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Technical Articles

Database

Mysql Tutorial

What is the difference between DATABASE and SCHEMA in MySQL?

InMySQL,thetermsdatabaseandschemaarenearlyinterchangeable,butcarrysubtlecontextualdifferences.2.Adatabaseisatop-levelcontainerfordataobjectsliketables,views,andprocedures,createdwithCREATEDATABASE.3.AschemainMySQLreferstoeitherthedatabaseitself—often

Jun 22, 2025 pm 04:45 PM

database schema

How to check and change a table's storage engine?

To view or modify the storage engine of MySQL tables, you can use the following methods: 1. Use SHOWCREATETABLEyour_table_name; view the storage engine of a single table; 2. Use SELECTTABLE_NAME,ENGINEFROMinformation_schema.TABLESWHERETABLE_SCHEMA='your_database_name'; batch view the storage engine of all tables in the database; 3. Use ALTERTABLEyour_table_nameENGINE=new_engine_name; modify the storage engine of tables, if changed to My

Jun 21, 2025 pm 01:41 PM

Why does ORDER BY sometimes make a query slow?

The main reasons for slowing SQL queries by adding ORDERBY include lack of indexes, excessive result sets, mixed use of JOIN and sorting, and temporary table processing problems. 1. The lack of index will cause the database to perform full sorting. Indexes should be created for the sorting sequence. Composite indexes should be used when WHERE is involved; 2. Large result sets increase the memory or disk I/O burden, and can limit the number of rows to be returned through LIMIT, avoid SELECT*, and use key set paging optimization; 3. The mixing of JOIN and ORDERBY may cause indexes to be invalid. It is necessary to ensure that the connection and sorting sequence have indexes, and try to adjust the JOIN order or get the primary key first and then associate; 4. The use of ORDERBY in subqueries may cause temporary tables to affect performance, and the sorting can be moved into subqueries and materialized derivatives.

Jun 20, 2025 pm 08:46 PM

order by Query optimization

How can I tell if my query is using an index?

You can check whether the query uses an index by looking at the execution plan. In most SQL systems, the query execution method can be analyzed using the EXPLAIN or EXPLAINANALYZE command; 1. If the output shows IndexScan or Usingindexcondition, it means that the index is used; 2. If SeqScan or type:ALL appears, the index is not used; 3. In MySQL, the Extra column shows Usingwhere; Usingindex means that the overlay index is used; 4. The key column is NULL, the index is not used; 5. The lower the rows value, the better, which means the number of rows that the optimizer expects to scan; 6. The composite index needs to be paid attention to

Jun 20, 2025 pm 01:33 PM

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.

Jun 20, 2025 pm 01:09 PM

What is the principle behind a database connection pool?

Aconnectionpoolisacacheofdatabaseconnectionsthatarekeptopenandreusedtoimproveefficiency.Insteadofopeningandclosingconnectionsforeachrequest,theapplicationborrowsaconnectionfromthepool,usesit,andthenreturnsit,reducingoverheadandimprovingperformance.Co

Jun 20, 2025 am 01:07 AM

principle Database connection pool

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.

Jun 20, 2025 am 01:06 AM

What is a B-Tree index?

B-Treeindexesmatterbecausetheyenablefastandefficientdataretrievalindatabasesbymaintainingsorteddataandallowinglogarithmictimecomplexityforsearch,insertion,anddeletionoperations.Theyautomaticallybalancethemselvestopreventperformancedegradationasdatais

Jun 20, 2025 am 01:02 AM

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

Jun 20, 2025 am 01:02 AM

CTE

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.

Jun 20, 2025 am 12:59 AM

How to use the CASE WHEN statement in a query?

TheSQLCASEWHENstatementisusedtohandleconditionallogicinqueriesbyreturningdifferentresultsbasedonspecifiedconditions.Itfunctionslikeanif-elsestatementandcanbeappliedinSELECT,WHERE,ORDERBY,andHAVINGclauses.Forexample,itcanclassifysalesas“Low”,“Medium”,

Jun 20, 2025 am 12:59 AM

sql query

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.

Jun 20, 2025 am 12:58 AM

innodb log

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.

Jun 20, 2025 am 12:55 AM

mysql lock lock type

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

Jun 20, 2025 am 12:55 AM