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Technical Articles

Database

How to find the length of a string in MySQL

TofindthelengthofastringinMySQL,useLENGTH()forbytesorCHAR_LENGTH()forcharacters.1.LENGTH()returnsthenumberofbytes,soSELECTLENGTH('Hello')returns5,butSELECTLENGTH('café')returns5inUTF8mb4because'é'uses2bytes.2.CHAR_LENGTH()returnsthenumberofcharacters

Aug 12, 2025 am 05:01 AM

Mastering the MongoDB Shell for Efficient Database Administration

MasteringtheMongoDBshellisessentialforfullcontroloverMongoDBenvironments.2.Keycommandslikeshowdbs,use,andshowcollectionsenablequicknavigationandbasicoperations.3.Usefind(),findOne(),andexplain("executionStats")toqueryandanalyzeperformance.4

Aug 12, 2025 am 04:49 AM

How to use the ANY and ALL operators in SQL?

TheANYoperatorreturnstrueifatleastonevalueinthesubquerysatisfiesthecondition,whiletheALLoperatorreturnstrueonlyiftheconditionissatisfiedforallvaluesinthesubquery;1.=ANYisequivalenttoINandchecksifavaluematchesanyvalueinthelist;2.ALLisequivalenttoNOTIN

Aug 12, 2025 am 04:39 AM

How to calculate a cumulative sum by group in SQL?

To calculate the accumulated sum by groups in SQL, use the SUM() window function to match the PARTITIONBY and ORDERBY clauses. 1.SUM(value_column) specifies the column to be accumulated and summed; 2.PARTITIONBYgroup_column defines the grouping basis (such as customer, category, etc.); 3.ORDERBYorder_column determines the accumulation order in the group (such as date or ID); 4.OVER() converts the aggregate function into a window function to realize ordered accumulation in the group. For example, grouping by salesperson in sales data and sorting by sales_date can result in the accumulated value of sales per person. If ORDERBY is omitted, return

Aug 12, 2025 am 04:37 AM

sql 累計(jì)求和

How to get the score of a specific member using ZSCORE?

Togetamember'sscoreinaRedissortedset,usetheZSCOREcommand.ItretrievesthescoreofaspecifiedmemberinO(1)time.1.Syntax:ZSCOREkeymember.2.Returnsthescoreasastring,ornilifthememberorkeydoesn'texist.3.Usecasesincludecheckinggamerankings,validatingmemberexist

Aug 12, 2025 am 04:35 AM

Navicat: what happens with my data?

Navicatensuresdatasecurityandintegritythroughsecureconnections,robustbackupsystems,andcarefuldatamanagement.1)ItusesSSHandSSL/TLSforsecuredatatransfer.2)Automaticbackupsandrecoveryoptionsprotectagainstdataloss.3)Usersmustmanagedataoperationscarefully

Aug 12, 2025 am 04:16 AM

navicat 數(shù)據(jù)庫(kù)數(shù)據(jù)

How to add a primary key to an existing table in MySQL?

To add a primary key to an existing table, use the ALTERTABLE statement with the ADDPRIMARYKEY clause. 1. Ensure that the target column has no NULL value, no duplication and is defined as NOTNULL; 2. The single-column primary key syntax is ALTERTABLE table name ADDPRIMARYKEY (column name); 3. The multi-column combination primary key syntax is ALTERTABLE table name ADDPRIMARYKEY (column 1, column 2); 4. If the column allows NULL, you must first execute MODIFY to set NOTNULL; 5. Each table can only have one primary key, and the old primary key must be deleted before adding; 6. If you need to increase it yourself, you can use MODIFY to set AUTO_INCREMENT. Ensure data before operation

Aug 12, 2025 am 04:11 AM

mysql primary key

How to use the SET data type in MySQL

TheSETdatatypeinMySQLstoreszeroormorevaluesfromapredefinedlist,idealformultiplechoiceslikepreferencesortags.2.DefineaSETcolumnbyspecifyingallowedstringvaluesinparenthesesduringtablecreation,suchasSET('email','sms','push','mail').3.Insertvaluesascomma

Aug 12, 2025 am 04:08 AM

How to create an audit trail for table changes in SQL?

To create an audit trail for SQL table changes, triggers, audit tables, and context information are used. 1. First create an audit table, including the original table structure and metadata fields such as action_type, changed_by, and changed_at; 2. Use the AFTER trigger to capture INSERT, UPDATE, and DELETE operations, and obtain new and old values through inserted and deleted temporary tables; 3. Optionally enhance audit information, such as application user, IP address and specific numerical differences before and after update; 4. Consider performance impact, avoid high-frequency recording, asynchronous batch writing and regular archives can be used; 5. Priority is given to the built-in functions of the database, such as SQLServer's Tempora

Aug 12, 2025 am 04:02 AM

How to add a primary key to a table in phpMyAdmin

ToaddaprimarykeywhencreatinganewtableinphpMyAdmin,entercolumndetails,checktheA_Iboxforauto-incrementifneeded,andselectthePrimarycheckboxunderIndex,thensave.2.Foranexistingtable,selectthedesiredcolumn,clickthe"Primary"linkunder"Withsele

Aug 12, 2025 am 04:01 AM

How to troubleshoot connection timeout issues in Navicat?

Connection timeout is usually caused by the network, configuration or database service status. Solutions include: 1. Use ping, telnet or NC to check whether the network is unobstructed; 2. Confirm whether the cloud server security group or LAN firewall releases the corresponding port; 3. Log in to the database server to check whether the service is running normally, and start the service if necessary and modify the binding address to 0.0.0.0; 4. Check the Navicat connection parameters such as host name, port, username and password and SSH tunnel settings; 5. Extend the connection timeout time in Navicat according to actual conditions to adapt to network conditions.

Aug 12, 2025 am 03:56 AM

How to use the ALTER TABLE statement in MySQL?

ALTERTABLEinMySQLisusedtomodifyanexistingtable’sstructure,andthemostcommonoperationsinclude:1.AddingacolumnusingADDCOLUMN,optionallyspecifyingpositionwithAFTERorFIRST;2.DroppingacolumnusingDROPCOLUMN,whichpermanentlyremovesthecolumnanditsdata;3.Modif

Aug 12, 2025 am 03:52 AM

mysql

How to count distinct values in SQL

To count the number of different values in SQL, COUNT (DISTINCT) is required. 1. Statistics for single columns: SELECTCOUNT(DISTINCTcolumn)FROMtable; 2. Multi-column combination statistics use SELECTCOUNT(DISTINCTcol1,col2) directly in MySQL, and subqueries are required in PostgreSQL or SQLServer: SELECTCOUNT(*)FROM(SELECTDISTINCTcol1,col2FROMtable)ASsub; 3. Use COALESCE to convert NULL into placeholders; 4. Combined with WH

Aug 12, 2025 am 03:48 AM

What are system privileges versus object privileges in Oracle security?

SystemprivilegesandobjectprivilegesinOracleservedifferentpurposes.1.Systemprivilegesgrantbroad,database-widepermissionslikecreatingsessionsortables.2.ObjectprivilegesarespecifictoindividualdatabaseobjectssuchasSELECTonacertaintable.3.Systemprivileges

Aug 12, 2025 am 03:36 AM

Permissions Oracle Security