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Technical Articles

Database

How to create a temporary table in MySQL?

To create a temporary MySQL table, use the CREATETEMPORARYTABLE statement, 1. The table only exists in the current session and is automatically deleted at the end of the session; 2. It can have the same name as the permanent table and is referenced first; 3. It supports most storage engines but the memory table does not support full-text indexing; 4. It can insert and query data like a normal table; 5. It can manually delete it with DROPTEMPORARYTABLE to avoid accidentally deleting permanent tables; it is suitable for complex queries, step-by-step data processing and other scenarios, providing session isolation and automatic cleaning functions.

Aug 28, 2025 am 12:29 AM

How to use table-level locks in MySQL

MySQL's table-level lock is implemented through LOCKTABLES and UNLOCKTABLES commands to control access to the entire table; 1. Use READ locks to prevent other sessions from modifying tables, which are suitable for backup or consistent reads; 2. Use WRITE locks to obtain exclusive access, which is suitable for maintenance operations; 3. If an alias is used in the query, the same alias must be specified during locking; it should be noted that in InnoDB, LOCKTABLES will implicitly submit the current transaction, and long-term holding of the lock should be avoided to reduce the impact on concurrency, and priority should be given to transaction and row-level locks to ensure concurrency performance. Table-level locks are only recommended for special scenarios such as batch import and maintenance tasks. UNLOCKTABLES must be called to release the lock after use.

Aug 27, 2025 am 08:11 AM

How to drop a trigger in MySQL

To delete triggers in MySQL, use the DROPTRIGGER statement; 1. The basic syntax is DROPTRIGGER[IFEXISTS][schema_name.]trigger_name; 2. IFEXISTS can prevent errors when the trigger does not exist; 3.schema_name can be omitted in the current database; 4. SUPER or ALTER permissions must be provided; 5. You can confirm that the trigger exists through SHOWTRIGGERS or information_schema.TRIGGERS; the deletion operation will not affect the association table, and there is no CASCADE option. The trigger will be removed after execution.

Aug 27, 2025 am 08:05 AM

How to work with JSON data in MySQL

When using MySQL to process JSON data, you should first create a JSON type column and insert valid JSON data. 1. Use the -> and -> operators to extract the JSON value, and recommend ->> to obtain a quoteless string; 2. Use ->> or JSON_EXTRACT to filter the data in the WHERE clause. Use JSON_CONTAINS_PATH to check whether the key exists, and use JSON_CONTAINS to query the array value; 3. Use JSON_SET, JSON_REPLACE or JSON_INSERT to update the JSON field, and JSON_ARRAY_APPEND can add elements to the array; 4

Aug 27, 2025 am 07:55 AM

mysql json

What is a CHECK constraint in MySQL and is it enforced?

Yes,CHECKconstraintsareenforcedinMySQLstartingfromversion8.0.16.Priortothisversion,CHECKconstraintswereparsedbutnotenforced,meaningtheyhadnoeffectondataintegrity.FromMySQL8.0.16onward,CHECKconstraintsareactivelyenforcedattheSQLlayer,ensuringthatonlyd

Aug 27, 2025 am 07:40 AM

What is the purpose of the USE statement in MySQL?

TheUSEstatementinMySQLselectsadefaultdatabaseforthecurrentsession,allowingsubsequentoperationstobeperformedwithinthatdatabasecontextwithoutneedingtofullyqualifytablenames;forexample,runningUSEsalessetsthesalesdatabaseasdefault,soquerieslikeSELECTFROM

Aug 27, 2025 am 07:02 AM

mysql USE語句

How to create a copy of a table in SQL?

Tocopybothstructureanddata,useCREATETABLEnew_tableASSELECTFROMoriginal_tableinPostgreSQL,Oracle,andMySQL,orSELECTINTOnew_tableFROMoriginal_tableinSQLServer,notingthatconstraintsandindexesarenotcopied.2.Tocopyonlythestructurewithoutdata,useCREATETABLE

Aug 27, 2025 am 06:36 AM

sql 復(fù)制表

What is the purpose of SQL_CALC_FOUND_ROWS in MySQL?

SQL_CALC_FOUND_ROWSwasusedtogetthetotalrowcountofaquerywithoutLIMITafterrunningaLIMITquery,enablingaccuratepaginationtotals;however,itwasdeprecatedinMySQL8.0andremovedin8.0.23duetoperformanceissues,asitforcedMySQLtoprocessallrowsdespitelimitingresult

Aug 27, 2025 am 06:34 AM

mysql

How to add a datafile to a tablespace in Oracle

To add data files to the tablespace, you need to use the ALTERTABLESPACE command to cooperate with the ADDDATAFILE clause; 1. First, confirm the table space and existing data file information by querying dba_tablespaces and dba_data_files; 2. Execute the ALTERTABLESPACE command to specify the path, size, automatic expansion and other parameters to add new data files; 3. Optionally, create a fixed-size data file that does not enable automatic expansion; 4. For large file table spaces, only unique data files can be expanded instead of adding new files; 5. Finally, query dba_data_files to verify that the new data file has been successfully added and taken effect. The entire process needs to ensure directory permissions and disk empty

Aug 27, 2025 am 06:28 AM

phpMyAdmin 'incorrect format parameter' solution

Make sure that the imported file format matches the actual content, and the SQL file should select SQL format; 2. Avoid manually forcing non-SQL formats such as CSV or Mediawiki; 3. Check and clear invalid format parameters in the URL to prevent errors; 4. Use a text editor to verify that the SQL file structure is correct and encoded as UTF-8 without BOM; 5. Select the corresponding compression method only when the file is in compressed formats such as .gz. Correctly matching the file content and import settings can resolve the "incorrectformatparameter" error.

Aug 27, 2025 am 06:23 AM

php

SQL Disaster Recovery in the Cloud

Understand SQL recovery options in the cloud environment, including automatic backup, point-in-time recovery (PITR), snapshot backup and multi-region deployment, and select appropriate methods based on RTO and RPO; 2. Design reasonable recovery strategies, regularly test processes, separate storage backups, configure monitoring and alarms, and standardize naming management; 3. Pay attention to recovery details, such as manufacturer differences, permission inheritance, encryption keys and cross-regional expenses; 4. Use cloud platform tools or third-party software to simplify the recovery process and improve efficiency. The above steps can ensure efficient and reliable SQL disaster recovery in the cloud.

Aug 27, 2025 am 06:13 AM

How to create and use user-defined functions in SQL?

User-defined functions (UDFs) can be used to encapsulate reusable logic to improve the readability and maintainability of SQL queries; 1. UDF can return a single scalar value or table, suitable for SELECT, WHERE, FROM clauses; 2. Example of scalar function: CREATEFUNCTIONdbo.CalculateDiscount returns a discount price; 3. Example of table value function: CREATEFUNCTIONdbo.GetProductsByCategory returns a product table of the specified classification; 4. Note when using: no side effects, avoid performance bottlenecks, and priority is given to the use of inline table value functions; 5. Different databases support differently: SQLServer supports complete, P

Aug 27, 2025 am 05:55 AM

How to get top N records per group in SQL?

To obtain the first N records of each group, you must use a window function to cooperate with CTE or subquery; 1. Use ROW_NUMBER()OVER (PARTITIONBY group column ORDERBY sort column DESC) to assign unique rankings to rows within each group; 2. Filter rows with ranking less than or equal to N in outer query; 3. If you need to deal with parallelism, you can replace it with RANK() or DENSE_RANK() to retain parallel records; 4. Ensure indexes are established on the grouping and sorting columns to improve performance; this method is suitable for scenarios such as obtaining the top N employees with the highest salaries in each department, and is a standard solution.

Aug 27, 2025 am 05:22 AM

How to use INSERT IGNORE in MySQL?

INSERTIGNOREinMySQLallowsinsertingrowswhileskippingerrorslikeduplicatekeysorNULLviolations,makingitidealforbulkinsertsordatasyncing;itconvertserrorsintowarningsinsteadofstoppingexecution,sousingSHOWWARNINGSafterwardrevealsskippedrowsduetoconstraints,

Aug 27, 2025 am 05:01 AM

mysql