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Technical Articles

Database

How to view table constraints?

To view the constraints of a table, the core method is to query the database's metadata or information schema. 1. Use INFORMATION_SCHEMA query. For example, in MySQL, you can view the primary key through SELECTCOLUMN_NAMEFROMINFORMATION_SCHEMA.KEY_COLUMN_USAGEWHERETABLE_NAME='your_table_name'ANDCONSTRAINT_NAME='PRIMARY', and change the condition to 'FOREIGNKEY' in foreign keys; 2. Use the database's own commands, such as DESCRIBEtable_name or SH in MySQL

Aug 31, 2025 am 01:24 AM

sql database

How to find points within a specific radius using GEORADIUS?

To use the GEORADIUS command in Redis to find geographical points within a specified radius, you must first add data through GEOADD, then execute GEORADIUS for querying and combine parameter expansion functions. 1. Use the GEOADD command to add geographic location data, such as the GEOADDcities latitude and longitude city name; 2. Use the GEORADIUS command to query by radius, the format is GEORADIUSkey latitude and longitude radius units; 3. You can select WITHCOORD and WITHDIST to obtain latitude and longitude and distance information; 4. Use COUNT to limit the number of returns and sort through ASC/DESC; 5. Pay attention to the latitude and longitude order, and do not directly modify the performance under stored data and large data volumes.

Aug 31, 2025 am 01:18 AM

How to configure phpMyAdmin with Apache

Install Apache, PHP, MySQL/MariaDB and related PHP extensions; 2. Install phpMyAdmin through the package manager and configure the database; 3. Enable the Apache configuration of phpMyAdmin and restart the service; 4. Access the http://localhost/phpmyadmin in the browser to log in to the management interface; 5. Create a symbolic link and enable configuration when encountering a 404 error, check MySQL credentials or adjust the authentication method when login fails, and install php-curl, php-gd and other modules when missing the extension; 6. In the production environment, it is recommended to rename the access path, use .htaccess password to protect or restrict IP access to enhance security.

Aug 31, 2025 am 12:54 AM

apache

What is the trade-off when using HyperLogLog? (space vs. accuracy)

HyperLogLogtradesaccuracyforspaceefficiencybyusingprobabilisticmethodstoestimateuniqueelementsinlargedatasets.Insteadofstoringallitems,ithashesvaluesandtracksleadingzerostoinfercardinalitywithminimalmemory—kilobytesinsteadofgigabytes.Thisintroducesan

Aug 30, 2025 am 08:56 AM

trade off

How to use ROWNUM in Oracle

Rownumisassignedbeforeorderby, SotogetorderDesults, SortinasubqueyThenaplyrownum; 2. Forpaginationlikerows6–10, Useanstedsubqueryytofirstsortandlimitto10, thenfilterfrownum> 5Viaanalias;

Aug 30, 2025 am 08:50 AM

How to connect to MySQL using PHP with mysqli

To use PHP's mysqli extension to connect to MySQL, it is recommended to use object-oriented methods: 1. Create a mysqli instance and pass it in the host, username, password and database name; 2. Check connect_error to ensure the connection is successful; 3. Use environment variables to store credentials and log errors instead of direct output; 4. Close the connection in time after the connection is completed; 5. Enable SSL and preprocessing statements to prevent SQL injection; 6. Check permissions, MySQL service status, firewall and host configuration when the connection fails. After correct configuration, you can perform database operations through mysqli_query or preprocessing statements. After successful connection, "Connected successfully" should be displayed.

Aug 30, 2025 am 08:48 AM

mysql php

How to check the available space in an ASM disk group in Oracle

UseSQLquery:SELECTname,total_mb,free_mb,(free_mb/total_mb)*100ASpct_freeFROMv$asm_diskgroup;togetdiskgroupname,totalspace,freespace,andpercentagefree.2.UseASMCMDcommand:asmcmdlsdgtodisplaydiskgroupstate,totalandfreespaceinMB,andredundancytype.3.Query

Aug 30, 2025 am 08:22 AM

oracle ASM磁盤(pán)組

How to Use Wildcard Indexes in MongoDB

WildcardindexesinMongoDBusethe"$"syntaxtoindexallfieldsunderaspecifiedpath,enablingefficientqueriesondynamicorunknownfieldnames.Forexample,creatinganindexwithdb.collection.createIndex({"attributes.$":1})automaticallyindexeseveryfi

Aug 30, 2025 am 07:31 AM

mongodb 通配符索引

How to use LAG() and LEAD() functions in MySQL

MySQL's LAG() and LEAD() functions can be used to access data from the previous or next row in the result set. 1. The LAG() function is used to obtain the row value of the specified offset before the current row. It is often used to compare the difference between the current row and the previous row, such as calculating monthly income changes; 2. The LEAD() function obtains the subsequent row value, which is suitable for predicting or comparing future values; 3. Both must be used in conjunction with the OVER() clause, and supports PARTITIONBY processing data by group partition, and is available since MySQL8.0. When using it, you need to ensure version compatibility, and finally achieve inter-row calculations without self-connection.

Aug 30, 2025 am 07:13 AM

mysql Window Functions

SQL CLR Integration: Extending Database Functionality

SQLServer's CLR integration refers to the technology of writing database objects in C# or VB.NET by loading .NET assembly. The advantages are handling complex calculations, string operations, and accessing external resources. The steps to enable: 1. Use the sp_configure command to enable CLR; 2. Set security policy to restrict permissions. The deployment process includes: 1. Create a SQLCLR project and compile and generate DLLs; 2. Load the assembly in SQLServer and create SQL object calling methods. Notes include: 1. Pay attention to performance impact; 2. Strengthen safety control; 3. Pay attention to maintenance complexity; 4. Check version compatibility.

Aug 30, 2025 am 07:11 AM

How to understand a query execution plan in SQL?

TointerpretaSQLqueryexecutionplaneffectively,startbyunderstandingthatthephysicalplanrevealsactualdatabaseoperationslikescans,seeks,andjoins;readtheplanbottom-upinmosttools,whereleafoperationsretrievedataandthetopmostoperatorreturnsthefinalresult;focu

Aug 30, 2025 am 06:57 AM

What are blocking list pop operations like BLPOP and BRPOP?

BLPOPandBRPOPareblockinglistcommandsinRedisusedtoretrieveandremoveelementsfromalist,waitingifnecessaryuntilanelementisavailableoratimeoutoccurs.BLPOPpopsfromtheleft(head)ofthelist,whileBRPOPpopsfromtheright(tail).Theyareidealfortaskqueuesandreal-time

Aug 30, 2025 am 06:36 AM

阻塞列表彈出操作

MySQL Database Testing Methodologies and Tools

To effectively test MySQL databases, unit testing, integration testing, use testing tools and performance stress testing should be used. 1. Unit testing: Verify the correctness of SQL statements and stored procedures through frameworks such as unittest or JUnit, and combine Mock data and transaction control to ensure a clean test environment. 2. Integration testing: Simulate real business scenarios, verify the interaction between the application and the database, use independent test databases and cooperate with slow query monitoring to improve efficiency. 3. Use MySQLWorkbench, DBeaver, dbForgeStudio and other tools to assist in testing to improve automation level and test coverage. 4. Performance and stress testing: Simulation of high concurrency using Sysbench and JMeter

Aug 30, 2025 am 06:20 AM

mysql database Test Methods

How to delete rows from a table in SQL?

TodeleterowsinSQL,usetheDELETEstatementwithaWHEREclausetospecifywhichrowstoremove;2.AlwaystesttheconditionwithaSELECTqueryfirsttoensureaccuracy;3.Usetransactionstoallowrollbackincaseoferrors;4.UseTRUNCATETABLEonlywhenremovingallrows,asitisfasterbutla

Aug 30, 2025 am 05:56 AM