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Technical Articles

Database

What is the difference between UNION and UNION ALL in MySQL?

UNIONremovesduplicateswhileUNIONALLkeepsallrowsincludingduplicates;1.UNIONperformsdeduplicationbysortingandcomparingrows,returningonlyuniqueresults,whichmakesitsloweronlargedatasets;2.UNIONALLincludeseveryrowfromeachquerywithoutcheckingforduplicates,

Aug 14, 2025 pm 05:25 PM

mysql union

How to handle BLOB and TEXT data types in MySQL?

BLOB and TEXT are used to store large amounts of data. BLOB stores binary data, and TEXT stores text data. Each has four sizes: TINY, ordinary, MEDIUM, and LONG, with a maximum of 4GB; 1. BLOB/TEXT should be avoided for small data, and VARCHAR should be used instead to improve performance; 2. It is not advisable to store files directly in the database. It is recommended to store file paths and store files in the file system or cloud storage; 3. BLOB is used for true binary data, and text classes such as JSON/XML should be stored as TEXT to support character sets and debugging; 4. It is impossible to index the complete BLOB/TEXT, and you can create a prefix index such as CREATEINDEXidx_contentONartic

Aug 14, 2025 pm 04:58 PM

How to work with JSON data in SQL?

UseJSONdatatypestostoreJSONincolumns,withautomaticvalidationindatabaseslikeMySQL.2.Extractvaluesusingdatabase-specificoperators:PostgreSQLuses->and->>forJSONandscalarvalues,MySQLuses->and->>withJSONpathsyntax,andSQLServerusesJSON_VA

Aug 14, 2025 pm 03:52 PM

How to find the top SQL statements in Oracle

To find the SQL statement with the highest resource consumption in Oracle, you should first query the V$SQLAREA view to obtain aggregate statistics, and then combine the AWR historical data for trend analysis; 1. Use V$SQL to sort the time, CPU, logical reading and other indicators of a single execution to identify real-time high-consumed SQL; 2. Use V$SQLAREA to locate high-frequency or inefficient statements based on aggregate indicators such as total logical reading and total execution times; 3. Query DBA_HIST_SQLSTAT and DBA_HIST_SNAPSHOT to count historical performance data such as total CPU time, total execution times in the past 7 days; 4. You can optionally use OEM or SQLDeveloper graphical tool to view TopSQL and analyze

Aug 14, 2025 pm 03:44 PM

oracle sql statement

How to remove non-alphanumeric characters from a string in SQL?

ForPostgreSQL,Oracle,andMySQL8.0 ,useREGEXP_REPLACE('[^a-zA-Z0-9]','',string)toremovenon-alphanumericcharactersinasingleoperation.2.InSQLServer,whereregexisnotnativelysupported,useaWHILEloopwithPATINDEXtolocatenon-alphanumericcharactersandSTUFFtoremo

Aug 14, 2025 pm 03:41 PM

How to configure the query cache in MySQL

MySQL's query cache is only available in version 5.7 and earlier, and version 8.0 and above has been removed; 1. Confirm support through SHOWVARIABLESLIKE'have_query_cache' and check query_cache_type, query_cache_size, etc. configurations; 2. Set query_cache_type=ON, query_cache_size=256M, query_cache_limit=2M, query_cache_min_res_unit=512K in my.cnf to enable and optimize cache; 3. Use SHOWSTATUSLIKE

Aug 14, 2025 pm 03:31 PM

How to restart the MySQL server from the command line?

UsesudosystemctlrestartmysqlorsudosystemctlrestartmysqldonmodernLinuxsystemswithsystemd.2.UsesudoservicemysqlrestartorsudoservicemysqldrestartonolderLinuxsystems.3.Usesudo/etc/init.d/mysqlrestartfordirectscript-basedrestarts.4.OnmacOSwithHomebrew,use

Aug 14, 2025 pm 03:27 PM

How to format numbers as currency in SQL

SQLformattingofnumbersascurrencyvariesbydatabasesystemandrequiresspecificfunctions:MySQLusesCONCAT('$',FORMAT(amount,2)),SQLServerusesFORMAT(amount,'C','en-US'),PostgreSQLusesTO_CHAR(amount,'L999G999G999D99'),OracleusesTO_CHAR(amount,'L999,999,999.99

Aug 14, 2025 pm 03:08 PM

sql 貨幣格式化

How to add a message to a Stream using XADD?

ToaddamessagetoaRedisstreamusingXADD,specifythekey,useforauto-generatedIDs,andprovidefield-valuepairs.1.Usetoauto-generatemessageIDs,ensuringuniquenessviatimestampandsequencenumber.2.SpecifycustomIDsintimestamp-sequenceformatwhenneeded,avoidingconfli

Aug 14, 2025 pm 02:52 PM

phpMyAdmin revoke privileges from a user

LogintophpMyAdminwithadministrativeprivileges.2.Gotothe"Useraccounts"tabandlocatethetargetuser.3.Click"Editprivileges"nexttotheusername.4.Intheappropriatetab(Global,Database,Table),unchecktheprivilegestorevoke.5.Forcompleteremoval

Aug 14, 2025 pm 02:49 PM

Permission revocation

How to perform a search and replace in phpMyAdmin

AlwaysbackupyourdatabaseinphpMyAdminbyselectingthedatabase,clickingExport,choosingQuickorCustom,andclickingGo.2.UsetheUPDATEandREPLACE()SQLfunctionstosearchandreplacetextwiththesyntax:UPDATEtable_nameSETcolumn_name=REPLACE(column_name,'search_string'

Aug 14, 2025 pm 02:47 PM

Search for replacement

How to execute a script file from the mysql command-line

ToexecuteascriptfilefromtheMySQLcommand-line,usetheSOURCEcommandor\.shortcutwithinMySQL,orrunitfromtheOScommandline.1.UseSOURCE/path/to/script.sql;inMySQLafterlogginginandselectingthedatabaseifneeded.2.Alternatively,use\./path/to/script.sqlasashortha

Aug 14, 2025 pm 02:44 PM

Automating MySQL Backups to AWS S3 or Google Cloud Storage

To automate backup of MySQL to AWSS3 or GoogleCloudStorage, you need to create backup scripts, upload backups, automate them with cron, and clean old files. 1. Create a mysqldump script containing database credentials and timestamps to generate backup files; 2. Use AWSCLI or gsutil tools to upload backups to S3 or GCS; 3. Automatically execute scripts and record logs through cron timing tasks every day at 2 a.m.; 4. Add cleaning commands to regularly delete old backups 7 days ago to save storage space. The entire process ensures that data is secure, automatic and recoverable.

Aug 14, 2025 pm 02:11 PM

How to find the median value in a MySQL column

UseROW_NUMBER()andCOUNT()OVER()torankrowsandgettotalcount;2.Filterrowswhererow_numequalsthemiddlepositionsusingFLOOR((total_count 1)/2)andCEIL((total_count 1)/2);3.WraptheresultandtakeAVG(amount)tocomputethemedian,whichworksforbothoddandevenrowcounts

Aug 14, 2025 pm 02:03 PM