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Technical Articles

Database

How do you add or delete a column in an existing SQL table?

To add or delete a column in a table, use the ALTERTABLE statement; 1. Use ALTERTABLEtable_nameADDCOLUMNcolumn_namedata_type[constraints] when adding a column, for example, add an email column and set the default value: ALTERTABLEemployeesADDCOLUMNstatusVARCHAR(20)DEFAULT'active'; 2. Use ALTERTABLEtable_nameDROPCOLUMNcolumn_name when deleting a column, for example, delete an email column: ALTERTABLEemployeesADDCOLUMNcolumn_name; 2. Use ALTERTABLEtable_nameDROPCOLUMNcolumn_name when deleting a column, for example, delete an email column: ALTERTABLEemployeesADDCOLUMNcolumn_name.

Aug 05, 2025 am 10:35 AM

How to create a trigger in Oracle?

To create an Oracle trigger, you must first clarify the trigger timing, table name and execution logic. 1. Use CREATEORREPLACETRIGGER to define the trigger name; 2. Specify the BEFORE, AFTER or INSTEADOF triggering timing; 3. Declare the INSERT, UPDATE or DELETE events; 4. Specify the associated table name; 5. Add FOREACHROW to implement row-level triggering; 6. Optional WHEN clause to set the trigger conditions; 7. Write PL/SQL logic between BEGIN and END; for example, use NEW and :OLD to reference new and old values to avoid mutating table errors, and finally use user_triggers to

Aug 05, 2025 am 09:54 AM

What are table-valued functions in SQL?

Table-valuedfunctions(TVFs)returnatableandcomeintwotypes:inline,whichusesasingleSELECTstatement,andmulti-statement,whichallowsmultipleSQLoperationstobuildacustomresultset.2.Theyofferreusability,parameterization,andseamlessintegrationintoqueriesliketa

Aug 05, 2025 am 09:44 AM

What is the difference between TRUNCATE, DELETE, and DROP in MySQL?

DELETEremovesspecificorallrows,keepstablestructure,allowsrollbackandtriggers,anddoesnotresetauto-increment;2.TRUNCATEquicklyremovesallrows,resetsauto-increment,cannotberolledbackinmostcases,doesnotfiretriggers,andkeepstablestructure;3.DROPremovesthee

Aug 05, 2025 am 09:39 AM

mysql Database operations

How to manage user accounts in phpMyAdmin

AccesstheUseraccountstabinphpMyAdminusinganadminaccounttomanageMySQLusers.2.Createanewuserbyclicking"Adduseraccount",settingausername,host,strongpassword,andassigningminimalnecessaryprivileges.3.Editexistingusersbyselecting"Editprivile

Aug 05, 2025 am 09:22 AM

User Management

How does SQL handle NULL in a comparison (e.g., WHERE column = NULL)?

Special attention is required when handling NULL values in SQL, because NULL represents unknown or missing values and does not follow conventional comparison logic. 1. Use ISNULL to check the NULL value, because =NULL will not take effect, it will return UNKNOWN; 2. Be careful when using NOTIN. If the subquery contains NULL value, the entire condition may return UNKNOWN. Use NOTEXISTS instead or filter NULL; 3. Aggregation functions such as COUNT, SUM, and AVG will ignore the NULL value, COUNT(*) counts all rows, and COUNT(column) only counts non-NULL values. Understanding these rules helps avoid common pitfalls in query results.

Aug 05, 2025 am 08:49 AM

Best Practices for Connection Pool Management in MongoDB Applications

UnderstandhowconnectionpoolsworkbyrecognizingthatMongoDBdriversreuseconnectionstoreduceoverhead,limitconcurrentoperationsviapoolsize,andrequirepropermanagementtoavoidtimeoutsorresourceexhaustion.2.Tuneconnectionpoolsettingsbasedonworkloadbyconfigurin

Aug 05, 2025 am 08:46 AM

What are the best practices for writing MySQL triggers?

Keeptriggerssimpleandfocusedbyperformingasingletaskanddelegatingcomplexlogictostoredproceduresorapplicationcode,ascomplextriggersarehardertodebugandtest.2.Avoidrecursiveorcascadingsideeffectsbypreventinginfiniteloopsthroughcarefuldesignandthoroughtes

Aug 05, 2025 am 08:25 AM

Best Practices mysql trigger

Optimizing MySQL for Real-time Stock Market Data

TooptimizeMySQLforreal-timestockmarketdata,focusonthefollowingsteps:1)UseInnoDBasthestorageenginefortransactions,crashrecovery,androw-levellocking,andenableinnodb_file_per_table;2)Designefficientindexes,especiallyacompoundindexon(symbol,timestamp),an

Aug 05, 2025 am 08:24 AM

Leveraging MongoDB Atlas Search for Powerful Full-Text Search Capabilities

CreateadeclarativeAtlasSearchindexusingJSONtospecifyfieldslikename,description,andcategorywithdynamic:falseforcontrol.2.Usethe$searchaggregationstageinsteadof$match,enablingtextsearchacrossspecifiedfieldswithrelevancescoring.3.Boostrelevancebyassigni

Aug 05, 2025 am 08:21 AM

How do you find the last inserted identity value in SQL?

Tofindthelastinsertedidentityvalue,usethedatabase-specificfunction:1.SQLServer:SCOPE_IDENTITY()issafestasitreturnsthelastidentityvaluewithinthesamescope,avoidingissueswithtriggers;2.MySQL:LAST_INSERT_ID()returnsthelastauto-incrementvalueforthecurrent

Aug 05, 2025 am 07:31 AM

Implementing MySQL Read-Write Splitting for Performance

MySQL read and write separation reduces the load on the main library and improves performance by distributing read requests to the slave library. 1. Reading and writing separation depends on the master-slave replication mechanism. The master library processes write operations and records binlog. The slave library plays the log synchronized data. Pay attention to the delay and consistency issues; 2. The implementation methods include application-level manual routing, middleware proxy (such as MyCat, ProxySQL) and ORM framework support, each with its advantages and disadvantages; 3. Precautions include avoiding dirty reads, reasonably managing connection pools, monitoring master-slave delays, reasonably allocating read requests and conducting sufficient tests and verification to ensure data consistency and system stability.

Aug 05, 2025 am 06:47 AM

How to find and remove orphaned records in MySQL?

Tofindorphanedrecords,useaLEFTJOINtoidentifychildrecordswithoutvalidparentreferences,suchas:SELECTo.*FROMordersoLEFTJOINusersuONo.user_id=u.idWHEREu.idISNULL.2.Todeleteorphanedrecords,useDELETEwithLEFTJOINorNOTEXISTS,forexample:DELETEoFROMordersoLEFT

Aug 05, 2025 am 06:35 AM

What is the Purpose of the Binary Log in MySQL?

ThebinaryloginMySQLiscriticalforreplication,recovery,andauditing;1.Inreplication,themasterrecordschangesinthebinarylog,andslavesuseI/OandSQLthreadstocopyandapplytheseeventsforsynchronization;2.Forpoint-in-timerecovery,itenablesrestoringdatatoaspecifi

Aug 05, 2025 am 06:09 AM