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Technical Articles

Database

SQL

How to compare a column to multiple values in SQL?

Use the IN operator to efficiently compare columns with multiple values, such as SELECT*FROMemployeesWHEREdepartment_idIN(1,2,3); use NOTIN if multiple values ??are excluded, but pay attention to NULL affecting the result.

Sep 16, 2025 am 02:12 AM

sql 比較列

How to use dynamic SQL

DynamicSQLallowsbuildingandexecutingSQLstatementsatruntime,whichisessentialwhenquerycomponentsliketablenamesorconditionsareunknownuntilexecution;itiscommonlyusedinstoredproceduresforflexiblequeries,optionalfilters,DDLstatements,anddynamicordering;inS

Sep 16, 2025 am 12:51 AM

sql 動(dòng)態(tài) SQL

How to convert rows to columns dynamically in SQL?

The core method of dynamically converting rows to columns is to use dynamic perspective, and the specific implementation depends on the database system. 1. In SQLServer, first obtain the unique column value through QUOTENAME() and splice it into dynamic SQL, and then use PIVOT to combine sp_executesql to ensure security and flexibility; 2. In MySQL, use GROUP_CONCAT to generate a list expression containing SUM(IF()) and perform dynamic query through PREPARE statement; 3. Dynamic perspective is suitable for scenarios where column names are unknown or frequently changed, such as report generation, but should be avoided in situations where performance is sensitive or can be processed through the application layer; pay attention to column sorting and data type

Sep 15, 2025 am 04:00 AM

How to use the INNER JOIN clause in SQL?

INNERJOINcombinesrowsfrommultipletablesbasedonarelatedcolumn,returningonlymatchingrows.ThesyntaxusesONtospecifythejoincondition,suchaslinkingcustomersandordersviacustomer_id.Forexample,joiningcustomersandordersreturnsonlycustomerswhoplacedorderswitht

Sep 15, 2025 am 03:11 AM

How to find rows that do not exist in another table using SQL?

UseLEFTJOINwithISNULLtofindunmatchedrows:SELECTa.FROMTableAaLEFTJOINTableBbONa.id=b.idWHEREb.idISNULL;2.Alternatively,useNOTEXISTSforcomplexconditions:SELECTa.FROMTableAaWHERENOTEXISTS(SELECT1FROMTableBbWHEREb.id=a.id);3.AvoidNOTINifTableBcontainsNUL

Sep 15, 2025 am 02:57 AM

sql 查找缺失行

How to manage user permissions with GRANT and REVOKE in SQL?

GRANTandREVOKEcommandsmanageuseraccessinSQLdatabases,enablingprecisecontroloverprivilegeslikeSELECT,INSERT,UPDATE,andDELETEondatabaseobjects.GRANTassignspermissions,optionallywiththeWITHGRANTOPTIONtoletusersdelegateaccess,whileREVOKEremovesthem,enhan

Sep 15, 2025 am 02:00 AM

SQL權(quán)限管理

How to use the CASE statement for conditional logic in SQL?

TheCASEstatementinSQLenablesconditionallogicinqueries.Itcomesintwoforms:simple,whichcomparesanexpressiontovalues,andsearched,whichevaluatesconditions.CommonlyusedinSELECTstatements,itcanclassifydatadynamically,suchasassigninglettergradesbasedonscores

Sep 15, 2025 am 01:08 AM

sql case statement

How to write a subquery in SQL?

AsubqueryisanestedqueryinSQLusedtofilterdata,appearinginSELECT,FROM,orWHEREclauseswithinparentheses.2.IntheWHEREclause,itfiltersrowsbasedonresultsfromaninnerquery,suchasfindingemployeeswithsalariesabovetheaverage.3.IntheFROMclause,itactsasatemporaryt

Sep 15, 2025 am 12:34 AM

sql Subquery

How to format a number with leading zeros in SQL?

UseLPAD(number,length,'0')inMySQL,PostgreSQL,andOracleforleft-paddingwithzeros.2.InSQLServer,useRIGHT('00000' CAST(numberASVARCHAR),length)orFORMAT(number,'00000')forsimilarresults.3.InPostgreSQLandOracle,TO_CHAR(number,'00000')formatsnumberswithlead

Sep 15, 2025 am 12:14 AM

How to use conditional aggregation with CASE in SQL?

Use CASE to perform conditional aggregation in SQL, and aggregation functions such as SUM and COUNT and CASE expressions can be combined in the SELECT clause to achieve dynamic calculation based on conditions. For example, through COUNT (CASEWHENamount>1000THEN1END), and at the same time, low-value orders are counted, without WHERE filtering, and it supports the analysis of multiple types of data in the same query.

Sep 15, 2025 am 12:13 AM

How to select a value from the previous row in SQL?

To get the value from the previous row of SQL, use the LAG() window function; its basic syntax is LAG(column_name,offset,default_value)OVER(ORDERBYsort_column), where column_name is the column to get the value, offset specifies the number of rows to trace back (default is 1), default_value is the value returned when there is no previous row (default is NULL). The ORDERBY clause is used to determine the logical order of rows; for example, in the sales table, it is SELECTdate, amount, LAG(amount,1)OVER(ORDERBYdate)

Sep 14, 2025 am 05:08 AM

How to find which tables contain a specific column name in SQL?

QueryINFORMATION_SCHEMA.COLUMNStofindtableswithaspecificcolumnname:SELECTTABLE_NAMEFROMINFORMATION_SCHEMA.COLUMNSWHERECOLUMN_NAME='your_column_name';2.OptionallyfilterbyschemaordatabaseusingTABLE_SCHEMA;3.InSQLServer,usesys.columnsandsys.tablesformor

Sep 14, 2025 am 04:38 AM

sql 查找列名

What are some common wait types in SQL Server and what do they indicate?

CXPACKETindicatesparallelquerycoordinationwaits,oftenduetouneventhreadworkloadsorexcessiveparallelism,andmayrequirequeryoptimizationorMAXDOPadjustment.2.PAGEIOLATCH_XXsignifiesdiskI/Olatency,pointingtoslowstorageormissingindexes,requiringquerytuning,

Sep 14, 2025 am 04:31 AM

How do you create and manage triggers in a SQL database?

AtriggerisastoredprogramthatautomaticallyexecutesinresponsetospecificeventslikeINSERT,UPDATE,orDELETEonatableorview.2.Tocreateatrigger,usetheCREATETRIGGERstatementwithBEFOREorAFTER,specifytheeventandtable,anddefinethelogicwithinaBEGIN...ENDblock,usin

Sep 14, 2025 am 04:07 AM