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Technical Articles

Database

SQL

How to select rows with the MAX value for a column in SQL

To select a row with a maximum value in a certain column, the most effective way is to use a window function; for a single row of the entire table, use ROW_NUMBER()OVER(ORDERBYcolumn_nameDESC) and filter rn=1; if there is a parallel situation and all maximum rows need to be returned, use RANK(); in grouping scenarios, use PARTITIONBY combined with RANK() or ROW_NUMBER() to obtain the row corresponding to the maximum value in each group; for old databases that do not support window functions, you can query SELECT*FROMyour_tablet1WHEREcolumn_name=(SELECTMAX(colu

Sep 10, 2025 am 04:47 AM

How to use linked servers to query remote data in SQL?

AlinkedserverinSQLServerenablesqueryingremotedatasourceslikeSQLServer,Oracle,orExcelusingdistributedqueries.2.Tocreatealinkedserver,useT-SQLcommandssuchassp_addlinkedserverandsp_addlinkedsrvlogin,specifyingtheremoteservername,product,andauthenticatio

Sep 10, 2025 am 04:28 AM

How to use the GROUP BY clause in SQL

TheGROUPBYclausegroupsrowswiththesamevaluesinspecifiedcolumnsforaggregatecalculations.1.Usethebasicsyntax:SELECTcolumn,AGGREGATE_FUNCTION()FROMtableGROUPBYcolumn,includingallnon-aggregatecolumnsinGROUPBY.2.Formultiplecolumns,listallnon-aggregatecolum

Sep 10, 2025 am 03:35 AM

SQL for Graph Databases: Neo4j and SQL Integration

Neo4j cannot use SQL directly, but it can be used in combination through integrated methods. Because Neo4j is a graph database and uses the Cypher query language, unlike SQL's relational data model, it operates on nodes and relationships, rather than tables and rows. For example, SQL query users and orders requires JOIN, while Cypher expresses relationships through MATCH. Despite this, Cypher is logically similar to SQL and is not very difficult to learn. In actual projects, Neo4j is often shared with SQL databases. Common integration methods include: 1. Use ETL tools (such as Talend and Neo4jETL) to convert SQL data into graph structures; 2. Connect SQ through scripting languages ??(such as Python)

Sep 10, 2025 am 01:48 AM

How to find overlapping date ranges in SQL

To determine the overlapping date range in SQL, the core condition is: start1start2, which is applicable to partial overlap, complete inclusion or complete match; for example, querying the subscription record with room_id of 101 and overlapping with the '2025-04-10' to '2025-04-15' time period, you can use SELECT*FROMbookingsWHEREroom_id=101ANDstart_date'2025-04-10'; if you need to find all overlapping date pairs in the table, you can use self-join and add a.id

Sep 09, 2025 am 06:52 AM

sql 重疊日期范圍

Building Recursive Queries with SQL CTEs

RecursivequeriesinSQLarebuiltusingrecursiveCommonTableExpressions(CTEs)tohandlehierarchicaldata.1.Startwithananchormemberselectingroot-leveldata.2.AddarecursivememberthatreferencestheCTEtotraversethehierarchy.3.CombinebothpartsusingUNIONALL.Forexampl

Sep 09, 2025 am 04:38 AM

recursive query SQL CTE

How to sort by a custom order in SQL

Use CASE expressions to implement custom sorting in SQL. 1. Use ORDERBY and CASE to assign sorting weights to each value, which is suitable for all mainstream databases; 2. Use FIELD() function to simplify the syntax in MySQL, but the values ??that are not in the list need to be processed; 3. It is recommended to use ARRAY_POSITION() instead of POSITION() in PostgreSQL to avoid matching errors; 4. For frequently used sorting logic, it is easier to maintain for creating mapping tables and associated queries. This method ensures that the custom order takes effect accurately and supports multi-column sorting and outlier processing, ultimately achieving flexible and reliable sorting results.

Sep 09, 2025 am 02:55 AM

How to disable a trigger in SQL?

InSQLServer,useDISABLETRIGGERtrigger_nameONtable_nametodisableatrigger;2.MySQLlacksdirectsupport,souseauser-definedvariableflaginthetriggerlogicordropandrecreatethetrigger;3.InPostgreSQL,useALTERTABLEtable_nameDISABLETRIGGERtrigger_name;4.InOracle,us

Sep 09, 2025 am 02:45 AM

SQL Server Change Data Capture (CDC) Implementation

To enable SQLServerCDC, follow the steps. First, confirm the version support, and then enable it for the database and table respectively. When querying change data, use built-in functions and LSN ranges. Cleaning up data can be done with default or custom policies. 1. Enable CDC: USEYourDatabaseName; EXECsys.sp_cdc_enable_db; and then enable: EXECsys.sp_cdc_enable_table@source_schema='dbo',@source_name='YourTableName'; 2. Query change: Use cdc.fn_cdc_get_all_chang

Sep 09, 2025 am 02:42 AM

How to concatenate strings in SQL

The use of the CONCAT function is the safest string splicing method across databases. It can automatically treat NULL values ??as empty string processing; this function is supported in MySQL, PostgreSQL, SQLServer, Oracle and SQLite; PostgreSQL, Oracle and SQLite also support splicing using the || operator, where the result of NULL and string splicing is the string itself; SQLServer uses operators, but it should be noted that if any operand is NULL, the result is NULL, so it needs to be used with ISNULL or COALESCE functions to ensure security; in summary, in order to ensure portability and reliability of NULL value processing,

Sep 09, 2025 am 02:22 AM

How to update a table from another table in SQL

ToupdateatableusingdatafromanothertableinSQL,useaJOIN-basedUPDATEsyntaxspecifictoyourdatabasesystem:inMySQL,useUPDATEwithJOIN;inSQLServer,includetheFROMclauseafterSET;inPostgreSQL,useFROMwithWHEREtojoin;alternatively,useasubquerywithWHEREEXISTSforbro

Sep 09, 2025 am 02:21 AM

sql 表更新

How to write a correlated subquery in SQL?

AcorrelatedsubqueryisaSQLsubquerythatdependsontheouterqueryandexecutesonceforeachrowfromtheouterquery.1.Itreferencesacolumnfromtheouterquery,typicallyintheWHERE,SELECT,orHAVINGclause,formingalinkthatmakesit"correlated."2.Example:Findemploye

Sep 09, 2025 am 01:03 AM

SQL Data Loss Prevention (DLP)

SQL Data Leakage Prevention (DLP) is a mechanism to prevent sensitive data such as user information and transaction records in the database from being illegally accessed, exported or deleted. The core lies in controlling internal permissions, identifying data flow, and discovering abnormal operations. 1. SQLDLP is a complete set of mechanisms, including identifying sensitive data, controlling access rights, monitoring data export behavior, automatic alarm or blocking abnormal operations; 2. Implementation methods include data classification and tagging, access control (RBAC/ABAC), database audit, real-time monitoring and alarm, and data desensitization; 3. When deploying, pay attention to policy flexibility, log retention time, application layer control, and periodic update of sensitive data definitions; 4. Recommended tools include Microsoft SQLServer audit Dynamic data mask

Sep 08, 2025 am 06:23 AM

How to handle deadlocks in SQL?

DeadlocksinSQLoccurwhentwoormoretransactionsarewaitingforeachothertoreleaselocksonresources,creatingacyclethatpreventsanyprogress.Whiletheycan'talwaysbeavoidedcompletely,theycanbemanagedeffectively.Here’showtohandl

Sep 08, 2025 am 06:05 AM

sql deadlock