and backup must be restored before repair; 4. Best practices include regular maintenance, priority use of InnoDB, and automated analysis, inspection and repair operations through mysqlcheck tool to ensure the number of

Aug 28, 2025 am 01:41 AM

How to create a temporary table in MySQL?

To create a temporary MySQL table, use the CREATETEMPORARYTABLE statement, 1. The table only exists in the current session and is automatically deleted at the end of the session; 2. It can have the same name as the permanent table and is referenced first; 3. It supports most storage engines but the memory table does not support full-text indexing; 4. It can insert and query data like a normal table; 5. It can manually delete it with DROPTEMPORARYTABLE to avoid accidentally deleting permanent tables; it is suitable for complex queries, step-by-step data processing and other scenarios, providing session isolation and automatic cleaning functions.

Aug 28, 2025 am 12:29 AM

How to use table-level locks in MySQL

MySQL's table-level lock is implemented through LOCKTABLES and UNLOCKTABLES commands to control access to the entire table; 1. Use READ locks to prevent other sessions from modifying tables, which are suitable for backup or consistent reads; 2. Use WRITE locks to obtain exclusive access, which is suitable for maintenance operations; 3. If an alias is used in the query, the same alias must be specified during locking; it should be noted that in InnoDB, LOCKTABLES will implicitly submit the current transaction, and long-term holding of the lock should be avoided to reduce the impact on concurrency, and priority should be given to transaction and row-level locks to ensure concurrency performance. Table-level locks are only recommended for special scenarios such as batch import and maintenance tasks. UNLOCKTABLES must be called to release the lock after use.

Aug 27, 2025 am 08:11 AM

How to drop a trigger in MySQL

To delete triggers in MySQL, use the DROPTRIGGER statement; 1. The basic syntax is DROPTRIGGER[IFEXISTS][schema_name.]trigger_name; 2. IFEXISTS can prevent errors when the trigger does not exist; 3.schema_name can be omitted in the current database; 4. SUPER or ALTER permissions must be provided; 5. You can confirm that the trigger exists through SHOWTRIGGERS or information_schema.TRIGGERS; the deletion operation will not affect the association table, and there is no CASCADE option. The trigger will be removed after execution.

Aug 27, 2025 am 08:05 AM

How to work with JSON data in MySQL

When using MySQL to process JSON data, you should first create a JSON type column and insert valid JSON data. 1. Use the -> and -> operators to extract the JSON value, and recommend ->> to obtain a quoteless string; 2. Use ->> or JSON_EXTRACT to filter the data in the WHERE clause. Use JSON_CONTAINS_PATH to check whether the key exists, and use JSON_CONTAINS to query the array value; 3. Use JSON_SET, JSON_REPLACE or JSON_INSERT to update the JSON field, and JSON_ARRAY_APPEND can add elements to the array; 4

Aug 27, 2025 am 07:55 AM

mysql json

What is a CHECK constraint in MySQL and is it enforced?

Yes,CHECKconstraintsareenforcedinMySQLstartingfromversion8.0.16.Priortothisversion,CHECKconstraintswereparsedbutnotenforced,meaningtheyhadnoeffectondataintegrity.FromMySQL8.0.16onward,CHECKconstraintsareactivelyenforcedattheSQLlayer,ensuringthatonlyd

Aug 27, 2025 am 07:40 AM

What is the purpose of the USE statement in MySQL?

TheUSEstatementinMySQLselectsadefaultdatabaseforthecurrentsession,allowingsubsequentoperationstobeperformedwithinthatdatabasecontextwithoutneedingtofullyqualifytablenames;forexample,runningUSEsalessetsthesalesdatabaseasdefault,soquerieslikeSELECTFROM

Aug 27, 2025 am 07:02 AM

mysql USE語句

What is the purpose of SQL_CALC_FOUND_ROWS in MySQL?

SQL_CALC_FOUND_ROWSwasusedtogetthetotalrowcountofaquerywithoutLIMITafterrunningaLIMITquery,enablingaccuratepaginationtotals;however,itwasdeprecatedinMySQL8.0andremovedin8.0.23duetoperformanceissues,asitforcedMySQLtoprocessallrowsdespitelimitingresult

Aug 27, 2025 am 06:34 AM

mysql

How to use INSERT IGNORE in MySQL?

INSERTIGNOREinMySQLallowsinsertingrowswhileskippingerrorslikeduplicatekeysorNULLviolations,makingitidealforbulkinsertsordatasyncing;itconvertserrorsintowarningsinsteadofstoppingexecution,sousingSHOWWARNINGSafterwardrevealsskippedrowsduetoconstraints,

Aug 27, 2025 am 05:01 AM

mysql

What are transactions in MySQL?

MySQLtransactionsaresequencesofSQLstatementstreatedasasingleunitofworkthateitherfullycompletesoriscompletelyundone,ensuringdataintegrityandconsistencythroughtheACIDproperties—Atomicity,Consistency,Isolation,andDurability—whereInnoDBstorageenginesuppo

Aug 27, 2025 am 02:43 AM

How to enable the slow query log in MySQL

To enable MySQL slow query logs, first check the current status: 1. Execute SHOWVARIABLESLIKE'slow_query_log'; if OFF, it needs to be enabled; 2. Check the log path and threshold: SHOWVARIABLESLIKE'slow_query_log_file'; and SHOWVARIABLESLIKE'long_query_time'; 3. It is recommended to modify the configuration file (such as /etc/my.cnf), and add it under [mysqld]: slow_query_log=ON, slow_query_log_file=/var/log/mysql/m

Aug 26, 2025 am 07:14 AM

How to find the Nth highest salary in a MySQL table?

The use of LIMIT and OFFSET is suitable for simple queries and N is known, but does not support dynamic variables; 2. The window function using DENSE_RANK() can correctly handle duplicate values, and it is recommended to use in dynamic N scenarios in modern MySQL versions; 3. The use of related subqueries is suitable for older versions of MySQL that do not support window functions, but have poor performance; the most recommended method is DENSE_RANK(), which is suitable for most production environments due to its accuracy, flexibility and efficiency.

Aug 26, 2025 am 06:42 AM

mysql salary

What is the SQL mode in MySQL?

SQLmodeinMySQLdefineshowtheserverinterpretsSQLstatementsbycontrollingdatavalidation,syntaxcompliance,andhandlingofinvalidormissingdata,withcommonmodesincludingSTRICT_TRANS_TABLESfordataintegrity,ONLY_FULL_GROUP_BYforstandardGROUPBYbehavior,NO_ZERO_DA

Aug 26, 2025 am 05:37 AM

mysql sql

What is the difference between a temporary table and a table variable in MySQL?

MySQLdoesnotsupporttablevariableslikeSQLServer;2.Theonlybuilt-inoptionfortemporaryresultsetsinMySQLisCREATETEMPORARYTABLE;3.Temporarytablesaresession-specific,supportindexesandjoins,andareautomaticallydroppedwhenthesessionends;4.User-definedvariables

Aug 26, 2025 am 04:51 AM