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Technical Articles

Database

How to Build a Recommendation Engine with MongoDB

Use MongoDB's flexible mode, indexing, and aggregation pipeline to build a lightweight recommendation engine. 2. Design user, item and interactive data models to support recommendations based on content or collaborative filtering. 3. Use the aggregation pipeline to analyze behavior, such as generating recommendations through co-occurrence calculations. 4. Create indexes for commonly used query fields to improve performance. 5. Combining application logic, implement user similarity calculation, cache recommended results, and trigger updates with change flow. 6. The user recommendation list can be pre-calculated and stored to speed up retrieval. MongoDB is suitable for implementing a simple recommendation system that is driven by real-time behavior.

Aug 30, 2025 am 05:41 AM

How to drop a view if it exists in SQL

To delete a view only if it exists, you should use the DROPVIEWIFEXISTS command, which is supported in most modern databases; 1. PostgreSQL: Execute DROPVIEWIFEXISTSmy_view; add CASCADE to delete dependent objects; 2. MySQL: syntax is the same as above, supports CASCADE option; 3. SQLServer2016 and above: Use DROPVIEWIFEXISTSmy_view directly, and old versions need to use IFOBJECT_ID to check; 4.SQLite3.7.11: Support DROPVIEWIFEXISTS; 5.Oracle: does not support IFEXI

Aug 30, 2025 am 03:49 AM

sql view

How to update multiple columns in a single statement in SQL?

Yes, multiple columns can be updated in one SQL statement. The method is to list the columns to be modified in the SET clause of the UPDATE statement and separate them with commas; 1. The basic syntax is: UPDATEtable_nameSETcolumn1=value1,column2=value2WHEREcondition; 2. Any number of columns can be updated, and the values ??can be literals, expressions or subqueries; 3. Use the WHERE clause to accurately define the target rows to avoid mistakenly updating all records; 4. In PostgreSQL and Oracle, support grouping multiple columns in parentheses for subquery updates, such as SET(col1,col2)=(subquery); 5

Aug 30, 2025 am 02:55 AM

phpMyAdmin empty a table (truncate)

ToemptyatableinphpMyAdminwhilekeepingitsstructure,usetheTRUNCATEcommandviathe"Empty"buttonorSQLtab:1.LogintophpMyAdmin.2.Selectthedatabaseandtable.3.Clickthe"Operations"tabandusethe"Empty"button,orgotothe"SQL"t

Aug 30, 2025 am 02:44 AM

What command-line options are available for Navicat?

Navicatsupportscommand-lineoperationsforautomationandscripting.1.Basicstructureusesnavicat[options]withOS-specificpaths.2.Commonoptionsinclude--open=,--new,--import=,--export=,and--query=forspecifictasks.3.ExecutionvariesslightlyacrossWindows,macOS,a

Aug 30, 2025 am 02:27 AM

navicat 命令行選項(xiàng)

How to optimize COUNT(*) for large tables in MySQL?

UseapproximatecountsfromSHOWTABLESTATUSorEXPLAINwhenprecisionisn’tneeded,astheyprovidefastestimateswithoutfullscans.2.Maintainacountertableupdatedviatriggersorapplicationlogicforexactcounts,enablinginstantretrieval.3.CachecountresultsinRedisortheappl

Aug 30, 2025 am 01:50 AM

How to find the longest and shortest string in a column in SQL?

Tofindthelongestandshorteststringlengths,useMAX(LENGTH(column_name))andMIN(LENGTH(column_name))inSQL,orMAX(LEN())andMIN(LEN())inSQLServer.2.Toretrievetheactuallongestandshorteststrings,useORDERBYLENGTH(column_name)DESCLIMIT1forthelongestandORDERBYLEN

Aug 30, 2025 am 01:39 AM

What is the CONCAT_WS() function in MySQL?

CONCAT_WS()inMySQLconcatenatesstringswithaspecifiedseparator,ignoringNULLvalueswhileapplyingtheseparatoronlybetweennon-NULLvalues;1.Thefunctionusesaseparator(e.g.,',','-')placedbetweeneachpairofnon-NULLarguments;2.IftheseparatorisNULL,theentireresult

Aug 30, 2025 am 01:35 AM

How to delete a table in phpMyAdmin

LogintophpMyAdminusingyourcredentials.2.Selectthetargetdatabasefromtheleft-handsidebar.3.Locateandselectthetableyouwanttodelete.4.Deletethetableeitherbycheckingtheboxandselecting"Drop"fromthe"Withselected"dropdown,orbyclickingthe&

Aug 30, 2025 am 12:59 AM

How to handle errors in MySQL stored procedures?

Use the DECLAREHANDLER statement to effectively handle errors in MySQL stored procedures, and to deal with exceptions such as SQLEXCEPTION by defining a processor of CONTINUE or EXIT type. Combined with GETDIAGNOSTICS to obtain error details, and use transactions and OUT parameters to ensure the integrity of operations and the accuracy of feedback, thereby improving the robustness of database applications.

Aug 30, 2025 am 12:50 AM

mysql stored procedure

How to merge two rows into one in SQL?

To merge two rows of data, you need to select methods according to the specific scenario: 1. If you merge rows according to the same ID, use GROUPBY to combine SUM, STRING_AGG and other aggregate functions to merge multiple row values ??into one row; 2. If you need to merge two specific rows (such as login/logout records), you can combine MAX and GROUPBY through CASE statements to realize row conversion; 3. If there is an association relationship between the two rows (such as employee and manager), use self-connection (SELF-JOIN) to merge the two rows into one row; 4. If you need to merge multiple rows of text into a single column string, you can use functions such as STRING_AGG (PostgreSQL), GROUP_CONCAT (MySQL) and other functions. The syntax of different databases is slightly different, so

Aug 30, 2025 am 12:49 AM

SQL Optimistic vs. Pessimistic Concurrency Control

Pessimistic concurrency control is suitable for scenarios with frequent write operations and high conflict probability, such as bank transfer systems, which avoid conflicts by adding locks; optimistic concurrency control is suitable for scenarios with more reads, fewer writes and fewer conflicts, such as e-commerce systems browsing products and detecting conflicts through version numbers. The choice of the two depends on the business scenario: 1. Pessimistic locks are preferred for high-conflict scenarios, pay attention to lock granularity and deadlock issues; 2. Optimistic locks are used in low-conflict scenarios to improve concurrency performance; 3. It can be mixed according to the business module, with pessimistic locks used for critical paths, and optimistic locks used for non-critical paths.

Aug 30, 2025 am 12:39 AM

How to clone a database in MySQL

Usemysqldumptoexportandimportthedatabaseforareliable,fullclone;2.UseCREATETABLE...LIKEandINSERTforcloningspecifictablesmanually;3.UseMySQLWorkbenchorphpMyAdminforaGUI-basedclone;4.Ensureproperpermissions,sufficientstorage,andhandleforeignkeysandlarge

Aug 30, 2025 am 12:28 AM

Could not connect to server Navicat: what if I have a local server?

To resolve the "Couldnotconnecttoserver" error when Navicat connects to the local server, you can follow the following steps: 1. Confirm the server's running status, use commands such as "sudoservicemysqlstatus" to check, and start the server if necessary; 2. Check the server configuration file (such as MySQL's my.cnf) to ensure that the bind-address is set correctly; 3. Adjust the firewall settings, temporarily disable the firewall or ensure that the server port is open; 4. Verify the connection configuration of Navicat, including the host name, port, user name and password. With these steps, you can effectively resolve connection issues.

Aug 30, 2025 am 12:12 AM