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Technical Articles

Database

Mysql Tutorial

How to clone a database in MySQL

Usemysqldumptoexportandimportthedatabaseforareliable,fullclone;2.UseCREATETABLE...LIKEandINSERTforcloningspecifictablesmanually;3.UseMySQLWorkbenchorphpMyAdminforaGUI-basedclone;4.Ensureproperpermissions,sufficientstorage,andhandleforeignkeysandlarge

Aug 30, 2025 am 12:28 AM

How to find duplicate values in a table in MySQL

To find duplicate values ??in MySQL table, you should use the GROUPBY and HAVING clauses; 1. Use SELECTemail,COUNT()AScountFROMusersGROUPBYemailHAVINGCOUNT()>1; to find duplicate mailboxes; 2. When expanding to multiple columns, use SELECTfirst_name,last_name,COUNT()AScountFROMusersGROUPBYfirst_name,last_nameHAVINGCOUNT()>1; to find duplicates based on multiple fields; 3. When looking for completely duplicate rows, all columns are performed

Aug 29, 2025 am 05:17 AM

How to use the CASE statement in MySQL?

There are two forms of CASE statements in MySQL: search type CASE and simple type CASE, among which search type is more commonly used; search type returns the corresponding results through the WHEN clause specified conditions, which are suitable for clauses such as SELECT, UPDATE, ORDERBY, etc., and can be used for data classification, custom sorting and condition update; simple type matches expressions with multiple values ??one by one and returns the result; CASE must end with END, ELSE is optional, but it is recommended to avoid NULL values; combined with aggregate functions, conditional statistics can be implemented, such as group counting with SUM (CASEWHEN...), thereby improving SQL's flexibility and functionality in data conversion and report generation.

Aug 29, 2025 am 04:33 AM

How to remove duplicate rows from a table in MySQL

Use the temporary table method to safely deduplicate. First create a temporary table with the same structure, insert the deduplicate data, rename the table and delete the backup; 2. MySQL8.0 can use the ROW_NUMBER() window function to mark duplicate rows, delete rows with a number greater than 1, and retain the first row sorted by the primary key; 3. Old version of MySQL can delete duplicates through self-connection, and retain rows with smaller ids; 4. Adding unique constraints can prevent future duplication. Before the operation, data must be backed up, queries must be tested first, and appropriate methods must be selected based on the MySQL version and primary key conditions. Temporary table method is the safest, and a unique index should be added to prevent duplicate data from appearing again.

Aug 29, 2025 am 03:59 AM

How to find circular references in foreign keys in MySQL

Circularreferencesinforeignkeyscancauseissueswithinserts,updates,anddeletes,especiallywithcascadingoperations.2.Theyoccurwhenachainofforeignkeyreferencesformsaloop,eitherdirectlyorindirectly.3.InMySQL8.0 ,usearecursiveCTEqueryonINFORMATION_SCHEMA.KEY

Aug 29, 2025 am 02:08 AM

How to deal with deadlocks in MySQL

MySQLautomaticallydetectsdeadlocksusingawait-forgraphandresolvesthembyabortingonetransaction,whichreceiveserror1213,whiletheotherproceeds;2.Applicationsmusthandletherollbackandretrythetransactionusingaretrymechanismwithlimitedattemptsandoptionalexpon

Aug 29, 2025 am 01:50 AM

How to configure a master-slave replication in MySQL

Configuring MySQL master-slave replication requires ensuring that the two servers are interconnected, version compatible, initial data consistent, and server-id is unique; 2. Enable binary logs on the master server and set server-id. After restarting MySQL, create a user with REPLICATIONSLAVE permission, and record the File and Position values ??output by SHOWMASTERSTATUS; 3. Configure the unique server-id on the slave server, enable relay-log, log-slave-updates and read-only, and execute CHANGEMASTERTO to specify the master server information and start STARTSL.

Aug 29, 2025 am 01:34 AM

How to handle foreign key constraints in MySQL?

ForeignkeyconstraintsinMySQLmaintainreferentialintegritybyensuringvalidrelationshipsbetweentables;theyaredefinedeitherduringtablecreationorviaALTERTABLE,canincludereferentialactionslikeCASCADEorSETNULL,andshouldbecarefullymanagedtopreventdatainconsis

Aug 29, 2025 am 12:25 AM

mysql foreign key constraints

What is the max_connections setting in MySQL?

max_connectionsinMySQLsetsthemaximumnumberofsimultaneousclientconnectionstheservercanhandle,withatypicaldefaultof151.1)ThevalueisadjustableviaconfigurationfileordynamicallyusingSETGLOBALmax_connections.2)Increasingitrequiressufficientsystemresourcesl

Aug 28, 2025 am 08:19 AM

mysql

How to pivot a table in MySQL

MySQL does not have a built-in PIVOT operator, but it can be combined with GROUPBY through CASE statements; 1. Understand the basic structure: use SUM (CASEWHEN) for each column conditional aggregation; 2. Use SUM or MAX: use SUM for multiple rows, and use MAX for a single row to avoid addition; 3. Dynamic row to column: When the column value is unknown, use GROUP_CONCAT to generate dynamic SQL and execute it; 4. Best practices: reasonably alias, handle null values, use MAX for text, and optimize indexes; 5. Text data examples: use MAX (CASE) to extract non-numeric fields; static or dynamic row to column can be flexibly implemented through conditional aggregation, and finally use GROUPBY to complete the data rotation, fully implement the row to column function.

Aug 28, 2025 am 08:06 AM

What is AUTO_INCREMENT in MySQL?

AUTO_INCREMENTinMySQLautomaticallygeneratesauniqueintegerforacolumn,typicallyusedforprimarykeys,ensuringeachnewrowreceivesasequentialidentifierwithoutmanualinput;itrequiresthecolumntobeaninteger,NOTNULL,andindexed,allowsonlyonepertable,incrementsfrom

Aug 28, 2025 am 07:35 AM

mysql