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Technical Articles

Database

Mysql Tutorial

How to create a trigger in MySQL

TocreateatriggerinMySQL,usethesyntaxwithDELIMITER,specifyBEFOREorAFTER,thetriggeringevent(INSERT,UPDATE,DELETE),thetablename,anddefinethelogicwithinBEGINandENDblocks.2.Forexample,anAFTERUPDATEtriggerontheuserstablecanlogemailchangesintoauser_audittab

Aug 20, 2025 am 12:41 AM

How to select data from a table in MySQL?

To select data from MySQL table, you should use SELECT statement, 1. Use SELECTcolumn1, column2FROMtable_name to obtain the specified column, or use SELECT* to obtain all columns; 2. Use WHERE clause to filter rows, such as SELECTname, ageFROMusersWHEREage>25; 3. Use ORDERBY to sort the results, such as ORDERBYageDESC, representing descending order of age; 4. Use LIMIT to limit the number of rows, such as LIMIT5 to return the first 5 rows, or use LIMIT10OFFSET20 to implement paging; 5. Use AND, OR and parentheses to combine

Aug 19, 2025 pm 01:47 PM

mysql data query

How to change a column's data type in MySQL?

Tochangeacolumn'sdatatypeinMySQL,useALTERTABLEwithMODIFYforsimpletypechangeswithoutrenaming,orCHANGEwhenrenamingisneeded;forexample,ALTERTABLEusersMODIFYageINTchangesthedatatypeoftheagecolumntoINT,whileALTERTABLEusersCHANGEageuser_ageINTrenamesthecol

Aug 19, 2025 pm 01:38 PM

How to use REGEXP for regular expression matching in MySQL?

MySQL supports basic regular expression matching using REGEXP or RLIKE operators, which can be used to perform pattern-based string searches in SQL queries. 1. Use ^ to start with a certain character, and $ to end with a certain character, directly match a specific pattern in the string, such as finding names starting with 'A'; 2. Support character classes and wildcard characters, including . (any single character), [abc] (match any character in brackets), 1 (not match any character in brackets), [a-z] (match range characters), * (zero or multiple times in previous item), (the previous item appears once or multiple times), ? (zero or once in previous item), (a|b) (match a or b), etc., which can be used to match email domain names or phone number formats

Aug 19, 2025 pm 01:07 PM

mysql regexp

How to add and drop indexes on a MySQL table?

ToaddordropindexesinMySQL,useCREATEINDEXorDROPINDEXwithpropersyntax.1.Toaddanindex:CREATEINDEXindex_nameONtable_name(column_list);2.Foracompositeindex:CREATEINDEXidx_nameONemployees(last_name,first_name);3.Forauniqueindex:CREATEUNIQUEINDEXidx_emailON

Aug 19, 2025 pm 12:09 PM

What is the role of the default value in a MySQL column?

ThedefaultvalueinaMySQLcolumnspecifiesthevalueautomaticallyinsertedwhennoexplicitvalueisprovidedduringanINSERToperation;itensuresdataconsistencybyfillingmissingvalueswithpredefinedstaticordynamicvalueslike'active'orCURRENT_TIMESTAMP,helpsavoidunwante

Aug 19, 2025 pm 12:02 PM

How to use LIMIT with an OFFSET in MySQL?

When using LIMIT and OFFSET for pagination, the answer should be clear: It is recommended to use LIMIT and OFFSET for pagination in combination with ORDERBY, but pay attention to the performance issues of large offsets. 1. Use ORDERBY to ensure the order of the results is consistent; 2. LIMIT controls the number of rows returned, and OFFSET specifies the number of skipped rows; 3. The recommended syntax is LIMITnOFFSETm to improve readability; 4. Avoid large OFFSET values, as it will cause performance degradation; 5. For deep paging, it is recommended to use cursor paging based on index columns instead (such as WHEREid>last_id). This method is suitable for small and medium-sized data volumes or shallow-level pagination, and the predictability and integrity of query results must be ensured.

Aug 19, 2025 am 10:29 AM

mysql limit

How to modify a column in a table in MySQL

UseMODIFYtochangeacolumn’sdatatype,size,orconstraintswithoutrenamingit,includingalldesiredattributesinthecommand;2.UseCHANGEtorenameacolumnormodifyitsdefinitionbyspecifyingbotholdandnewnames,whichoffersmoreflexibilitybutisslower;3.Commonmodifications

Aug 18, 2025 am 11:28 AM

mysql 修改列

How to start and stop the MySQL server

OnmodernLinuxsystemsusingsystemd,startMySQLwithsudosystemctlstartmysql(ormysqldonsomesystems),stopwithsudosystemctlstopmysql,restartwithsudosystemctlrestartmysql,checkstatuswithsudosystemctlstatusmysql,andenableordisableauto-startwithsudosystemctlena

Aug 18, 2025 am 11:26 AM

What is a primary key in MySQL?

AprimarykeyinMySQLisacolumnorsetofcolumnsthatuniquelyidentifieseachrow,ensuringdataintegritybyenforcinguniquenessandprohibitingnullvalues;itmustbeunique,notnull,limitedtoonepertable(thoughitcanspanmultiplecolumns),andautomaticallycreatesaclusteredind

Aug 18, 2025 am 11:14 AM

What is database normalization in MySQL?

NormalizationinMySQLisadatabasedesignprinciplethatorganizestablestoreduceredundancyandimprovedataintegritybyeliminatingduplicatedataandensuringproperdependencies.Itinvolvesdividinglargetablesintosmaller,relatedonesandlinkingthemthroughkeys,followinga

Aug 18, 2025 am 10:32 AM

mysql 數(shù)據(jù)庫規(guī)范化

How to create a database in MySQL?

To create a MySQL database, first use mysql-uusername-p to log in to the server, and then execute the CREATEDATABASEdatabase_name; command to create a database, such as CREATEDATABASEmyapp_db; to avoid duplicate name errors, use CREATEDATABASEIFNOTEXISTSmyapp_db;, and then select the database through USEmyapp_db;, optionally specify the character set and proofreading rules at creation time such as CREATEDATABASEmyapp_dbCHARACTERSETutf8mb4COLLATEutf8mb4_unic

Aug 18, 2025 am 10:02 AM

How to extract the year, month, or day from a date in MySQL

Use the YEAR() function to extract the year, such as SELECTYEAR('2025-04-05') to return 2025; 2. Use the MONTH() function to extract the month (1-12), or MONTHNAME() to obtain the month name, such as SELECTMONTH('2025-04-05') to return 4; 3. Use DAY() or DAYOFMONTH() to extract the date (1-31), DAYOFWEEK() to return the week value (1-7), DAYNAME() to return the week name, such as SELECTDAY('2025-04-05') to return 5, all functions return NULL when the date is invalid or NULL, which can be used in combination with the query

Aug 18, 2025 am 08:57 AM

How to select data from a table in MySQL

To select data from MySQL table, you need to use SELECT statement. The basic syntax is SELECTcolumn1, column2FROMtable_name; 1. List the column names when selecting a specific column, such as SELECTname, emailFROMusers; 2. Select all data to use SELECTFROMusers, but you need to handle large tables with caution; 3. Use WHERE clause to filter the results, such as SELECTname, ageFROMusersWHEREage>25, and support operators such as =, , IN, LIKE, BETWEEN, etc.; 4. Use ORDERBY to sort the results, such as SELECTna

Aug 18, 2025 am 08:42 AM