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Home

Technical Articles

Database

Mysql Tutorial

How to query for points within a certain distance in MySQL

To query points within a specific distance in MySQL, use the ST_Distance_Sphere() function to calculate the distance of geographic coordinates (latitude and longitude). 1. Create a table containing POINT type columns and specify SRID4326; 2. Use the ST_Distance_Sphere() function to filter points in the specified radius, with distance units in meters; 3. To improve performance, add spatial indexes and use bounding box filtering in combination with MBRWithin() to trigger the index. For example: SELECTid, name, ST_Distance_Sphere(coord,POINT(40.7128,-74.0060))ASdistanceFROMl

Aug 14, 2025 pm 12:22 PM

How to use subqueries in MySQL

A subquery is a SELECT statement nested in another SQL statement and can be used in WHERE, FROM, and SELECT clauses. 1. Using subqueries in WHERE can filter data based on another query result, such as finding employees with salary above average; they can also use IN, EXISTS and other operators to combine subquery filtering. 2. Use subqueries in the FROM clause to create temporary derived tables and must be given an alias for them. 3. The subquery in the SELECT clause must be a scalar subquery, which only returns a single row and single column value, but may affect performance. 4. Related subqueries rely on external queries and are executed once per line, which is less efficient and should be replaced by JOIN as much as possible. It is recommended to always wrap subqueries in brackets to avoid NOTIN and

Aug 14, 2025 am 11:54 AM

How to use the AVG function in MySQL

TheAVG()functioninMySQLisanaggregatefunctionusedtocalculatetheaveragevalueofanumericcolumn.It’scommonlyusedwiththeSELECTstatementandisespeciallyhelpfulwhenanalyzingdatasuchasaverageprices,scores,salaries,oranymeasu

Aug 14, 2025 am 11:41 AM

How to drop a column from a table in MySQL?

To delete columns from MySQL table, you need to use the ALTERTABLE statement and the DROPCOLUMN clause. The specific syntax is ALTERTABLEtable_nameDROPCOLUMNcolumn_name; the COLUMN keyword can also be omitted; before deletion, you need to confirm that the column exists, otherwise an error will be reported, and IFEXISTS can be used to avoid errors; data deletion cannot be restored; if the column is referenced by index, foreign key, trigger or view, the dependency must be processed first; multiple columns can be deleted at once, and each column needs to be separated by commas; data should be backed up before operation and tested in the development environment to prevent data loss due to incorrect operations.

Aug 14, 2025 am 10:56 AM

mysql Delete column

How to set a primary key in MySQL

Tosetaprimarykeywhencreatingatable,usethePRIMARYKEYconstraintintheCREATETABLEstatement,suchasdefiningasinglecolumnlikeidINTAUTO_INCREMENTPRIMARYKEYoracompositekeywithPRIMARYKEY(column1,column2).2.Toaddaprimarykeytoanexistingtable,useALTERTABLEtable_n

Aug 13, 2025 pm 12:15 PM

What is the maximum size of a MySQL database?

ThemaximumsizeofaMySQLdatabaseisnotfixedanddependsonmultiplefactors,withInnoDBsupportingupto64terabytespertableanddatabasesexceeding100 TBpossibleinpractice,whileMyISAMtablescanreachupto256terabytesbutarelimitedbyfilesystemconstraintsandlackmodernfea

Aug 13, 2025 pm 12:11 PM

How to combine results from multiple tables with UNION

When using UNION to merge the results of multiple tables, it is necessary to ensure that the number of columns of each SELECT statement is the same and the data types of the corresponding columns are compatible. The column names of the result sets come from the first SELECT; using UNION will automatically remove duplicate rows, while UNIONALL retains all rows (including duplicates) and has higher performance; if sorting is required, ORDERBY should be used at the end of the entire UNION query; for complex queries, you can wrap subqueries with brackets; pay attention to implicit type conversion issues, and explicitly convert data types if necessary; this operation is suitable for data merging with similar structures, such as records divided by region, time, or role.

Aug 13, 2025 am 11:54 AM

MySQL Enterprise Edition vs Community Edition Features

MySQLCommunityEdition is a free, open source version suitable for medium-sized applications; EnterpriseEdition is a paid version that provides additional tools, plug-ins and official support. 1.EnterpriseEdition includes performance monitoring tools (such as MySQLEnterpriseMonitor), online backup tools (MySQLEnterpriseBackup), and security enhancements (MySQLEnterpriseSecurity), while CommunityEdition lacks these advanced tools. 2.EnterpriseEdition provides database auditing and fine-grained access

Aug 13, 2025 am 10:10 AM

How to implement a search with multiple optional parameters in MySQL

UseconditionalWHEREclauseswithOR?ISNULLtohandleoptionalparametersbyallowingeachfiltertobeskippediftheparameterisNULL,ensuringsafepreparedstatementusage.2.BuilddynamicSQLinapplicationcodetoincludeonlytheprovidedfilters,improvingindexutilizationandquer

Aug 13, 2025 am 10:09 AM

MySQL Data Dictionary and Information Schema Exploration

Information Schema is a system database provided by MySQL, which contains read-only virtual tables, used to display metadata, such as querying which tables the database can execute SELECTTABLE_NAMEFROMinformation_schema.TABLESWHERETABLE_SCHEMA='your_database_name'; Common uses include: 1. Use COLUMNS table to check column information; 2. Use STATISTICS table to check index information; 3. Use VIEWS table to check view definitions; 4. Use TABLES and COLLATIONS tables to check character sets and sorting rules. Data Dictionary (DataD

Aug 13, 2025 am 09:09 AM

How to schedule a job using the MySQL event scheduler

To use the MySQL event scheduler to schedule tasks, you must first enable the event scheduler. You can turn on by executing SETGLOBALevent_scheduler=ON; and it is recommended to add event_scheduler=ON to the configuration file to persist. Then create a scheduled event, such as performing a log cleanup task once a night at midnight every day, and use a CREATEEVENT statement definition, such as CREATEEVENTIFNOTEXISTSdaily_log_cleanupONSCHEDULEEVERY1DAY1DAYSTARTSTIMESTAMP(CURDATE() INTERVAL1DAY,'00:00:

Aug 13, 2025 am 07:12 AM

How to configure MySQL to run on a different port?

Modify the MySQL running port to edit the configuration file my.cnf (Linux/macOS) or my.ini (Windows), and set port=new port number under the [mysqld] paragraph; 2. Restart MySQL service after saving the file: Linux uses sudosystemctlrestartmysql, Windows uses netstopmysql and netstartmysql; 3. Verify that the port has taken effect through sudonestat-tulnp|grepmysqld or mysql-uroot-p-P new port-host=127.0.0.1; 4. Ensure that the firewall opens the new port

Aug 13, 2025 am 06:07 AM

mysql 端口配置

How to analyze and repair tables in MySQL?

Analyzeandrepairtableswhenqueriesslowdown,errorsindicatecorruption,oraftercrashes,especiallyforMyISAMtables.2.RunANALYZETABLEtoupdateindexstatisticsandimprovequeryoptimizerdecisions,lockingthetablebriefly.3.UseREPAIRTABLEtofixMyISAMcorruption,withopt

Aug 13, 2025 am 04:32 AM

How to rollback a transaction in MySQL

To roll back transactions in MySQL, you must use a transaction-enabled storage engine such as InnoDB. 1. Confirm the table to use InnoDB: SHOWCREATETABLEyour_table_name; if not InnoDB, execute ALTERTABLEyour_table_nameENGINE=InnoDB; 2. Start the transaction explicitly: use STARTTRANSACTION or BEGIN; 3. Execute SQL statements such as UPDATEaccountsSETbalance=balance-100WHEREuser_id=1; and UPDATEaccountsSETbalance

Aug 13, 2025 am 03:26 AM

mysql 事務回滾