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Home

Technical Articles

Database

Mysql Tutorial

What are the most common string functions in MySQL?

The most commonly used string functions in MySQL include: 1. CONCAT() is used to concatenate strings, such as merging names or URLs; 2. SUBSTRING() extracts substrings by position and length, suitable for obtaining file extensions, etc.; 3. UPPER() and LOWER() are case-based to standardize comparison or output; 4. TRIM(), LTRIM() and RTRIM() remove spaces to clean up user input data. These functions can efficiently handle the formatting, splicing and cleaning tasks of text data.

Jun 19, 2025 am 12:43 AM

mysql String functions

Where is the MySQL configuration file my.cnf (or my.ini) located?

MySQL configuration files are usually located in standard paths, such as Linux in /etc/my.cnf or /etc/mysql/my.cnf, macOS (Homebrew) in /usr/local/etc/my.cnf, Windows in the installation directory or Windows directory. You can confirm the specific path by command mysql--help|grep"Defaultoptions" or in MySQL shell; if it is not found, you can manually create and set basic content, pay attention to permission issues and rings.

Jun 19, 2025 am 12:39 AM

mysql Configuration file

What is a Primary Key and what is its purpose?

Aprimarykeyensuresuniqueidentificationofrecords,supportstablerelationships,andimprovesqueryperformance.Ituniquelyidentifieseachrowinatableusingasinglecolumnorcompositekey,disallowingduplicatesandNULLvalues.1.Itenforcesdataintegritybypreventingduplica

Jun 19, 2025 am 12:37 AM

database primary key

Why is it recommended to use the utf8mb4 character set?

MySQL recommends using utf8mb4 character set because it can fully support four-byte characters such as emojis. Traditional utf8 only supports three-byte characters, which will cause errors or garbled codes when storing Emoji or special text. Therefore, if the application involves user input emojis or minority texts, utf8mb4 must be used to ensure the data is stored correctly. For example, the VARCHAR (255) field can store 255 emoji characters normally under utf8mb4. The content will not be lost. It is recommended to set the database table and field character set to utf8mb4. At the same time, the connection layer should also set charset=utf8mb4 to fully support more Unicode characters, including ancient characters, mathematical symbols, musical symbols and rare Chinese characters, but you need to pay attention to utf8mb4.

Jun 19, 2025 am 12:35 AM

What are Generated Columns and what are their use cases?

The generated columns are used in the database to automatically calculate values ??based on other list expressions, simplifying queries and improving performance. They avoid repeated complex calculations, such as automatically generating total_price through unit_price and quantity; they can improve the efficiency of querying, such as pre-calculating order_year to accelerate annual filtering; ensure logical consistency between multiple applications, such as unified calculation of after-tax prices; they are divided into two types: virtual (calculating when reading) and storage (calculating when writing), and should be selected based on the use case.

Jun 18, 2025 am 12:31 AM

How do COMMIT and ROLLBACK work?

COMMITpermanentlysaveschangesmadeduringatransaction,whileROLLBACKundoesthem.AtransactionisasequenceofSQLoperationstreatedasasingleunittoensuredataintegrity,followingACIDproperties.Forexample,inamoneytransfer,ifoneaccountisdebitedbuttheotherisn'tcredi

Jun 18, 2025 am 12:28 AM

commit ROLLBACK

What does LIMIT 10, 5 mean in a MySQL query?

LIMIT10,5meansskipthefirst10rowsandreturnthenext5rows.Thissyntaxisusedforpaginationwherethefirstnumberistheoffset(rowstoskip)andthesecondisthecount(rowstoreturn).ItisusefulfordisplayingdatainpagessuchasPage1:LIMIT0,5,Page2:LIMIT5,5,andsoon.Commonusec

Jun 18, 2025 am 12:28 AM

mysql limit

Is it always better to set the max_connections parameter higher?

Improving max_connections is not always better. Blindly raising it up will lead to resource contention and performance degradation. max_connections is a parameter that limits the number of simultaneous connections in the database. Each connection occupies memory and CPU. If it is set too high, it may exhaust resources. If it is too low, it may limit concurrency. Reasons for not being able to be raised blindly include: 1. Each connection consumes resources; 2. Too many connections cause competition and waiting; 3. Restricted by system file descriptors and thread count; 4. It is difficult to run stably without a connection pool. Reasonable setup methods include: 1. Evaluate connection requirements based on load; 2. Use connection pools to reduce direct connections; 3. Monitor system resource bottlenecks; 4. Distinguish between active and idle connections. Suitable cases for raising the height are: 1. The connection pool is not used and concurrent

Jun 18, 2025 am 12:26 AM

database

How does semi-synchronous replication work in MySQL?

MySQL's semi-synchronousreplication balances performance with data security by ensuring at least one replica receives transactions. 1. When the transaction is submitted, the master server waits for at least one replica to confirm receipt and writes the relay log; 2. Once confirmed, the master server submits the transaction and returns it to the client successfully; 3. If the timeout does not receive a response, it will automatically fall back to asynchronous mode to maintain the system operation; 4. Enable this function requires installing the plug-in on the master and slave server and setting the corresponding parameters; 5. Its advantage is that it provides stronger data integrity than asynchronous replication, but has slight performance loss and network latency impact. This replication method is suitable for scenarios where high data consistency is required but cannot accept full synchronization performance overhead.

Jun 18, 2025 am 12:24 AM

mysql semisynchronous replication

What is Index Condition Pushdown (ICP)?

IndexConditionPushdown(ICP)isaMySQLoptimizationthatimprovesqueryperformancebypushingWHEREclauseconditionsintothestorageengine.ICPworksbyallowingthestorageenginetoevaluatepartsoftheWHEREconditionduringindexscanning,reducingunnecessaryrowlookupsanddisk

Jun 18, 2025 am 12:23 AM

What are Window Functions and how to use the OVER() clause?

Window functions are tools in SQL that are used to calculate data while preserving the original row. Common usages include defining window scopes with the OVER() clause. For example, use AVG (salary)OVER (PARTITIONBYdepartment) to calculate the average salary of the department, or use ROW_NUMBER(), RANK(), etc. to rank. 1. The window function groups data through PARTITIONBY, such as calculating the average value by department grouping; 2. Use ORDERBY to sort in the window and combine FRAMEclause to define window frames, such as adding the cumulative sum from the first row to the current row; 3. Common scenarios include grouping statistics retention details, ranking functions and moving average calculations,

Jun 18, 2025 am 12:22 AM

What are the differences between ANY, ALL, IN, and EXISTS?

The difference between ANY, ALL, IN and EXISTS in SQL queries is their purpose and behavior. 1.IN is used to check whether the value matches any value in the list, which is suitable for scenarios where specific values ??are known; 2. EXISTS is used to determine whether there are return rows in the subquery, which is often used for associative subquery; 3. ANY compares the value with any value in the set and meets the conditions; 4. ALL requires that the value be compared with all values ??in the set and all meet the conditions. Correct choices can improve query efficiency and clarity.

Jun 18, 2025 am 12:13 AM

sql Inquire

What is the difference between utf8 and utf8mb4 character sets in MySQL?

MySQL's utf8 does not fully support UTF-8 encoding, while utf8mb4 supports it in full. Specifically, utf8 only supports up to 3 bytes of characters, and cannot correctly process 4-byte characters such as emojis, some rare Chinese characters and mathematical symbols, which may lead to data loss or errors; utf8mb4 supports all Unicode characters, accurately covering all symbols required for modern communications, and maintaining backward compatibility. Switching to utf8mb4 requires updating the character set of database, tables and columns, setting the connection character set, and repairing the converted data. In addition, you need to pay attention to whether the connection encoding, backup files and sorting rules match utf8mb4 to avoid potential problems.

Jun 18, 2025 am 12:11 AM

mysql character set

What is SQL Injection and how to prevent it simply?

The key to preventing SQL injection is to standardize input and use the database operation correctly. The main methods include: 1. Use parameterized queries to separate SQL statements from user input to prevent malicious code execution; 2. Filter and verify user input, limit and verify data types; 3. Follow the principle of minimum permissions, control database account permissions and hide detailed error information; 4. Use mature frameworks and libraries, relying on default security mechanisms such as ORM or parameterized queries. As long as it is developed according to the recommended method, it can effectively prevent the risk of SQL injection.

Jun 18, 2025 am 12:09 AM

sql injection Safety precautions