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Technical Articles

Database

How to update a view in MySQL

Toupdateaview'sdefinitioninMySQL,useALTERVIEWorCREATEORREPLACEVIEWtomodifyitsunderlyingquery.2.Toupdatedatathroughaview,useUPDATE,INSERT,orDELETEstatementsiftheviewisupdatable,whichrequiresittonotuseDISTINCT,GROUPBY,HAVING,aggregates,JOINs,subqueries

Sep 11, 2025 am 11:01 AM

How to write a recursive query with a CTE in SQL

ArecursiveCTEinSQLisusedtoqueryhierarchicaldatabydefiningananchormemberasthestartingpoint,arecursivememberthatreferencestheCTEitself,andaUNIONALLtocombinethem,enablingtraversaloftree-likestructuressuchasorganizationalchartsorfilesystems,withtherecurs

Sep 11, 2025 am 10:51 AM

How to get table schema information in MySQL?

UseDESCRIBEorDESCtoquicklyviewcolumndetailslikename,type,andkeyinfo.2.QueryINFORMATION_SCHEMA.COLUMNSforcustomizable,detailedmetadata.3.UseSHOWCREATETABLEtoseethefulltablecreationSQL.4.UseSHOWINDEXtoretrieveindexinformation.

Sep 11, 2025 am 10:48 AM

How to perform a LEFT JOIN in Oracle?

ALEFTJOINreturnsallrowsfromthelefttableandmatchedrowsfromtherighttable,withNULLsforunmatchedright-sidecolumns.InOracle,useLEFTJOINorLEFTOUTERJOINintheFROMclausewithproperONconditions.Forexample,joiningemployeesanddepartmentsensuresallemployeesappear,

Sep 11, 2025 am 10:35 AM

oracle

What is the performance_schema database in MySQL?

Theperformance_schemadatabaseinMySQLprovidesreal-timeperformancemonitoringanddiagnosticsbytrackingserveractivitiessuchasSQLstatementexecution,threadbehavior,fileI/O,tableaccess,locks,memoryusage,andwaitevents;unlikeINFORMATION_SCHEMA,whichstoresmetad

Sep 11, 2025 am 10:26 AM

mysql

How to perform bitwise operations in SQL?

SQL supports bit operation, 1. Use & for bit and operation, to check whether a specific bit is set; 2. Use | for bit or operation, to combine flags; 3. Use ^ for bit exclusive or operation, to switch specific bits; 4. Use ~ for bit non-operation, to reverse all bits, pay attention to the complement representation of signed integers; 5. Use left and right displacement, to realize multiply or divide by powers of 2, but PostgreSQL and Oracle need special processing; it is common in permission management and other scenarios, and the operation is efficient, but attention should be paid to the integer size and symbolic characteristics of the database system.

Sep 11, 2025 am 09:59 AM

sql Bit operations

How to Deal with 'Too Many Connections' Error in MySQL?

CheckcurrentconnectionusagewithSHOWVARIABLESLIKE'max_connections'andSHOWSTATUSLIKE'Threads_connected'toconfirmifthelimitisreached.2.TemporarilyincreasethelimitusingSETGLOBALmax_connections=500toallowimmediateconnections.3.Optimizeapplicationbehaviorb

Sep 11, 2025 am 09:36 AM

How to create a foreign key in MySQL?

TocreateaforeignkeyinMySQL,defineacolumnreferencinganothertable’sprimaryoruniquekeyusingFOREIGNKEYinCREATETABLEorALTERTABLE,ensuringmatchingdatatypes,InnoDBengine,andindexedreferencedcolumns.

Sep 11, 2025 am 09:25 AM

mysql foreign key

How to implement pagination in SQL

PaginationinSQLisimplementedusingmethodslikeLIMITOFFSET,ROW_NUMBER(),OFFSETFETCH,orkeysetpaginationdependingonthedatabase;forefficiencywithlargedatasets,keysetpaginationispreferredasitavoidsperformanceissuesfromlargeoffsets,whileLIMITOFFSETworkswellf

Sep 11, 2025 am 12:28 AM

sql Pagination

How to check the status of MySQL replication

RunSHOWSLAVESTATUS\Gonthereplicatocheckreplicationstatus.2.VerifySlave_IO_Running:Yes,Slave_SQL_Running:Yes,Seconds_Behind_Masterislowor0,andnoerrorsinLast_Errororrelatedfields.3.Useperformance_schemaqueriesinMySQL5.7 fordetailedthreadstatus.4.Option

Sep 11, 2025 am 12:01 AM

Building ETL Pipelines with MySQL and Other Data Sources

TobuildanETLpipelinethatpullsdatafrommultiplesources,transformsit,andloadsitintoMySQL,followthesesteps:1)Understandyourdatasources,includingMySQL(assourceortarget),APIs,CSVfiles,andotherdatabases.2)ChooseappropriatetoolslikePythonwithPandas/SQLAlchem

Sep 10, 2025 am 06:53 AM

How to use cursors in MySQL stored procedures

When using cursors, you must first declare the variable, then declare the cursor, and finally declare the processor. 1. Declare the variable and NOTFOUND processor; 2. Declare the cursor and associate the SELECT statement; 3. Open the cursor; 4. Get data in the loop and process it; 5. Close the cursor; it must be executed in this order and ensure that resources are cleaned. Cursors are only used in scenarios that must be processed line by line. Due to the low performance, set operations should be used to complete tasks first and end with a complete sentence structure.

Sep 10, 2025 am 06:52 AM

How to create a primary key in MySQL?

Defining primary keys can ensure data integrity. When creating tables, you can use PRIMARYKEY constraints to specify single column or multiple column key combinations, such as CREATETABLEusers(idINTPRIMARYKEY, nameVARCHAR(50)); for existing tables, use ALTERTABLEusersADDPRIMARYKEY(id), but the columns must be unique and non-empty; compound primary keys are suitable for multi-column combination unique scenarios, such as PRIMARYKEY(order_id, product_id); use ALTERTABLEusersDROPPRIMARYKEY to remove primary keys.

Sep 10, 2025 am 06:50 AM

mysql primary key

How to use regular expressions in MySQL with REGEXP

MySQL supports basic regular expression matching using REGEXP or RLIKE operators for performing pattern-based string searches in SQL queries. 1. You can use REGEXP to implement pattern filtering in the WHERE clause. For example, SELECTFROMusersWHEREemailREGEXP'gmail' can match the lines in which the email field contains "gmail", which is not case sensitive by default; 2. Common regular elements include. (any single character), ^ (starting of string), $ (end of string), (zero or more precedent characters), (one or more),? (zero or one), [abc] (character set), [a-z] (character specimen

Sep 10, 2025 am 06:34 AM

mysql regular expression