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Technical Articles

Database

Troubleshooting MySQL Service Unavailable Errors

The troubleshooting of MySQL service is not available requires the following steps. 1. First check whether the MySQL service is running, use systemctlstatusmysql to view the status, if it is not running, try to start, and view the error log if it fails; 2. Confirm the monitoring status of port 3306, check the firewall settings and adjust the bind-address configuration to allow remote access; 3. Check user permissions, grant remote access permissions by modifying the host field, and pay attention to security; 4. Troubleshoot insufficient resources or configuration errors, check memory and disk usage, adjust buffer pool parameters and restart the service. Gradually check the root cause of the problem in accordance with the above directions.

Aug 06, 2025 pm 05:05 PM

How to find columns with a specific data type in SQL?

To find columns with specific data types, you should query the metadata table of the database; 1. Use INFORMATION_SCHEMA.COLUMNS to query the specified data type and database name, which is suitable for MySQL, PostgreSQL, SQLServer, etc.; 2. PostgreSQL can use pg_catalog system table to obtain more detailed information; 3. SQLServer can obtain column and type information through sys.columns and sys.tables query; 4. MySQL can add CHARACTER_MAXIMUM_LENGTH and other conditions in INFORMATION_SCHEMA.COLUMNS.

Aug 06, 2025 pm 05:02 PM

sql type of data

What are constraints in SQL and why are they used?

ConstraintsinSQLareusedtoenforcedataintegrity,ensureaccuracy,andmaintainconsistencybypreventinginvaliddataentry.1.NOTNULLensuresacolumncannothavenullvalues;2.UNIQUEguaranteesallcolumnvaluesaredistinct;3.PRIMARYKEYcombinesNOTNULLandUNIQUEtouniquelyide

Aug 06, 2025 pm 05:01 PM

Monitoring MySQL Database Health with Percona Toolkit

Monitoring MySQL health can be implemented using the PerconaToolkit tool set. 1. Check the data consistency of the master-slave replication status by pt-table-checksum, and then repair it through pt-table-sync after discovering an exception; 2. Use pt-query-digest to analyze slow query to find time-consuming SQL and optimize it by analyzing the log; 3. Real-time monitoring can use pt-heartbeat to detect master-slave delays, and pt-stalk automatically grabs diagnostic information when system abnormalities; 4. Other tools such as pt-online-schema-change support online modification of table structure, pt-index-usage analyzes index usage, p

Aug 06, 2025 pm 04:53 PM

How to Manage Auto-Incrementing Primary Keys in MySQL?

Auto-incrementingprimarykeysinMySQLaremanagedeffectivelybyfollowingkeypractices:1.UnderstandthatAUTO_INCREMENTensuresunique,monotonicallyincreasingIDs,withgapsbeingnormalduetorollbacks,deletions,orfailedinserts.2.SetthestartingvalueusingALTERTABLEuse

Aug 06, 2025 pm 04:50 PM

How to use check constraints to enforce data rules in MySQL?

MySQL supports CHECK constraints to force domain integrity, effective from version 8.0.16; 1. Add constraints when creating a table: Use CREATETABLE to define CHECK conditions, such as age ≥18, salary > 0, department limit values; 2. Modify the table to add constraints: Use ALTERTABLEADDCONSTRAINT to limit field values, such as name non-empty; 3. Use complex conditions: support multi-column logic and expressions, such as end date ≥start date and completion status must have an end date; 4. Delete constraints: use ALTERTABLEDROPCONSTRAINT to specify the name to delete; 5. Notes: MySQL8.0.16, InnoDB or MyISAM needs to be quoted

Aug 06, 2025 pm 04:49 PM

mysql

What is the role of the database administrator (DBA) in a SQL environment?

Theroleofadatabaseadministrator(DBA)inaSQLenvironmentistoensuredatabasesaresecure,available,performant,andwell-maintained.1.DBAsdesignandimplementefficientdatabaseschemasbycreatingtables,indexes,views,andstoredprocedures,enforcingdataintegritythrough

Aug 06, 2025 pm 04:43 PM

How to write dynamic SQL?

Dynamic SQL should be used when flexibility is required, such as running the query structure, executing DDL statements, or building reports with optional filtering conditions; in SQLServer, QUOTENAME() and sp_executesql should be used for parameterization to prevent injection; in Oracle, DBMS_ASSERT and EXECUTEIMMEDIATE should be used to combine binding variables; in application code, parameterized queries should be used and table names and column names should be verified through whitelists; always avoid direct splicing of user input, strictly verify input, use minimum permission accounts, and give priority to static SQL to ensure security.

Aug 06, 2025 pm 04:30 PM

What are the different types of subqueries in SQL?

There are many subquery types in SQL, each suitable for different scenarios: 1. Scalar subquery returns a single value of a single row and a column, which is often used in SELECT, WHERE or HAVING clauses, such as combined with aggregate functions; 2. Row subquery returns multiple columns for a row and multiple columns, which are used for compound value comparison, and supports varying from database, such as MySQL supports but SQLServer may not support it; 3. Column subquery (or multi-row subquery) returns a single column and multiple rows, which are often used in conjunction with IN, NOTIN, ANY, SOME, ALL and other operators, such as filtering department employees belonging to a specific region; 4. Table subquery (or derived tables, inline views) returns multiple rows and columns, which are used as temporary tables in the FROM clause, and usually use AS to enter

Aug 06, 2025 pm 04:28 PM

How to calculate a running total in MySQL?

ForMySQL8.0andlater,usetheSUM()windowfunctiontocalculatearunningtotalefficientlyandreliably.2.Forversionsbefore8.0,simulatetherunningtotalusinguservariableswithaCROSSJOINtoinitializethevariable.3.Avoidcorrelatedsubqueriesastheyareinefficientforlarged

Aug 06, 2025 pm 04:21 PM

mysql 運(yùn)行總計(jì)

What is the difference between authentication and authorization in a SQL database?

Authenticationverifiesauser'sidentityduringloginusingcredentialslikeusername/passwordorexternalproviders,occurringattheserverlevel;1.ItconfirmswhoyouarewhenconnectingtoaSQLdatabase.Authorizationdetermineswhatanauthenticatedusercando,2.byenforcingperm

Aug 06, 2025 pm 04:19 PM

sql database 身份驗(yàn)證授權(quán)

How to get the number of affected rows after a query in MySQL

UseROW_COUNT()inMySQLtogetthenumberofaffectedrowsafterINSERT,UPDATE,orDELETEstatementswhenworkingdirectlyinSQL;2.Inprogramminglanguages,usebuilt-inmethodssuchas$pdo->rowCount()inPHP,cursor.rowcountinPython,orresults.affectedRowsinNode.jstoretrieve

Aug 06, 2025 pm 04:08 PM

How do you filter results using the WHERE clause in SQL?

TheWHEREclausefiltersrowsinaSELECTstatementbasedonspecifiedconditions.1.ItfollowstheFROMclauseandprecedesORDERBY,GROUPBY,orLIMIT.2.Commonoperatorsincludecomparisonoperators(=,,>,=,

Aug 06, 2025 pm 04:03 PM

How to Use Views to Simplify Complex Queries in MySQL?

Using views can simplify complex queries, improve readability and reduce redundancy; 1. Avoid repeated writing of complex SQL by encapsulating multi-table joins and subqueries in the view; 2. Pre-aggregate data with views to simplify report generation; 3. Restrict access to sensitive fields through views to achieve security control; 4. Improve maintenance, only modify the view definition when structural changes are made; it is recommended to name specifications, avoid too deep nesting, pay attention to basic table index optimization, and not suitable for scenarios that require parameterization or extremely sensitive performance. The final view should focus on standardizing common query logic to enhance maintainability.

Aug 06, 2025 pm 03:53 PM

Complex query mysql view