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Technical Articles

Database

How to perform conditional inserts in SQL?

UseINSERT...SELECTwithWHERENOTEXISTStoinsertonlyifnoduplicateexists,whichiswidelysupportedacrossdatabases;2.UseMERGE(upsert)inSQLServer,Oracle,orPostgreSQLtoinsertorupdatebasedonrowexistence;3.InMySQL,useINSERT...ONDUPLICATEKEYUPDATEtoconditionallyin

Aug 07, 2025 pm 09:34 PM

sql 條件插入

How to calculate the interquartile range (IQR) in SQL?

TocalculateIQRinSQL,usePERCENTILE_CONT(0.25)andPERCENTILE_CONT(0.75)inPostgreSQL,SQLServer,orOracletogetQ1andQ3,thensubtractQ1fromQ3toobtaintheIQR;2.InMySQL,sincePERCENTILE_CONTisnotavailable,approximateQ1andQ3usingROW_NUMBER()withpercentile-basedrow

Aug 07, 2025 pm 09:32 PM

How to work with geography and geometry data types in SQL?

Usegeographyforglobal,real-worldaccuratedataandgeometryforlocal,high-performanceapplications;2.StorespatialdatawithappropriatetypesandSRIDs;3.Insertdatausingdatabase-specificfunctionslikeST_GeogFromTextinPostGISorgeography::PointinSQLServer;4.Querywi

Aug 07, 2025 pm 09:28 PM

sql geographical data

Writing and Executing Stored Procedures in SQL Databases

Stored procedures are suitable for scenarios with high repetition, multi-table association, high security and performance requirements. For example: 1. Business logic with high repetition, such as timed statistical reports; 2. Multi-table correlation and complex logic; 3. Access is controlled through stored procedures when security requirements are high; 4. Performance optimization requirements. When writing basic stored procedures, taking MySQL as an example, you need to use DELIMITER to define the ending character, CREATEPROCEDURE to declare the process name and parameters, and BEGIN...END wrap the logic body. When calling, use CALL statements and pass parameters, and debugging is performed according to different database characteristics. Notes include: avoiding conflicts between parameters and field names, using transactions reasonably, ensuring execution permissions, controlling logic complexity to reduce dimensions

Aug 07, 2025 pm 08:49 PM

How to create a function that returns a table in SQL?

Yes, functions that return tables can be created in SQLServer and PostgreSQL. 1. Use RETURNSTABLE in SQLServer to create inline table value functions, such as CREATEFUNCTIONdbo.GetEmployeesByDepartment(@DeptIDINT)RETURNSTABLEASRETURN(SELECTEmployeeID,Name,Salary,DepartmentIDFROMEmployeesWHEREDepartmentID=@DeptID); 2. For multi-statement functions, use RETURNS@ta

Aug 07, 2025 pm 08:47 PM

sql function 表值函數(shù)

How to set a default value for a column in MySQL

Tosetadefaultvaluewhencreatingatable,usetheDEFAULTkeywordinthecolumndefinition,suchasDEFAULT'active'orDEFAULTCURRENT_TIMESTAMP.2.Toaddorchangeadefaultonanexistingcolumn,useALTERTABLEusersALTERCOLUMNstatusSETDEFAULT'inactive'inMySQL8.0 ,oruseMODIFYfor

Aug 07, 2025 pm 08:44 PM

How to Fix a Corrupted MySQL Table?

First, use the CHECKTABLE command or myisamchk tool to check whether the table is corrupted, and then select the repair method according to the storage engine after confirmation; 2. For MyISAM table, you can use the REPAIRTABLE command or run myisamchk-r-f offline for repair; 3. For InnoDB table, you should configure innodb_force_recovery to restart MySQL, export the data and rebuild the table; 4. If the repair fails, you need to restore data from backup or library; 5. To prevent future damage, you should give priority to using InnoDB engine, ensure normal shutdown, enable checksum, regularly backup and monitor disk health. All repair operations must be done before

Aug 07, 2025 pm 08:24 PM

How to update existing records in a table in SQL?

To update existing records in SQL tables, you need to use the UPDATE statement; 1. Use UPDATEtable_name to specify the target table; 2. Use SETcolumn1=value1,column2=value2 to set a new value; 3. The rows to be updated must be limited with the WHERE condition, otherwise all records will be affected; 4. The WHERE condition can be tested through SELECT to ensure accuracy; 5. It is recommended to perform critical updates in the transaction so that errors can be rolled back; 6. Multiple columns or rows can be updated at the same time, supporting calculations based on the current value; be sure to verify carefully before operation to avoid error updates.

Aug 07, 2025 pm 08:13 PM

sql Update records

How to use SHOW PROCESSLIST to see running queries in MySQL

ToseecurrentlyrunningqueriesinMySQL,usetheSHOWPROCESSLISTcommand;thisdisplaysactivethreadswithdetailslikeuser,host,querystate,andexecutiontime,whereIdisthethreadID,Usertheaccountrunningthequery,Hosttheclientaddress,dbtheselecteddatabase,Commandtheope

Aug 07, 2025 pm 07:48 PM

What are the benefits of using a connection pool with MySQL?

UsingaconnectionpoolwithMySQLimprovesperformancebyreusingexistingconnections,reducingtheoverheadofrepeatedconnectionestablishmentandloweringlatency.2.Itenablesbetterresourcemanagementbylimitingconcurrentconnections,preventingthedatabasefromhittingmax

Aug 07, 2025 pm 07:42 PM

How to use the IN operator in MySQL

Use the IN operator to simplify multi-value matching query and improve readability; 1. Use IN to filter records in the specified list with literal values; 2. Use NOTIN to exclude specific values, but be careful that NULL will cause abnormal results; 3. Dynamic filtering can be implemented in combination with subqueries, such as obtaining a list of IDs that meet the conditions through subqueries; 4. In terms of performance, small to medium lists are suitable for IN, and large lists are recommended to replace them with JOIN, and it is necessary to ensure that the subquery column is indexed to avoid NOTIN and subqueries that may return NULL. At this time, NOTEXISTS should be used instead. In short, IN makes multi-value logic more concise, but NULL and performance issues need to be handled with caution.

Aug 07, 2025 pm 07:21 PM

mysql IN操作符

How to alter an existing table in SQL?

Use the ALTERTABLE statement to modify the table structure, 1. Add columns: ALTERTABLEtable_nameADDcolumn_namedata_type[constraints]; 2. Delete columns: ALTERTABLEtable_nameDROPCOLUMNcolumn_name (the data will be deleted permanently); 3. Modify column data type: MODIFY for MySQL, ALTERCOLUMNTYPE for PostgreSQL/SQLServer/Oracle; 4. Rename columns: RENAMECOLUMN for MySQL and PostgreSQL, SQLSe

Aug 07, 2025 pm 07:16 PM

sql 修改表

What is the purpose of the DISTINCT keyword in SQL?

ThepurposeoftheDISTINCTkeywordinSQListoremoveduplicaterowsfromtheresultset,returningonlyuniquevalues.1.DISTINCTfiltersoutrepeatedcombinationsofvaluesintheselectedcolumns,ensuringeachrowintheoutputisunique.2.Itoperatesontheentirecombinationofselectedc

Aug 07, 2025 pm 07:09 PM

What are Common Table Expressions (CTEs) in SQL and how are they used?

CTEsimproveSQLreadabilityandsimplifycomplexqueriesbybreakingthemintologicalsteps.1.Theysimplifycomplexqueriesbyreplacingnestedsubquerieswithcleartemporaryresultsets,asshownwhenfindingemployeesearningabovetheirdepartment’saveragesalary.2.Theyenablerec

Aug 07, 2025 pm 07:07 PM