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Technical Articles

Database

Mysql Tutorial

How to configure the query cache in MySQL

MySQL's query cache is only available in version 5.7 and earlier, and version 8.0 and above has been removed; 1. Confirm support through SHOWVARIABLESLIKE'have_query_cache' and check query_cache_type, query_cache_size, etc. configurations; 2. Set query_cache_type=ON, query_cache_size=256M, query_cache_limit=2M, query_cache_min_res_unit=512K in my.cnf to enable and optimize cache; 3. Use SHOWSTATUSLIKE

Aug 14, 2025 pm 03:31 PM

How to restart the MySQL server from the command line?

UsesudosystemctlrestartmysqlorsudosystemctlrestartmysqldonmodernLinuxsystemswithsystemd.2.UsesudoservicemysqlrestartorsudoservicemysqldrestartonolderLinuxsystems.3.Usesudo/etc/init.d/mysqlrestartfordirectscript-basedrestarts.4.OnmacOSwithHomebrew,use

Aug 14, 2025 pm 03:27 PM

How to execute a script file from the mysql command-line

ToexecuteascriptfilefromtheMySQLcommand-line,usetheSOURCEcommandor\.shortcutwithinMySQL,orrunitfromtheOScommandline.1.UseSOURCE/path/to/script.sql;inMySQLafterlogginginandselectingthedatabaseifneeded.2.Alternatively,use\./path/to/script.sqlasashortha

Aug 14, 2025 pm 02:44 PM

Automating MySQL Backups to AWS S3 or Google Cloud Storage

To automate backup of MySQL to AWSS3 or GoogleCloudStorage, you need to create backup scripts, upload backups, automate them with cron, and clean old files. 1. Create a mysqldump script containing database credentials and timestamps to generate backup files; 2. Use AWSCLI or gsutil tools to upload backups to S3 or GCS; 3. Automatically execute scripts and record logs through cron timing tasks every day at 2 a.m.; 4. Add cleaning commands to regularly delete old backups 7 days ago to save storage space. The entire process ensures that data is secure, automatic and recoverable.

Aug 14, 2025 pm 02:11 PM

How to find the median value in a MySQL column

UseROW_NUMBER()andCOUNT()OVER()torankrowsandgettotalcount;2.Filterrowswhererow_numequalsthemiddlepositionsusingFLOOR((total_count 1)/2)andCEIL((total_count 1)/2);3.WraptheresultandtakeAVG(amount)tocomputethemedian,whichworksforbothoddandevenrowcounts

Aug 14, 2025 pm 02:03 PM

How to find duplicate values in a MySQL table?

Tofindduplicatesinasinglecolumnlikeemail,useGROUPBYwithHAVINGCOUNT()>1toshowvaluesappearingmorethanonce.2.Forduplicatesacrossmultiplecolumnssuchasfirst_nameandlast_name,applyGROUPBYonthecombinedcolumnswithHAVINGCOUNT()>1.3.Todisplayallduplicate

Aug 14, 2025 pm 02:02 PM

What are the different storage engines in MySQL?

InnoDB is MySQL default storage engine, suitable for applications that require transactions, high concurrency and data integrity, and supports row-level locks, foreign keys and crash recovery; MyISAM is suitable for read-intensive non-transactional scenarios but is no longer recommended; Memory engine is used for temporary data storage in memory, fast but not durable; CSV is used for data exchange, Archive is suitable for appended log data; Blackhole is used for testing and replication, NDB is used for high availability cluster environments, Federated is used for accessing remote tables, and Merge is the deprecated MyISAM merge engine. The selection engine should be based on specific needs. InnoDB is suitable for most modern applications.

Aug 14, 2025 pm 01:47 PM

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