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Technical Articles

Database

MySQL Data Dictionary and Information Schema Exploration

Information Schema is a system database provided by MySQL, which contains read-only virtual tables, used to display metadata, such as querying which tables the database can execute SELECTTABLE_NAMEFROMinformation_schema.TABLESWHERETABLE_SCHEMA='your_database_name'; Common uses include: 1. Use COLUMNS table to check column information; 2. Use STATISTICS table to check index information; 3. Use VIEWS table to check view definitions; 4. Use TABLES and COLLATIONS tables to check character sets and sorting rules. Data Dictionary (DataD

Aug 13, 2025 am 09:09 AM

How to pivot data in SQL

PivotingdatainSQLtransformsrowsintocolumnsforcategory-basedsummaries,achievableviaconditionalaggregationwithCASEandSUMforbroadcompatibility,usingthePIVOToperatorindatabaseslikeSQLServerandOracleforcleanersyntax,employingdynamicSQLwhencolumnvaluesareu

Aug 13, 2025 am 09:00 AM

sql 數(shù)據(jù)透視

How to start and stop the Oracle database

Connecting to the database requires sqlplus/assysdba to log in with SYSDBA permission; 2. It is recommended to stop the database by SHUTDOWNIMMEDIATE, which will terminate the session and roll back the transaction, and only use SHUTDOWNABORT in emergency situations; 3. Start the database by using the STARTUP command, which will automatically go through the NOMOUNT, MOUNT and OPEN stages, or use STARTUPMOUNT or STARTUPUPGRADE as needed; 4. After starting, confirm that the database status is OPE through SELECTstatusFROMv$instance and SELECTopen_modeFROMv$database

Aug 13, 2025 am 08:41 AM

What is the difference between a view and a materialized view in Oracle?

Aviewdoesnotstoredataphysicallyandexecutestheunderlyingqueryeachtimeitisaccessed,whileamaterializedviewstoresthequeryresultasaphysicaltable.2.Materializedviewsgenerallyofferfasterqueryperformancebecausetheyaccessprecomputeddata,whereasviewscanbeslowe

Aug 13, 2025 am 08:29 AM

How to perform a cross join in SQL?

ACROSSJOINreturnstheCartesianproductoftwotables,combiningeveryrowfromthefirsttablewitheveryrowfromthesecond.1.Useittogenerateallpossiblecombinations,suchasproductvariants.2.Itisusefulforcreatingtestdataorfillingmissingcombinations.3.ThesyntaxisSELECT

Aug 13, 2025 am 08:27 AM

sql 交叉連接

How to generate a GUID or UUID in SQL

The method of generating GUID/UUID in SQL database varies from database system: SQLServer uses NEWID() to generate random GUIDs, or NEWSEQUENTIALID() to generate ordered GUIDs (default constraints only); MySQL uses UUID() to generate UUIDs in string format. It is recommended to store efficiently in binary form with UUID_TO_BIN() and BIN_TO_UUID(); PostgreSQL needs to enable pgcrypto extension and use gen_random_uuid() to generate UUIDs; SQLite does not have a built-in UUID function, and it needs to be generated and inserted in external applications, or randomb

Aug 13, 2025 am 08:21 AM

How to get the first day and last day of the week in SQL?

To get the first and last day of a week in SQL, you need to select the corresponding method according to the database system and weekly start day (such as Sunday or Monday); 1. PostgreSQL uses date_trunc('week',date) to obtain the weekly interval at the beginning of Sunday, or adjust it to Monday through EXTRACT(DOW); 2. SQLServer uses DATEADD and DATEDIFF to calculate the weekly beginning of Sunday, or combines SETDATEFIRST1 or arithmetic to adjust it to Monday; 3. MySQL uses DAYOFWEEK() to subtract the offset to start on Sunday, and WEEKDAY() is used on Monday; 4. SQLite uses strftime('%w','now')

Aug 13, 2025 am 08:00 AM

How MongoDB Handles Joins

MongoDB does not support JOIN of traditional SQL, but implements data association in three ways: 1. Embedded design, directly store relevant data in the same document, suitable for scenarios where a pair of small amounts of data is often read together, with the fastest reading performance but may lead to excessive document size; 2. Reference method, manual association between multiple collections through application logic, suitable for one-to-many or shared data scenarios, maintaining data standardization but requires multiple queries and slow performance; 3. Use the $lookup aggregation pipeline to achieve a left-out connection, suitable for analytical or temporary queries, with high flexibility but moderate performance. The selection strategy should be based on the query pattern rather than forcing a certain model to effectively achieve data association.

Aug 13, 2025 am 07:58 AM

SQL Deadlock Resolution Strategies

When SQL deadlock occurs, the database will automatically roll back a transaction, and the application should catch the error and try again; it is recommended to set a reasonable number of retries and intervals to avoid looping. Methods to reduce deadlocks include: 1. Unified access order; 2. Shorten transaction time; 3. Reduce lock granularity; 4. Use index reasonably. To analyze deadlock logs, you can use commands such as SHOWENGINEINNODBSTATUS to view competing resources, waiting relationships and transaction context information, thereby optimizing logic and index design.

Aug 13, 2025 am 07:50 AM

Top 5 Mistakes to Avoid with MongoDB

Thetop5MongoDBmistakestoavoidare:1.TreatingMongoDBlikearelationaldatabase—instead,useembeddeddocumentsforone-to-feworread-heavyrelationshipsanddesignschemasaroundquerypatterns;2.Ignoringoroverusingindexes—createtargetedindexesforcommonqueryfieldsandd

Aug 13, 2025 am 07:20 AM

Basic CRUD Operations in MongoDB

The core of MongoDB's CRUD operation is Insert→Find→Update→Delete, which can efficiently manage data using mongosh; 2. Use insertOne()/insertMany() to automatically create a collection; 3. Use find()/findOne() to support conditions and projection in query; 4. Use updateOne()/updateMany() to update with $set and other operators; 5. Use deleteOne()/deleteMany() or drop() to remove the document or the entire collection. Before the operation, be sure to use find() to verify the conditions to avoid error deletion. This process completely covers the basic number of MongoDB.

Aug 13, 2025 am 07:19 AM

How to schedule a job using the MySQL event scheduler

To use the MySQL event scheduler to schedule tasks, you must first enable the event scheduler. You can turn on by executing SETGLOBALevent_scheduler=ON; and it is recommended to add event_scheduler=ON to the configuration file to persist. Then create a scheduled event, such as performing a log cleanup task once a night at midnight every day, and use a CREATEEVENT statement definition, such as CREATEEVENTIFNOTEXISTSdaily_log_cleanupONSCHEDULEEVERY1DAY1DAYSTARTSTIMESTAMP(CURDATE() INTERVAL1DAY,'00:00:

Aug 13, 2025 am 07:12 AM

How to change phpMyAdmin theme

CheckifphpMyAdminsupportsthemesbylookingfora"Themes"optionnearthelogo;ifvisible,clickittoaccessthethemeselector.2.ChangethethemeviathewebinterfacebyselectingfromavailableoptionslikeOriginal,PMASA,orMetro,withthechoicesavedincookies.3.IftheT

Aug 13, 2025 am 06:26 AM

What are some best practices for writing clean and maintainable SQL code?

Useconsistentformattingandindentationtoenhancereadability,clearlyseparatingSQLclausesandlogicalstructures.2.Choosedescriptiveandconsistentnamesfortables,columns,andaliases,followingauniformnamingconventionlikesnake_case.3.Breakdowncomplexqueriesusing

Aug 13, 2025 am 06:22 AM

sql Code specifications