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Technical Articles

Database

Mysql Tutorial

How to use the JSON_OBJECT function in MySQL

JSON_OBJECT() is used in MySQL to create JSON objects from a list of key-value pairs, and is suitable for generating structured JSON data directly from a query. 1. The basic syntax is JSON_OBJECT(key1, value1, key2, value2,...), where the key must be a string or an expression that can be converted into a string, and the value can be any expression; 2. Simple examples such as SELECTJSON_OBJECT('name','Alice','age',30) return {"name":"Alice","age":30}; 3. Can combine table count

Aug 25, 2025 am 07:14 AM

How to use CROSS JOIN in MySQL?

CROSSJOIN is used in MySQL to generate Cartesian products of two tables, that is, each row of the first table and each row of the second table; 1. Used when all combinations of two sets of data need to be generated (such as product color and size); 2. Applicable to tables that have no direct association but need to be fully combined; 3. The basic syntax is SELECT*FROMtable1CROSSJOINtable2, equivalent to implicit writing but clearer; 4. In the example, the color table and the size table cross-connection generate 6 combinations; 5. Use it with caution, because 1000×1000 rows will produce millions of rows; 6. Do not use the ON clause because there is no matching key required; 7. The results can be filtered through the WHERE clause, but excessive filtering may indicate that

Aug 25, 2025 am 04:33 AM

Troubleshooting MySQL Performance Schema Configuration

PerformanceSchema has not collected data, so instruments and consumers need to be checked and enabled; 2. The performance declines significantly after activation, and eventhistory should be enabled and restricted as needed; 3. The table structure is missing or inaccessible, so version and permissions need to be confirmed; 4. The configuration restart is invalid, and global switches need to be set in the configuration file. If the query is empty, first confirm whether SELECT*FROMperformance_schema.setup_instruments and setup_consumers enable the corresponding modules. If the file I/O needs to be turned on, UPDATE; if the performance is degraded, you should avoid full opening of low-level instru.

Aug 25, 2025 am 04:12 AM

How to Use Common Table Expressions (CTEs) in MySQL?

CTEsinMySQL8.0 aretemporaryresultsetsdefinedwiththeWITHclausethatenhancequeryreadabilityandmaintainability.1.AsimpleCTEcalculatesaveragesalaryandfiltersemployeesearningmoreusingCROSSJOIN.2.MultipleCTEscanbeusedinonequery,suchascalculatingdepartmentav

Aug 25, 2025 am 03:45 AM

What is the difference between != and in MySQL?

InMySQL,!=andarefunctionallyidentical,bothmeaning"notequalto"andproducingthesameresultswithnoperformancedifference;forexample,SELECTFROMusersWHEREstatus!='active'andSELECTFROMusersWHEREstatus'active'returnthesamerowswherestatusisnot'active'

Aug 25, 2025 am 12:35 AM

How to manage sessions in MySQL?

MySQLdoesnothaveabuilt-insessionmanagementsystemlikewebapplications,butitsupportssession-levelvariablesandconnections,andcanstoreapplicationsessiondata.1.Formanagingdatabasesessions,useSHOWPROCESSLISTtoviewactiveconnections,KILL[thread_id]toterminate

Aug 24, 2025 pm 02:11 PM

mysql

What is a subquery in MySQL?

AsubqueryinMySQLisaquerynestedwithinanotherquery,usedtoretrievedataforfilteringorcomputationintheouterquery.Itexecutesfirstandreturnsresultsthattheouterqueryuses,appearinginclauseslikeSELECT,FROM,WHERE,orHAVING.Subqueriescanreturnasinglevalue(scalar)

Aug 24, 2025 pm 01:22 PM

mysql Subquery

How to use the BETWEEN operator in MySQL?

TheBETWEENoperatorinMySQLisusedtofilterresultswithinaninclusiverangeofvalues,workingwithnumbers,text,anddates;forexample,"SELECT*FROMproductsWHEREpriceBETWEEN10AND50"retrievesproductswithpricesfrom10to50,includingbothbounds,andthesamelogica

Aug 24, 2025 pm 01:02 PM

How to grant privileges to a user in MySQL

ConnecttoMySQLasaprivilegeduserlikerootusingmysql-uroot-p.2.UsetheGRANTstatementtoassignspecificprivilegessuchasSELECT,INSERT,UPDATE,DELETE,orALLPRIVILEGESonspecifieddatabasesortablestoauser,forexample:GRANTSELECT,INSERTONmydb.*TO'john'@'localhost'ID

Aug 24, 2025 pm 12:05 PM

How to use row-level locks in MySQL

Row-levellocksinMySQLareautomaticallyappliedbyInnoDBduringwriteoperationsonindexedcolumns,lockingonlyspecificrows;2.UseSELECT...FORUPDATEtoexplicitlylockrowsforexclusiveaccessduringatransaction,preventingothertransactionsfrommodifyingthem;3.UseSELECT

Aug 24, 2025 am 09:58 AM

How to use variables in MySQL queries?

Use@prefixtodeclareuser-definedvariablesinMySQL,whicharesession-specificandpersistduringtheconnection.2.AssignvaluesusingSETorSELECTwiththe:=operatorinSELECTstatementstodistinguishassignmentfromcomparison.3.ReusevariablesinSELECT,INSERT,UPDATE,andDEL

Aug 24, 2025 am 09:46 AM

mysql variable

How to Handle Large Objects (BLOBs and TEXT) in MySQL?

ChoosetheappropriateBLOBorTEXTtypebasedonsizeneeds,usingMEDIUMBLOB/MEDIUMTEXTformostcasesandreservingLONGtypesforgigabyte-scaledata,whileusingTEXTforcharacterdataandBLOBforbinarydata.2.Storelargefilesexternallyinfilesystemsorobjectstorage(e.g.,AWSS3)

Aug 24, 2025 am 09:43 AM

mysql blob

How to use the REGEXP operator for pattern matching in MySQL?

MySQL's REGEXP operator is used to perform regular expression pattern matching, which is more powerful than LIKE. 1. The basic syntax is SELECTcolumn_nameFROMtable_nameWHEREcolumn_nameREGEXP'pattern', which can be replaced by RLIKE; 2.^ represents the beginning of a string, $ represents the end,. To match any single character, special characters such as \\., such as \\.; 3. Common patterns include [abc] matching characters in brackets, [a-z] matching range, [^abc] matching characters in non-branches, (abc|def) matching multiple options; 4. Example: ^john matches strings starting with john, \\.

Aug 24, 2025 am 09:20 AM

What is database replication in MySQL?

MySQLreplicationimprovesdataavailability,reliability,andperformancebycopyingdatafromamasterservertooneormoreslaveservers;itenableshighavailabilitywithfailoversupport,offloadsbackupsandreadqueriestoslaves,andreduceslatencythroughgeographicdistribution

Aug 24, 2025 am 08:50 AM