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Technical Articles

Database

How do you handle NULL values in SQL?

To correctly handle NULL values ??in SQL, you must use ISNULL or ISNOTNULL to judge, because NULL is different from normal values, you cannot compare with = or!=; 1. Use ISNULL or ISNOTNULL to check NULL values, such as SELECTFROMemployeesWHEREphoneISNULL; 2. Use COALESCE or ISNULL to replace NULL, such as COALESCE(email,'N/A') to return the first non-NULL value; 3. Note that NULL is ignored in the aggregate function, COUNT() counts all rows, while COUNT(column) only counts non-NULL values; 4. Use CASE

Aug 31, 2025 am 06:04 AM

Advanced Data Modeling Techniques for MongoDB

Embeddatawhenaccessedtogetherfrequently,small,andrequireconsistency;referencewhenlarge,shared,orgrowing.2.Usepolymorphicschemaswithatypediscriminatorforheterogeneousdocuments,enablingqueryflexibilityandefficiency.3.Createcompoundindexesinorderofequal

Aug 31, 2025 am 05:57 AM

mongodb Data modeling

How to restore a database from a backup in MySQL

Torestorefromamysqldumpfile,firstcreatethedatabaseifitdoesn’texistusingCREATEDATABASEIFNOTEXISTSyour_database_name,thenrunmysql-uusername-pyour_database_name

Aug 31, 2025 am 05:52 AM

How to create a clustered and non-clustered index in SQL?

Aclusteredindexdefinesthephysicalorderoftabledataandallowsonlyonepertable,typicallycreatedautomaticallyviaaPRIMARYKEYconstraintormanuallyusingCREATECLUSTEREDINDEX,whileanon-clusteredindexcreatesaseparatestructurepointingtodatarowsandallowsupto999pert

Aug 31, 2025 am 05:51 AM

How to rethrow an error in SQL

The correct way to re-throw an error in SQLServer is to use the THROW statement. 1. Use THROW (without parameters) in the CATCH block to re-throw the original error, retaining the error number, message, severity, status and call stack. 2. Optionally, first record the error message, such as using PRINTERROR_MESSAGE(). 3. To throw a custom error, use THROWerror_number,message,state, where the user-defined error number should be greater than or equal to 50000. 4. Avoid using RAISERROR because it truncates messages, does not retain the original error number and is not recommended for re-throwing errors. 5.THROW can only be used in CAT

Aug 31, 2025 am 05:27 AM

SQL Bulk Insert Techniques for Large Datasets

ToefficientlyhandlelargedatasetsusingSQL'sBULKINSERTcommand,firstensureproperdatafileformattingwithconsistentdelimiters,correctdatatypes,andoptionalpre-sortingforclusteredindexes.1.Useaformatfileifdatastructuredoesn'talignwiththetable.2.ApplyBULKINSE

Aug 31, 2025 am 05:12 AM

What is the innodb_file_per_table option in MySQL?

Theinnodb_file_per_tableoptionshouldbeenabledtoalloweachInnoDBtabletohaveitsown.ibdfile,improvingspacereclamation,manageability,andstorageoptimization,withbenefitsoutweighingminorfilesystemoverhead,anditisenabledbydefaultinMySQL5.6andlaterversions.

Aug 31, 2025 am 04:30 AM

mysql innodb

What formats can Navicat export data to?

Navicat supports a variety of data export formats, including CSV, Excel, JSON, XML, SQLdump files, HTML and PDF. ① CSV and Excel are suitable for sharing data with non-technical personnel. CSV is used for flat data exchange. Excel supports multiple table Sheet exports to facilitate organization of data; ② SQLdump files are suitable for database migration or backup, retaining structure and data, but paying attention to the import size limitation; ③ JSON and XML are suitable for developers and API use, retaining data types and structures, and clearly expressing nested relationships; ④ HTML and PDF are used to quickly generate visual reports or documents, HTML supports browser viewing and style applications, and PDF is easy to print and cross

Aug 31, 2025 am 03:49 AM

How to compare and synchronize two MySQL databases?

Compareschemasusingmysqldumpwith--no-dataanddiffortoolslikept-schema-sync;2.CompareandsyncdatausingCHECKSUMTABLE,SELECTCRC32withGROUP_CONCAT,orpreferablypt-table-checksumandpt-table-syncforaccuracy;3.AutomatewithMySQLreplication,customscriptsusingPyt

Aug 31, 2025 am 03:30 AM

Can you run a command that accesses multiple keys on a Redis Cluster?

Yes, you can run commands to access multiple keys on RedisCluster, but with some limitations. RedisCluster distributes keys to different nodes through data sharding, so if a command requires multiple keys to operate, these keys must be on the same node to be executed. 1. Prerequisite for supporting multi-key operations: the key must divide the data into 16384 hash slots (hashslots) in the same slot, and each key is mapped to a slot according to its hash value. Only when multiple keys are mapped to the same slo

Aug 31, 2025 am 03:03 AM

多鍵訪問

How to run a JOIN query in phpMyAdmin

After logging in to phpMyAdmin and selecting the database, click the "SQL" tag at the top to enter the query interface; 2. Write a JOIN statement in the SQL query box, such as using INNERJOIN, LEFTJOIN or RIGHTJOIN to ensure that the matching is based on the associated columns (such as users.id=orders.user_id); 3. Enter the correct syntax query, such as SELECTusers.name, orders.order_totalFROMusers INNERJOINordersONusers.id=orders.user_id; 4. You can add WHERE and ORDERBY

Aug 31, 2025 am 02:39 AM

Join查詢

How to delete duplicate rows from a table in SQL

The method of deleting duplicate rows depends on the database system. Use CTE and ROW_NUMBER() to identify and retain the first row in each group, which is suitable for SQLServer, PostgreSQL and MySQL8.0; if CTE is not supported, you can create a temporary table to store deduplicate data and replace the original table; in MySQL or SQLite, you can delete duplicate records through self-connection, and keep the rows with the smallest primary key; if there is no primary key, you should first add a unique identity and then perform a deduplication operation. Before all operations, you should back up the data and preview the results with SELECT, and finally select a method suitable for the specific database and table structure to complete deduplication.

Aug 31, 2025 am 02:27 AM

sql Delete duplicate lines

How to grant permissions on a specific database only in MySQL

TograntpermissionsonaspecificdatabaseonlyinMySQL,usetheGRANTstatementwiththedatabasenameexplicitlyspecified.1.SpecifythedatabaseintheGRANTstatementusingtheformat:GRANTpermission_typeONdatabase_name.TO'username'@'host';forexample,GRANTALLPRIVILEGESONs

Aug 31, 2025 am 02:20 AM

What are the Best Practices for Naming Tables and Columns in MySQL?

Usedescriptiveandmeaningfulnamesfortablesandcolumnstoenhanceclarityandmaintainability.2.Adoptsnake_casewithunderscoresforalltableandcolumnnamestoensurereadabilityandcompatibilityacrosssystems.3.Prefersingulartablenames(e.g.,user,order)torepresententi

Aug 31, 2025 am 01:58 AM

mysql 數(shù)據(jù)庫命名