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  • What are the information_schema and performance_schema databases used for?
    What are the information_schema and performance_schema databases used for?
    information_schema and performance_schema are MySQL system databases used to store metadata and performance metrics respectively. information_schema provides database structure information, such as tables, columns, permissions, etc., which cannot be modified and only contains structural metadata; performance_schema records performance data during the server runtime, such as query waiting, resource consumption, etc., and specific instruments are required to enable specific instruments to obtain detailed information. Use the former to dynamically query the database object structure, while the latter can be used to troubleshoot performance bottlenecks. The two have different uses but complementary, and mastering their usage is crucial to managing and optimizing MySQL.
    Mysql Tutorial . Database 584 2025-06-20 13:09:10
  • What is the principle behind a database connection pool?
    What is the principle behind a database connection pool?
    Aconnectionpoolisacacheofdatabaseconnectionsthatarekeptopenandreusedtoimproveefficiency.Insteadofopeningandclosingconnectionsforeachrequest,theapplicationborrowsaconnectionfromthepool,usesit,andthenreturnsit,reducingoverheadandimprovingperformance.Co
    Mysql Tutorial . Database 839 2025-06-20 01:07:31
  • What are the ACID properties of a MySQL transaction?
    What are the ACID properties of a MySQL transaction?
    MySQL transactions follow ACID characteristics to ensure the reliability and consistency of database transactions. First, atomicity ensures that transactions are executed as an indivisible whole, either all succeed or all fail to roll back. For example, withdrawals and deposits must be completed or not occur at the same time in the transfer operation; second, consistency ensures that transactions transition the database from one valid state to another, and maintains the correct data logic through mechanisms such as constraints and triggers; third, isolation controls the visibility of multiple transactions when concurrent execution, prevents dirty reading, non-repeatable reading and fantasy reading. MySQL supports ReadUncommitted and ReadCommi.
    Mysql Tutorial . Database 288 2025-06-20 01:06:01
  • What is a B-Tree index?
    What is a B-Tree index?
    B-Treeindexesmatterbecausetheyenablefastandefficientdataretrievalindatabasesbymaintainingsorteddataandallowinglogarithmictimecomplexityforsearch,insertion,anddeletionoperations.Theyautomaticallybalancethemselvestopreventperformancedegradationasdatais
    Mysql Tutorial . Database 452 2025-06-20 01:02:50
  • What are Common Table Expressions (CTEs) and how to use the WITH clause?
    What are Common Table Expressions (CTEs) and how to use the WITH clause?
    CTE (CommonTableExpression) is a way in SQL for defining temporary result sets, which are defined by the WITH keyword and exist only during the current query execution. Its core role is to simplify complex query structures and improve readability and maintenance. The main uses of CTE include: 1. Simplify nested queries to make multi-layer logic clear and separate; 2. Support recursive queries, suitable for processing hierarchical or tree-like data structures; 3. Replace views, providing temporary logical abstraction without changing the database structure. When using it, you should pay attention to: the scope of action of CTE is limited to the queries that follow. Multiple CTEs can be defined and naming conflicts can be avoided. The performance is comparable to subqueries and does not guarantee improvement in execution efficiency. Choose CTE or temporary table
    Mysql Tutorial . Database 818 2025-06-20 01:02:11
  • How to check the MySQL server version?
    How to check the MySQL server version?
    To view the MySQL server version, it can be implemented in various ways, as follows: 1. Execute mysql-V using the command line; 2. Log in to the MySQL client and run SELECTVERSION(); or enter status; (abbreviated as \s); 3. Execute SHOWVARIABLESLIKE'version'; obtain more accurate version information; 4. Execute SQL query version number through database connection in the program, as shown in the Python sample code.
    Mysql Tutorial . Database 948 2025-06-20 00:59:31
  • How to use the CASE WHEN statement in a query?
    How to use the CASE WHEN statement in a query?
    TheSQLCASEWHENstatementisusedtohandleconditionallogicinqueriesbyreturningdifferentresultsbasedonspecifiedconditions.Itfunctionslikeanif-elsestatementandcanbeappliedinSELECT,WHERE,ORDERBY,andHAVINGclauses.Forexample,itcanclassifysalesas“Low”,“Medium”,
    Mysql Tutorial . Database 889 2025-06-20 00:59:11
  • What are the roles of the redo log and undo log in InnoDB?
    What are the roles of the redo log and undo log in InnoDB?
    InnoDB's redolog and undolog guarantee the persistence, atomicity and MVCC of transactions respectively. Redolog is a physical log that is written before data modification, records data page changes, supports crash recovery, and uses loop writing to improve performance; Undolog is a logical log that records reverse operations, is used for transaction rollback and implementation of MVCC, organizes multi-version data snapshots through linked lists, and is cleaned by purge threads. The two work together to ensure the complete implementation of transaction ACID characteristics.
    Mysql Tutorial . Database 286 2025-06-20 00:58:31
  • What types of locks does MySQL use, like table locks, row locks, and gap locks?
    What types of locks does MySQL use, like table locks, row locks, and gap locks?
    MySQL manages concurrent access using table locks, row locks, and gap locks. Table locks lock the entire table, suitable for scenarios with more reads and fewer writes; row locks allow multiple transactions to operate different rows, improving concurrency; gap locks prevent phantom reading and lock index gaps. These three locks work according to different storage engines and isolation levels.
    Mysql Tutorial . Database 787 2025-06-20 00:55:50
  • How to check the current number of connections and server status?
    How to check the current number of connections and server status?
    To view the current number of connections and server status, you can use the following methods: 1. View the number of server connections: Use ss or netstat commands, such as ss-tuln or netstat-tuln to list the listening ports, and combine ss-tn|wc-l to count the number of TCP connections; 2. Monitor the overall status of the server: use uptime to view the load and runtime, and use top and htop to view the CPU and memory usage in real time; 3. Use monitoring tools to achieve long-term observation: Deploy Grafana Prometheus, Netdata or Zabbix to graphically display data and set alarms; 4. Notes: Handle TIME_WAIT status connection, optimize kernel parameters and query different commands
    Mysql Tutorial . Database 145 2025-06-20 00:55:31
  • What is the functional difference between the WHERE and HAVING clauses?
    What is the functional difference between the WHERE and HAVING clauses?
    In SQL, the main difference between WHERE and HAVING is the execution timing and the type of data filtered. 1.WHERE filters a single row before grouping, and cannot use an aggregation function; 2.HAVING filters the aggregation results after grouping, allowing the use of an aggregation function. For example, when querying departments with more than 10 high-paying employees, WHERE first filters low-paying employees, then uses GROUPBY to group them, and finally uses HAVING to filter groups that meet the criteria. In terms of performance, WHERE should be used to reduce the amount of data, and HAVING should only be used when filtering the aggregate results.
    Mysql Tutorial . Database 854 2025-06-20 00:55:11
  • Which is more efficient: COUNT(*), COUNT(1), or COUNT(column_name)?
    Which is more efficient: COUNT(*), COUNT(1), or COUNT(column_name)?
    InmodernSQLdatabases,COUNT(),COUNT(1),andCOUNT(column_name)havelittletonoperformancedifferenceinbasicqueries.1.COUNT()countsallrows,includingNULLs,andisbestfortotalrowcount.2.COUNT(1)behavesthesameasCOUNT(),withnoperformanceadvantage,andisusedmainlyb
    Mysql Tutorial . Database 273 2025-06-20 00:53:51
  • What are optimistic and pessimistic locks, and how to implement them in MySQL?
    What are optimistic and pessimistic locks, and how to implement them in MySQL?
    Pessimistic locks and optimistic locks are two strategies for handling concurrent database access. Pessimistic locking assumes conflicts and locks are immediately added when the data is modified, such as in MySQL using SELECT...FORUPDATE or SELECT...LOCKINSHAREMODE, which is suitable for high-competitive scenarios but may degrade performance. Optimistic locks assume fewer conflicts and do not lock immediately, but check version numbers or timestamps when updated. They are suitable for low-competitive scenarios and avoid lock overhead, but the application layer needs to handle conflicts. If you choose a pessimistic lock, if you write frequently and have high data consistency requirements; if you choose an optimistic lock, if you have fewer conflicts, you hope to improve concurrency and can handle it elegantly. In addition, pessimistic locks may lead to deadlocks, and optimistic locks require additional logic to handle conflicts.
    Mysql Tutorial . Database 894 2025-06-20 00:51:20
  • What is read-write splitting and how is it implemented?
    What is read-write splitting and how is it implemented?
    Read-writesplittingimprovesdatabaseperformancebyseparatingreadandwriteoperationsacrossdifferentservers.Itworksbydirectingwritestotheprimarydatabaseandreadstoreplicas,reducingload,improvingresponsetime,andenhancingfaulttolerance.Commonimplementationme
    Mysql Tutorial . Database 261 2025-06-20 00:37:31

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