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Technical Articles

Database

Mysql Tutorial

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

How to get the list of databases on a MySQL server?

To get the database list on the MySQL server, the most common method is to use the SHOWDATABASES command, and after logging in to MySQL, execute SHOWDATABASES; it can display all databases that the current user has permission to access, and system databases such as information_schema, mysql, performance_schema and sys are usually listed; in addition, the same results can be obtained by querying the INFORMATION_SCHEMA.SCHEMATA table, which is suitable for scenarios where filtering or scripting operations are required, such as using SELECTSCHEMA_NAMEFROMINFORMATION_SCHEMA_SCHEMA_

Aug 26, 2025 am 04:17 AM

What is the role of my.cnf or my.ini file in MySQL?

Themy.cnformy.inifileisessentialforcustomizingMySQLbehavior,asitallowsadministratorstocontrolserversettings,optimizeperformance,managelogs,andapplyconfigurationstospecificcomponents;withoutit,MySQLrunsondefaults,limitingperformance,security,andmonito

Aug 26, 2025 am 03:49 AM

What is a foreign key in MySQL?

AforeignkeyinMySQLisacolumnorsetofcolumnsthatreferencesaprimaryoruniquekeyinanothertabletoenforcereferentialintegrity;itensuresdataconsistencybyallowingonlyvalidvaluesfromthereferencedtable,preventsaccidentaldeletionofrelatedrecordsdependingonconstra

Aug 26, 2025 am 02:49 AM

What is the difference between COUNT(*) and COUNT(column) in MySQL?

COUNT()countsallrowsincludingthosewithNULLvalues,whileCOUNT(column)onlycountsrowswherethespecifiedcolumnisnotNULL.1.COUNT()includeseveryrowregardlessofNULLsandisusedtogetthetotalnumberofrows.2.COUNT(column)excludesNULLsinthespecifiedcolumnandisusedto

Aug 26, 2025 am 12:56 AM

How to use IF statements in MySQL

MySQL supports two conditional logic processing methods: 1. Use the IF() function in the query, with the syntax of IF(condition, value_if_true, value_if_false), which is suitable for SELECT, UPDATE and other statements, such as returning "Pass" or "Fail" based on student scores; 2. Use IF statements in stored procedures, with the syntax of IFTHENELSEIFELSEENDIF, which is used for complex control processes, such as judging the inventory quantity and returning different messages; the two cannot be mixed, IF() is recommended for simple conditions, IF statements are used for complex logic, and CASE is recommended for multi-layer conditions to improve readability.

Aug 25, 2025 pm 02:55 PM

mysql if statement

How to partition a large table in MySQL

To effectively partition MySQL large tables, first select the appropriate partition type, such as RANGE for time series data; secondly select the partition key that matches the query pattern and make sure it is included in the primary key; then regularly manage partitions, such as adding new partitions and deleting old partitions; finally ensure performance through query optimization and monitoring. 1. Select RANGE, LIST, HASH or KEY partitions, and the time data is recommended RANGE or RANGECOLUMNS; 2. The partition key should match high-frequency query conditions and avoid frequent update columns; 3. Regularly use ALTERTABLE to add, delete or reorganize partitions; 4. Ensure that the query contains partition keys to enable partition pruning, and verify the execution plan through EXPLAIN; pay attention to Inn

Aug 25, 2025 pm 02:17 PM

Troubleshooting MySQL High Availability Failover

MySQL failover problems are usually caused by master-slave replication exceptions, inaccurate detection mechanisms, VIP configuration errors, or inconsistent data. 1. The abnormal master-slave replication status will cause no available switching nodes. You need to check the Slave_IO/SQL_Running status, replication delay and errors; 2. The inaccurate fault detection mechanism may cause misjudgment, and multi-dimensional detection should be used and reasonable timeout and retry should be set; 3. The VIP does not drift correctly or the application connection configuration will cause the new master library to be still unable to access after the switch. You need to confirm the VIP binding, DNS update and connection pool reconnection mechanism; 4. The risk of data inconsistency after the switch can be alleviated through semi-synchronous replication, GTID checking and relaylog compensation mechanisms to ensure that the data before and after the switch is complete and consistent.

Aug 25, 2025 pm 02:10 PM