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Technical Articles

Database

Mysql Tutorial

How to implement a hierarchical data structure (tree) in MySQL

AdjacencyListModelusesparent-childreferencesandissimpletoimplementbutrequiresMySQL8 CTEsforefficienttraversal.2.PathEnumerationstoresfullpathsasstrings,enablingfastsubtreeandancestorquerieswithoutrecursion,thoughupdatesarecostly.3.NestedSetModelusesl

Sep 09, 2025 am 12:02 AM

What is the purpose of the HAVING clause in MySQL?

HAVING is used to filter grouped data after GROUPBY, especially when the conditions involve aggregation functions such as COUNT and SUM; for example, if you look for departments with more than 5 employees, you need to use HAVINGCOUNT(*)>5; unlike WHERE, WHERE filters a single row before grouping and cannot use an aggregate function, HAVING filters after grouping and supports an aggregate function; the two can be used in combination, such as first using WHERE to screen employees with a salary of more than 30,000, then group them by department, and finally use HAVING to screen departments with an average salary of more than 50,000, so as to achieve effective filtering of the aggregated data and fully support complex query needs.

Sep 08, 2025 am 04:26 AM

mysql HAVING子句

How to create an index in MySQL

Creating indexes can improve query performance. It should be created on columns frequently used in WHERE, JOIN, ORDERBY or GROUPBY; 2. Use CREATEINDEXindex_nameONtable_name(column_name) to create a single column index; 3. Use composite index for multi-column joint queries, with the syntax CREATEINDEXindex_nameONtable_name(column1,column2), note that the column order affects index usage; 4. The index can be directly defined when CREATETABLE; 5. Use CREATEUNIQUEINDEX to ensure that the column values ??are unique and prevent duplication; 6. Pass A

Sep 08, 2025 am 04:19 AM

How to add a column to a table in MySQL?

To add columns to an existing table, you need to use the ALTERTABLE statement to the ADDCOLUMN clause; for example, ALTERTABLEusersADDCOLUMNemailVARCHAR(100) can add email columns to the users table; you can add constraints such as NOTNULL and DEFAULT at the same time, such as automatically recording time when creating a time column; you can specify the location of the column through FIRST or AFTER; support adding multiple columns at a time to improve efficiency; be careful when operating large tables to avoid locking tables affecting performance, and backup data should be backed up before modification. If you add non-empty columns, the default value must be set to prevent the operation from failing.

Sep 08, 2025 am 03:23 AM

mysql database

What is a NATURAL JOIN in MySQL?

ANATURALJOINinMySQLautomaticallyjoinstablesbasedoncolumnswiththesamenameandcompatibledatatypes,returningonlyonecopyofeachcommoncolumn;itrequiresnoexplicitONorUSINGclause,makingitconcisebutriskyduetoimplicitbehaviorthatcanleadtounexpectedresultsifsche

Sep 08, 2025 am 03:04 AM

mysql join

How to use user-defined variables in MySQL

User-defined variables start with @, and are assigned through SET or :=, which are only valid in the current session. They can be used to store values, simulate line numbers, etc. 1. Set variables using SET or SELECT; 2. Reference variables in subsequent statements; 3. Pay attention to the session scope and initialization, avoid undefined use, and finally end with a complete statement.

Sep 08, 2025 am 02:19 AM

How to connect to MySQL using Node.js

Install the mysql2 package: Run npminstallmysql2 to obtain a MySQL client with better performance and support Promise; 2. Create a connection: Use the mysql.createConnection() method and pass in the correct host, user name, password and database name to establish a connection; 3. Execute query: execute SQL statements through the connection.query() method and process the results; 4. Close the connection: Call the connection.end() method to safely close the connection; 5. Use Promise: Introduce the 'mysql2/promise' module to support async/await syntax to improve the readability of asynchronous operations

Sep 08, 2025 am 02:10 AM

What is the information_schema database in MySQL?

information_schemaisaread-onlyvirtualdatabaseinMySQLthatprovidesmetadataaboutallotherdatabases;itcontainssystemviewslikeTABLES,COLUMNS,andSCHEMATAforqueryingdatabasestructure,enablingtaskssuchasfindingtableswithspecificcolumnsorlistingindexinformatio

Sep 08, 2025 am 12:57 AM

How to query a parent-child hierarchy in MySQL

MySQL supports querying parent-child hierarchy structures depending on the version: ① MySQL8.0 can use recursive CTE to traverse downwards (such as querying all subordinates of a manager) or trace upwards (such as the path from employees to CEO); ② Old versions need to use stored procedures to simulate recursion with temporary tables; ③ can load full data at the application layer to build a tree structure; ④ Optimization scheme includes using closure tables or path enumerations (such as storage /1/3/7/paths) to improve query performance, where path enumerations quickly obtain subtrees through LIKE matching, which is suitable for large-scale hierarchical data.

Sep 08, 2025 am 12:25 AM

How to use indexes to improve performance in MySQL

Using indexes can significantly improve MySQL query performance and avoid full table scanning; 2. Indexes are suitable for accelerating WHERE, JOIN, ORDERBY and GROUPBY operations, but will increase write operation overhead and storage consumption; 3. Single columns, composite, prefix, unique or overlay indexes should be created according to query requirements, pay attention to the column order of composite indexes; 4. Avoid using functions and data types on index columns to mismatch or violate the leftmost prefix principle to ensure effective use of indexes; 5. Regularly use EXPLAIN to analyze query execution plans, remove unused or duplicated indexes, and update table statistics to optimize index effects.

Sep 07, 2025 am 06:04 AM

mysql database index

What is binary logging in MySQL?

BinarylogginginMySQLisessentialforreplication,point-in-timerecovery,andauditingasitrecordsalldataandstructurechangeslikeINSERT,UPDATE,DELETE,andDDLstatements,whileexcludingSELECTandnon-modifyingtransactions;itstoreseventsinbinaryformatcontainingSQLst

Sep 07, 2025 am 05:58 AM

mysql

How to perform an UPDATE from a SELECT statement in MySQL?

SELECT cannot be used directly in UPDATE in MySQL, but it can be implemented by combining UPDATE with JOIN, subquery or derived tables. The most effective method is to use UPDATE...JOIN, for example, UPDATEemployeesJOINsalariesONemployees.id=salaries.employee_idSETemployees.salary=salaries.amount; if you need to avoid the limitation of querying the same table when modifying the table, you can wrap the subqueries in the derived table; it is usually recommended to use the JOIN method, because it is concise, efficient and easy to understand, and you should always test SELEC first in the end.

Sep 07, 2025 am 05:44 AM

How to monitor a MySQL server

Enable slow query logs and analysis to monitor query performance; 2. Monitor key indicators such as connection number, buffer pool hit rate, lock waiting and replication delay; 3. Use tools such as PerformanceSchema, PMM, Prometheus to achieve visual monitoring; 4. Set alarm rules to promptly discover CPU, disk, connection number and replication delay abnormalities; 5. Regularly conduct log rotation, query mode review and index usage analysis to optimize long-term performance; effective MySQL monitoring should combine real-time observation, historical trend analysis and automated alarms to prevent problems in advance.

Sep 07, 2025 am 05:04 AM

What are user-defined functions in MySQL?

User-definedfunctions(UDFs)inMySQLarecustomfunctionscreatedbyuserstoextenddatabasefunctionality,withthemostcommontypebeingstoredfunctionswritteninSQL.Thesefunctionsreturnasinglescalarvalue,canbedeterministicornon-deterministic,andsupportinputparamete

Sep 07, 2025 am 04:25 AM

mysql 用戶自定義函數(shù)