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Technical Articles

Database

How to use comments in MySQL queries?

MySQL supports a variety of comment syntax to improve the readability and maintainability of SQL code. 1. Use single-line comments - (subsequent to spaces) or #, the content from the mark to the end of the line will be ignored; 2. Use /.../ for multiple lines and can be used to comment or disable code blocks, or can be used to inline comments; 3. Use /!.../ for conditional comments, the contents will be executed by MySQL but ignored by other databases, and can also be specified versions such as /!50001.../ only in MySQL 5.0.1 and above; 4. Comments can be used to document descriptions of complex queries, temporarily disable code, debugging, or writing tutorial examples, so as to help others and future selves understand SQL logic more clearly.

Sep 10, 2025 am 05:20 AM

mysql Comment

SQL for Data Analysis: A Deep Dive into Real-World Applications

SQLfordataanalysisisessentialfortransformingrawdataintoactionableinsightsbyaskingtherightquestionsandapplyingstructuredqueryingtechniques.1.DatacleaningandpreparationinvolvehandlingNULLswithCOALESCE(),standardizingtextusingTRIM()andUPPER()/LOWER(),pa

Sep 10, 2025 am 04:49 AM

How to select rows with the MAX value for a column in SQL

To select a row with a maximum value in a certain column, the most effective way is to use a window function; for a single row of the entire table, use ROW_NUMBER()OVER(ORDERBYcolumn_nameDESC) and filter rn=1; if there is a parallel situation and all maximum rows need to be returned, use RANK(); in grouping scenarios, use PARTITIONBY combined with RANK() or ROW_NUMBER() to obtain the row corresponding to the maximum value in each group; for old databases that do not support window functions, you can query SELECT*FROMyour_tablet1WHEREcolumn_name=(SELECTMAX(colu

Sep 10, 2025 am 04:47 AM

How to use linked servers to query remote data in SQL?

AlinkedserverinSQLServerenablesqueryingremotedatasourceslikeSQLServer,Oracle,orExcelusingdistributedqueries.2.Tocreatealinkedserver,useT-SQLcommandssuchassp_addlinkedserverandsp_addlinkedsrvlogin,specifyingtheremoteservername,product,andauthenticatio

Sep 10, 2025 am 04:28 AM

How to find the Nth highest value in a table in MySQL

To find the N-highest value in the table, it is recommended to use the DENSE_RANK() function, 1. Use LIMIT and OFFSET: suitable for scenarios without duplicates or small datasets, the syntax is SELECTDISTINCT column name FROM table name ORDERBY column name DESCLIMIT1OFFSETN-1; 2. Use DENSE_RANK(): suitable for processing duplicate values ??to ensure that there is no gap in ranking, the syntax is SELECT column name FROM (SELECT column name, DENSE_RANK() OVER (ORDERBY column name DESC) ASrnkFROM table name) tWHERErnk=N; 3. Use subquery: suitable for MySQL8.0 below

Sep 10, 2025 am 04:18 AM

How to analyze Redis memory usage with the MEMORY USAGE command?

TheRedisMEMORYUSAGEcommandestimatesthememoryconsumptionofaspecifickey.1.Itaccountsforboththevalueandinternalmetadata,providingresultsinbytes.2.Youcanuseittoidentifymemory-heavykeys,comparedatastructureefficiency,ortroubleshoothighmemoryusage.3.Forlar

Sep 10, 2025 am 04:08 AM

Redis內(nèi)存

How to drop a database in MySQL?

UseDROPDATABASEdatabase_nametodeleteadatabaseinMySQL;thispermanentlyremovesthedatabaseandallitscontents.UseDROPDATABASEIFEXISTSdatabase_nametoavoiderrorsifthedatabasedoesnotexist.ThisactionisirreversibleandrequiresDROPprivileges.Alwaysverifythedataba

Sep 10, 2025 am 03:41 AM

mysql database

How to use the GROUP BY clause in SQL

TheGROUPBYclausegroupsrowswiththesamevaluesinspecifiedcolumnsforaggregatecalculations.1.Usethebasicsyntax:SELECTcolumn,AGGREGATE_FUNCTION()FROMtableGROUPBYcolumn,includingallnon-aggregatecolumnsinGROUPBY.2.Formultiplecolumns,listallnon-aggregatecolum

Sep 10, 2025 am 03:35 AM

Mastering Queries in MongoDB

Usefind()toretrievemultipledocumentsandfindOne()togetthefirstmatch,optionallyusingprojectiontolimitfields.2.Applycomparisonoperatorslike$gt,$lt,$inandlogicaloperatorslike$and,$orforcomplexfiltering.3.Querynesteddocumentswithdotnotationandarraysusinge

Sep 10, 2025 am 03:25 AM

mongodb Inquire

How to update phpMyAdmin to the latest version

CheckyourcurrentphpMyAdminversionbyviewingthefooterintheinterfaceortheVersion.phpfile,thencompareittothelatestreleaseontheofficialsite.2.Alwaysbackuptheconfig.inc.phpfile,theentirephpMyAdmindirectory,andanycustomizationsbeforeproceeding.3.Chooseyouru

Sep 10, 2025 am 03:21 AM

renew

How to execute a Lua script using the EVAL command?

ToexecuteaLuascriptusingtheEVALcommandinRedis,usethesyntaxEVALscriptnumkeyskey[key...]arg[arg...];1)specifytheLuascriptasastring,2)definethenumberofkeys,3)listthekeysaccessibleviaKEYStable,and4)includeargumentsaccessibleviaARGVtable;youcanalsocallRed

Sep 10, 2025 am 03:03 AM

What Port Does Navicat Use to Connect to Servers?

Navicatusesspecificdefaultportsfordifferentdatabases:1)MySQL:3306,2)PostgreSQL:5432,3)Oracle:1521,4)SQLServer:1433,5)MongoDB:27017;understandingtheseportsiscrucialforsecureandefficientdatabaseconnections.

Sep 10, 2025 am 02:26 AM

How to enable auditing in Oracle?

EnablestandardauditingbysettingAUDIT_TRAIL=DB,restartingthedatabase,andauditingspecificactionslikeSESSIONorSELECT;2.ForOracle12c ,useunifiedauditingbyenablingitviamakecommand,creatingauditpolicies,andqueryingUNIFIED_AUDIT_TRAILforlogs.

Sep 10, 2025 am 02:18 AM

oracle audit

How to add a column to an existing table in Oracle?

UseALTERTABLEwithADDtoaddacolumninOracle.Syntax:ALTERTABLEtable_nameADD(column_namedata_type[DEFAULTvalue][constraints]).ExamplesincludeaddingVARCHAR2,NUMBERwithdefault,NOTNULLDATE,ormultiplecolumns.Newcolumnsappearattheend,defaulttoNULLunlessspecifi

Sep 10, 2025 am 02:15 AM

oracle 添加列