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Technical Articles

Database

Troubleshooting Common MongoDB Errors

"E11000duplicatekeyerror": Check the unique index, use upsert to update or adjust the data; 2. "Connectionrefused": Confirm that the service is running, the connection string is correct, and the network is not blocked; 3. "Cursornotfound": add noCursorTimeout, pagination or projection to reduce the amount of data; 4. "Documentfailedvalidation": Check the schema rules, temporarily close verification or pre-validation with ODM; 5. Slow query: use explain to analyze and create necessary indexes to avoid full table scanning. This list covers 90% of common problems in MongoDB production and asks

Aug 15, 2025 pm 01:50 PM

mongodb mistake

How to enable archive log mode in Oracle

First,checkthecurrentarchivelogstatususingSELECTlog_modeFROMv$database;orARCHIVELOGLIST;toconfirmifthedatabaseisinNOARCHIVELOGmode.2.ShutdownthedatabasewithSHUTDOWNIMMEDIATE;.3.StartthedatabaseinmountmodeusingSTARTUPMOUNT;.4.Enablearchivelogmodebyrun

Aug 15, 2025 pm 12:24 PM

oracle 歸檔日志模式

How to handle emoji characters in MySQL

To correctly handle emoji characters in MySQL, you must use the utf8mb4 character set. 1. Make sure that the database, table and column use utf8mb4 and utf8mb4_unicode_ci; 2. Set the client and server character set to utf8mb4 in the MySQL configuration file, and enable innodb_large_prefix; 3. Specify the charset to utf8mb4 when applying the connection; 4. Verify the configuration through SHOWVARIABLES and SHOWCREATETABLE; 5. Pay attention to the utf8mb4 index length limit and limit the index column length to within 191 characters. Only use utf8m in the entire data link

Aug 15, 2025 am 11:14 AM

What is the difference between MyISAM and InnoDB in MySQL?

InnoDB supports transaction and ACID features, but MyISAM does not support it; 2. InnoDB uses row-level locks to improve concurrency performance, while MyISAM uses table-level locks to easily cause blockage; 3. InnoDB supports foreign key constraints to ensure data integrity, but MyISAM does not support it; 4. InnoDB has crash recovery capabilities, and MyISAM is prone to table damage; 5. MyISAM may be faster in pure read scenarios, but InnoDB performs better under mixed loads; 6. Since MySQL5.6, InnoDB has supported full-text indexing and no longer lag behind; 7. InnoDB optimizes data access through buffer pools, and MyISAM only caches indexes. In summary, InnoDB is suitable for most modern applications, and My

Aug 15, 2025 am 11:06 AM

How to import a database using mysqldump

mysqldumpisusedtoexport,notimport,adatabase;toimport,usethemysqlcommand-lineclient.1.Ensureyouhavea.sqldumpfilecreatedviamysqldump.2.Createthetargetdatabaseifitdoesn’texistusingCREATEDATABASE.3.ImporttheSQLfilewithmysql-uusername-pdatabase_name

Aug 15, 2025 am 10:56 AM

Database import

What is a full-text search in MySQL?

Full-textsearchinMySQLenablesefficient,relevance-rankedtextsearchingincharacter-basedcolumnsusingFULLTEXTindexesandMATCH()...AGAINST()syntax,supportingnaturallanguage,boolean,andqueryexpansionmodesprimarilyonInnoDBandMyISAMengines,makingitidealforlar

Aug 15, 2025 am 10:43 AM

mysql research all

How to run an UPDATE query in phpMyAdmin

TorunanUPDATEqueryinphpMyAdmin,firstaccessphpMyAdminandselectyourdatabasefromtheleft-handpanel,thenchoosethetargettable.Next,clicktheSQLtabandwriteyourUPDATEstatementusingthesyntax:UPDATEtable_nameSETcolumn1='new_value'WHEREcondition;forexample:UPDAT

Aug 15, 2025 am 09:34 AM

How to implement full-text search in MySQL

Create a FULLTEXT index to enable full-text search, which can be added when creating a table or using ALTERTABLE; 2. Use MATCH()...AGAINST() syntax to perform searches, support natural language mode, Boolean mode and query extensions; 3. Pay attention to restrictions and best practices such as minimum word length, stop words, differences between InnoDB and MyISAM; 4. By combining WHERE conditions, using EXPLAIN to analyze and reasonably maintain index optimization performance, MySQL full-text search can meet the needs of most application scenarios.

Aug 15, 2025 am 09:27 AM

mysql research all

How to delete from a table using a join in SQL

Todeleterowsfromatableusingconditionsfromanothertable,useaJOINinDELETEforMySQLandSQLServerwithsyntaxlikeDELETEt1FROMtable1t1INNERJOINtable2t2ONt1.id=t2.table1_idWHEREt2.some_column='value';forPostgreSQL,useDELETEFROMtable1USINGtable2WHEREtable1.id=ta

Aug 15, 2025 am 09:20 AM

How to optimize a MySQL query?

Use appropriate indexes to create indexes for columns involved in WHERE, JOIN, ORDERBY and GROUPBY, give priority to compound indexes but avoid over-index, and verify index usage through EXPLAIN; 2. Write efficient queries, avoid SELECT*, do not use functions on index columns, use LIMIT reasonably, handle OR conditions carefully, and give priority to EXISTS rather than IN subqueries; 3. Optimize JOIN and table structure, ensure that the join columns are indexed, select the correct JOIN type, de-normalize appropriately to reduce JOIN, and use smaller data types to improve efficiency; 4. Use EXPLAIN and EXPLAINFORMAT=JSON to analyze query execution plans,

Aug 15, 2025 am 09:02 AM

How to import a SQL file into phpMyAdmin

AccessphpMyAdminviayourbrowserandlogin.2.Selectanexistingdatabaseorcreateanewonewithapropercollationlikeutf8mb4_unicode_ci.3.Clickthe"Import"tab,chooseyourSQLfile,ensureformatisSQL,andverifycharactersetsettings.4.Ifthefileexceedssizelimits,

Aug 15, 2025 am 09:00 AM

How to use the CROSS APPLY and OUTER APPLY operators in SQL

CROSSAPPLYreturnsonlyrowsfromthelefttablethathavematchingresultsintheappliedsubqueryorfunction,whileOUTERAPPLYreturnsallrowsfromthelefttable,includingthosewithnomatches,fillingunmatchedcolumnswithNULLs;bothallowcorrelatedsubqueriesandtable-valuedfunc

Aug 15, 2025 am 08:58 AM

sql

How to use transactions in MySQL

To use MySQL transactions correctly, it is necessary to ensure that the transaction is started explicitly through STARTTRANSACTION or BEGIN, and relevant SQL operations are performed. If all operations are successful, COMMIT commit, if there is an error, ROLLBACK rollback. You can use SAVEPOINT to set a save point to achieve partial rollback. In the application, you need to manage exceptions in combination with the try-catch mechanism. At the same time, pay attention to turning off the autocommit mode to avoid automatic commit, keep the transaction short to improve performance, and select the appropriate isolation level according to business needs to ensure data consistency and integrity.

Aug 15, 2025 am 08:31 AM

How to generate JSON output from a query in SQL?

PostgreSQL uses row_to_json, json_build_object and json_agg to generate JSON; 2.MySQL uses JSON_OBJECT, JSON_ARRAYAGG and JSON_OBJECTAGG to build a single or aggregate JSON object; 3. SQLServer formats the query results into JSON through FORJSONAUTO or PATH clauses, which can contain NULL values; 4.SQLite supports basic JSON output with the json_object and json_group_array functions extended by JSON1; SQL requires the corresponding syntax to implement SQL

Aug 15, 2025 am 07:52 AM