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Technical Articles

Database

Mysql Tutorial

How to get the list of users in a MySQL database?

TogetalistofusersinaMySQLdatabase,executeSELECTUser,HostFROMmysql.user;afterconnectingwithaprivilegedaccount.2.Thisqueryretrievestheusernameandallowedconnectionhostfromthemysql.usertable,showingdetailslikeroot@localhostoradmin@%.3.Foradditionaluserin

Aug 24, 2025 am 06:35 AM

What is the table_open_cache setting in MySQL?

Thetable_open_cachesettinginMySQLspecifiesthemaximumnumberofopentableinstancesthatcanbecachedtoreducetheoverheadofrepeatedlyopeningandclosingtables,improvingperformancebyreusingtablehandles;itisdistinctfromtable_definition_cache,whichcachestablemetad

Aug 24, 2025 am 06:05 AM

How to audit user activity in MySQL

EnableGeneralQueryLogfortemporary,full-queryloggingwithhighperformanceoverhead.2.UseMySQLEnterpriseAuditpluginforrobust,filtered,JSON-formattedauditinginEnterpriseEdition.3.InstallMariaDBAuditPlugininCommunityEditionforcomprehensiveloggingofconnectio

Aug 24, 2025 am 03:38 AM

What is the MySQL Slow Query Log and How to Use It?

TheMySQLSlowQueryLogshouldbeenabledtoidentifyandoptimizeslow-performingqueries,asitrecordsqueriesexceedingaspecifiedexecutiontimeandhelpsimprovedatabaseperformance.1.Enabletheslowquerylogbysettingslow_query_log=ONintheconfigurationfileorviaSETGLOBALs

Aug 24, 2025 am 01:34 AM

How to perform a self-join in MySQL?

Aself-joininMySQLisaJOINwhereatableisjoinedwithitselfusingaliasestocomparerowswithinthesametable,commonlyusedforhierarchicaldatalikeemployeesandmanagersorfindingrelatedrecordssuchasemployeessharingthesamemanager,anditrequirestablealiases(e.g.,t1,t2)t

Aug 24, 2025 am 12:26 AM

How to create a foreign key in MySQL

TocreateaforeignkeyinMySQL,useCREATETABLEwithFOREIGNKEYreferencingaparenttable’sprimarykey,ensuringreferentialintegrity.2.Forexistingtables,useALTERTABLEADDCONSTRAINTtoaddaforeignkeywithoptionalnaming.3.RequirementsincludeusingInnoDBstorageengine,mat

Aug 23, 2025 pm 04:41 PM

mysql foreign key

How to Create and Manage Users and Privileges in MySQL?

Create a new user requires the use of the CREATEUSER statement to specify the user name, host and password, such as CREATEUSER'jane'@'localhost'IDENTIFIEDBY'SecurePass123!'; 2. Grant permissions to use GRANT statements, such as GRANTSELECT, INSERTONmydb.productsTO'jane'@'localhost', and execute FLUSHPRIVILEGES to make the changes take effect immediately; 3. View user permissions to use SHOWGRANTSFOR'username'@'host'; 4. Modify permissions to revoke specific or

Aug 23, 2025 pm 04:13 PM

mysql User rights

How to get the number of rows in a MySQL table?

To get the number of rows in the MySQL table, the most common method is to use the COUNT() function; 1. Use COUNT() to count all rows, which is the most standard and reliable method, including rows containing NULL values. For example, SELECTCOUNT()FROMusers will return the total number of rows in the users table; 2. Use COUNT(1), the result is the same as COUNT(). Some developers use it out of habit, but there is no difference in performance in MySQL; 3. If you only count the number of rows that are not NULL in a specific column, you can use SELECTCOUNT(column_name)FROMtable_name, which will exclude rows with the column NULL; 4. You can combine WHER

Aug 23, 2025 pm 04:10 PM

mysql Number of lines

How to use the ENUM data type in MySQL

ENUMinMySQLisastringobjectthatrestrictsacolumntoapredefinedlistofvalues.1.DefineENUMbylistingallowedvalues:CREATETABLEtasks(statusENUM('pending','in_progress','completed','cancelled')DEFAULT'pending');2.Insertonlyvalidstringvalues,oranerroroccursfori

Aug 23, 2025 pm 03:54 PM

How to use aggregate functions in MySQL

MySQL's aggregation function is used to calculate data and return a single value, often used in combination with GROUPBY. 1. The main functions include COUNT() (count), SUM() (sum), AVG() (average), MAX() (maximum) and MIN() (minimum), where NULL values ??are ignored except COUNT(). 2. Basic usages are as follows: COUNT() counts the total number of rows, SUM(amount) calculates the total sales, AVG(amount) calculates the average sales, MAX and MIN obtain the highest and lowest values ??respectively. 3. Use GROUPBY to group and aggregate by category, such as counting total sales by region and calculating average prices by product. 4. Make

Aug 23, 2025 pm 02:24 PM

How to import a CSV file into a MySQL table?

ThemostefficientwaytoimportaCSVfileintoaMySQLtableisusingtheLOADDATAINFILEstatement,providedthefileisaccessibletotheserverandsecure_file_privsettingsallowit;2.Beforeimporting,ensuretheCSVisproperlyformattedwithconsistentdelimiters,thetargettableexist

Aug 23, 2025 am 09:27 AM

What is the difference between TIMESTAMP and DATETIME in MySQL?

TIMESTAMPistimezoneawareandautomaticallyconvertsbetweenUTCandthesessiontimezone,whileDATETIMEistimezonenaiveandstoresvaluesexactlyasprovided;TIMESTAMPhasalimitedrangefrom'1970-01-0100:00:01'UTCto'2038-01-1903:14:07'UTC,whereasDATETIMErangesfrom'1000-

Aug 23, 2025 am 08:21 AM

mysql 日期類型

What are the different isolation levels in MySQL transactions?

MySQL supports four transaction isolation levels: READUNCOMMITTED allows dirty reading; READCOMMITTED avoids dirty reading but does not have repetitive reading; REPEATABLEREAD is the default level of InnoDB, which solves dirty reading and non-repetitive reading, and reduces phantom reading through MVCC and gap locks; SERIALIZABLE eliminates dirty reading, non-repetitive reading and phantom reading through forced serial execution, but has the lowest concurrency performance; the appropriate level can be selected according to the application's requirements for consistency and performance. Usually REPEATABLEREAD has the best balance, while READCOMMITTED can be used in high concurrency scenarios.

Aug 23, 2025 am 07:46 AM

How to drop a column from a table in MySQL

To delete a column from a MySQL table, you need to use the ALTERTABLE statement to match the DROPCOLUMN clause; the specific syntax is ALTERTABLEtable_nameDROPCOLUMNcolumn_name, where the COLUMN keyword can be omitted; for example, if you delete the age column in the employees table, you can perform ALTERTABLEemployeesDROPCOLUMNage; the deletion operation is irreversible, which will cause all data in the column to be permanently lost; if there are dependent objects (such as views, triggers, foreign keys or indexes), the constraints must be released first; deleting multiple columns at one time requires chain writing: ALTERTABLEtable_nameDROPC

Aug 23, 2025 am 06:29 AM