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Home

Technical Articles

Database

How to write a conditional WHERE clause in SQL

The conditional WHERE clause is implemented through Boolean logic or CASE expression. The most effective method is to use AND/OR combination conditions, such as selecting different filter conditions according to the parameter value, filtering according to LastName when the parameter is 1, and filtering according to Department when it is 2, otherwise all rows will be returned; CASE return value can also be compared, but the readability and performance are poor; dynamic SQL can be used for complex situations, but SQL injection needs to be prevented; it is recommended to avoid OR conditions that lead to index scans, use COALESCE to process optional filtering, test execution plans, and split the query when the logic is large, and ultimately, it should be kept simple, clear and optimized.

Aug 27, 2025 am 01:49 AM

sql 條件WHERE

How does Redis handle a restart when both RDB and AOF are enabled?

WhenRedisrestartswithbothRDBandAOFenabled,theAOFfiletakespriorityfordatarecoverybecauseitoffersgreaterdurabilitybyloggingeverywriteoperation.1.RedisfirstchecksforthepresenceofanAOFfileandloadsdatafromitifavailable.2.IftheAOFfileiscorruptedorempty,Red

Aug 27, 2025 am 01:23 AM

How to group by year in SQL

To group by year, you need to use the corresponding functions to extract the year and group it according to the database system; 1. Use the YEAR() function in MySQL or SQLServer: SELECTYEAR(order_date)ASyear,COUNT()AStotal_ordersFROMordersGROUPBYYEAR(order_date)ORDERBYyear; 2. Use the EXTRACT() function in PostgreSQL, Oracle, etc.: SELECTEXTRACT(YEARFROMorder_date)ASyear,COUNT()AStotal_order_order; 2. Use the EXTRACT() function in PostgreSQL, Oracle, etc.: SELECTEXTRACT(YEARFROMorder_date)ASyear,COUNT()AStotal_order

Aug 27, 2025 am 01:16 AM

How to use the Data Generator for specific data types?

TouseAdatagereratorerator effectiveForForpecificatypes, first choosetheritebuilt-intypelikikenamsorhonenumbers, Thencustomizeformatsusingplaceholtersorregex, NextAdjustStRASTSSUSMIN/MAXVALLUSPEVERSHOUSTEGESUTE ANDFINALL

Aug 27, 2025 am 12:44 AM

How to use the CAST function to convert data types in SQL

The CAST function is used to explicitly convert data types to ensure data compatibility. Its basic syntax is CAST (expressionAStarget_data_type). It can convert strings into numbers such as SELECTCAST ('123'ASINT), convert numbers into strings for splicing such as SELECT'Total:' CAST (totalASVARCHAR), and supports date conversion such as CAST ('2023-10-01'ASDATE), but invalid conversion will report an error. It is recommended to use safe alternative methods such as TRY_CAST to deal with exceptions. It is often used for querying, formatting output and data aggregation of types that are not matched. Different databases have slightly named data types.

Aug 27, 2025 am 12:10 AM

sql CAST函數(shù)

What is the difference between UNION and UNION ALL in Oracle?

UNIONremovesduplicatesbyperformingaDISTINCToperation,makingitslowerduetosortingandduplicatechecking,while2.UNIONALLincludesallrowswithoutduplicatechecking,makingitfasterandmoreefficient,souseUNIONwhenuniqueresultsareneededandUNIONALLwhenpreservingall

Aug 26, 2025 am 07:17 AM

How to enable the slow query log in MySQL

To enable MySQL slow query logs, first check the current status: 1. Execute SHOWVARIABLESLIKE'slow_query_log'; if OFF, it needs to be enabled; 2. Check the log path and threshold: SHOWVARIABLESLIKE'slow_query_log_file'; and SHOWVARIABLESLIKE'long_query_time'; 3. It is recommended to modify the configuration file (such as /etc/my.cnf), and add it under [mysqld]: slow_query_log=ON, slow_query_log_file=/var/log/mysql/m

Aug 26, 2025 am 07:14 AM

How to find the Nth highest salary in a MySQL table?

The use of LIMIT and OFFSET is suitable for simple queries and N is known, but does not support dynamic variables; 2. The window function using DENSE_RANK() can correctly handle duplicate values, and it is recommended to use in dynamic N scenarios in modern MySQL versions; 3. The use of related subqueries is suitable for older versions of MySQL that do not support window functions, but have poor performance; the most recommended method is DENSE_RANK(), which is suitable for most production environments due to its accuracy, flexibility and efficiency.

Aug 26, 2025 am 06:42 AM

mysql salary

How to analyze a table in Oracle

To analyze Oracle tables, first use DBMS_STATS.GATHER_TABLE_STATS to collect statistical information, 1. Execute GATHER_TABLE_STATS to collect statistical information; 2. Query DBA_TABLES, DBA_TAB_COLUMNS and DBA_INDEXES to view statistical details; 3. Check stale_stats in DBA_TAB_STATISTICS to determine whether the statistical information has expired; 4. Understand the table structure, constraints and index by querying the data dictionary; 5. Use EXPLAINPLAN to verify the query execution plan to ensure that the optimizer behaves reasonably, thereby comprehensively analyzing the table and improving performance.

Aug 26, 2025 am 06:37 AM

How to filter grouped data with the HAVING clause in SQL?

HAVINGisusedtofiltergroupsafteraggregation,unlikeWHEREwhichfiltersrowsbeforegrouping.1.UseHAVINGwithGROUPBYtoapplyconditionsonaggregateslikeCOUNT,SUM,orAVG.2.WHEREcannotbeusedwithaggregatefunctions,soexpressionslikeCOUNT()>5mustappearinHAVING.3.Mu

Aug 26, 2025 am 06:18 AM

What is the SQL mode in MySQL?

SQLmodeinMySQLdefineshowtheserverinterpretsSQLstatementsbycontrollingdatavalidation,syntaxcompliance,andhandlingofinvalidormissingdata,withcommonmodesincludingSTRICT_TRANS_TABLESfordataintegrity,ONLY_FULL_GROUP_BYforstandardGROUPBYbehavior,NO_ZERO_DA

Aug 26, 2025 am 05:37 AM

mysql sql

Best Practices for MongoDB in Production

Alwaysusereplicasetswithatleastthreemembersforhighavailabilityandautomaticfailover.2.Enableauthentication,useTLS/SSLencryption,bindtoprivateIPs,andaudituseraccesstosecuredeployments.3.Optimizequerieswithproperindexing,avoidindexoveruse,andanalyzeslow

Aug 26, 2025 am 05:36 AM

mongodb Production Environment

How to use window functions in SQL

WindowfunctionsinSQLenablecalculationsacrossrelatedrowswithoutcollapsingthem,allowingforrankings,runningtotals,movingaverages,androwcomparisonswithinpartitions.Thebasicsyntaxusesfunction_name(expression)OVER(PARTITIONBYpartition_expressionORDERBYsort

Aug 26, 2025 am 05:31 AM

How to clear command history?

Clearing the command history can be achieved through the following steps: 1. Clear the Bash history in the current session memory and write back to the file, execute history-c and history-w; 2. Completely delete the history file, run rm~/.bash_history and create an empty file to prevent recovery; 3. If you use Zsh, replace it with the ~/.zsh_history path; 4. Block future history recording, add unsetHISTFILE in ~/.bashrc or ~/.zshrc and take effect configuration. The above methods can be used for different cleaning requirements and shell types respectively to ensure privacy and security.

Aug 26, 2025 am 05:21 AM