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Technical Articles

Database

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

How to use the Report Builder feature in Navicat?

Use Navicat's ReportBuilder to create professional reports from database data without writing complex code. The steps include: 1. Ensure the database connection and select the data source; 2. Drag and drop fields in the layout editor to the Header, Detail and Footer areas and add static content; 3. Use GroupExpert to implement data grouping; 4. Format the output style, such as currency symbols, font colors and conditional formats; 5. Use the export function to save the report to PDF, Excel and other formats; 6. Save and name it clearly for subsequent use or team sharing.

Sep 09, 2025 am 12:58 AM

navicat Report generation

Can I use Navicat from the command line?

Yes,youcanuseNavicatfromthecommandlinethroughitsbuilt-incommand-linetooltoexecutesavedtasks.1.Usenavicat.exe(Windows),Navicat.app/Contents/MacOS/navicat(macOS),or/usr/lib/navicat/navicat(Linux)withswitcheslike--backup,--restore,--synchronize,--import

Sep 09, 2025 am 12:04 AM

How to implement a hierarchical data structure (tree) in MySQL

AdjacencyListModelusesparent-childreferencesandissimpletoimplementbutrequiresMySQL8 CTEsforefficienttraversal.2.PathEnumerationstoresfullpathsasstrings,enablingfastsubtreeandancestorquerieswithoutrecursion,thoughupdatesarecostly.3.NestedSetModelusesl

Sep 09, 2025 am 12:02 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 import a table into Oracle

UseSQLLoaderforimportingdatafromflatfileslikeCSVbycreatingatargettable,preparingacontrolfile,andrunningsqlldrfromthecommandline;2.UseOracleDataPump(IMPDP)for.DMPfilesbycreatingadirectoryobject,grantingpermissions,andexecutingimpdpwithappropriateparam

Sep 08, 2025 am 06:12 AM

How to handle deadlocks in SQL?

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

Sep 08, 2025 am 06:05 AM

sql deadlock

How to add a column to an existing table in SQL?

ToaddacolumntoanexistingtableinSQL,usetheALTERTABLEstatementwiththeADDCOLUMNclause;1.Specifythetablename,2.Definethenewcolumnname,3.Assignadatatype,and4.OptionallyincludeconstraintslikeNOTNULLorDEFAULT;syntaxvariesslightlybydatabase—MySQLandPostgreSQ

Sep 08, 2025 am 05:24 AM

sql 添加列

How to use the OVER clause with PARTITION BY in SQL

PARTITIONBYintheOVERclausedividesrowsintogroupsforwindowfunctionswithoutcollapsingthem,enablingper-rowcalculationswithincategories.ItallowswindowfunctionslikeSUM,RANK,andAVGtooperatewithindefinedpartitions(e.g.,regions),preservingalloriginalrowsandpr

Sep 08, 2025 am 05:18 AM

How to find rows that exist in one table but not another in SQL?

Themostreliablemethodstofindrowsinonetablebutnotanotherare:1.UsingLEFTJOINwithISNULL,whichreturnsallrowsfromthelefttablewherenomatchexistsintherighttable;2.UsingNOTEXISTS,whichiseffectiveforcorrelatedsubqueriesandhandlesNULLsbetter;3.UsingNOTINcautio

Sep 08, 2025 am 04:44 AM

How to rename a table in SQL

To rename SQL tables, different syntaxes need to be used according to the database system: 1. In MySQL, PostgreSQL and SQLite, use ALTERTABLEold_table_nameRENAMETOnew_table_name; 2. In SQLServer, use EXECsp_rename'old_table_name','new_table_name'; before the operation, you need to ensure that you have appropriate permissions, check and update dependent objects such as views or stored procedures. It is recommended to test and back up the data in the development environment before executing to avoid production environment problems.

Sep 08, 2025 am 04:37 AM

What is the purpose of the HAVING clause in MySQL?

HAVING is used to filter grouped data after GROUPBY, especially when the conditions involve aggregation functions such as COUNT and SUM; for example, if you look for departments with more than 5 employees, you need to use HAVINGCOUNT(*)>5; unlike WHERE, WHERE filters a single row before grouping and cannot use an aggregate function, HAVING filters after grouping and supports an aggregate function; the two can be used in combination, such as first using WHERE to screen employees with a salary of more than 30,000, then group them by department, and finally use HAVING to screen departments with an average salary of more than 50,000, so as to achieve effective filtering of the aggregated data and fully support complex query needs.

Sep 08, 2025 am 04:26 AM

mysql HAVING子句

How to create an index in MySQL

Creating indexes can improve query performance. It should be created on columns frequently used in WHERE, JOIN, ORDERBY or GROUPBY; 2. Use CREATEINDEXindex_nameONtable_name(column_name) to create a single column index; 3. Use composite index for multi-column joint queries, with the syntax CREATEINDEXindex_nameONtable_name(column1,column2), note that the column order affects index usage; 4. The index can be directly defined when CREATETABLE; 5. Use CREATEUNIQUEINDEX to ensure that the column values ??are unique and prevent duplication; 6. Pass A

Sep 08, 2025 am 04:19 AM

How to check tablespace size in Oracle

To check the Oracle tablespace size, SQL query must be performed using the data dictionary view; 1. Pass SELECTdf.tablespace_name,ROUND(SUM(df.bytes)/1024/1024/1024,2)AStotal_size_gb,ROUND(SUM(NVL(fs.bytes,0))/1024/1024/1024,2)ASfree_space_gb,ROUND((SUM(df.bytes)-SUM(NVL(fs.bytes,0)))/1024/1024/1024,2)ASused_space_gb,

Sep 08, 2025 am 03:54 AM