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Home

Technical Articles

Database

Mysql Tutorial

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

What are the numeric data types in MySQL?

MySQLprovidesinteger,fixed-point,andfloating-pointnumericdatatypes;useTINYINTforsmallrangesandflags,INTforgeneralwholenumbers,DECIMALforexactvalueslikemoney,andBIGINTforlargenumbers,ensuringoptimalstorageandperformance.

Aug 23, 2025 am 05:48 AM

What is the difference between BIGINT and INT in MySQL?

BIGINTuses8bytesandsupportsvaluesfrom-9,223,372,036,854,775,808to9,223,372,036,854,775,807,whileINTuses4bytesandsupports-2,147,483,648to2,147,483,647,makingBIGINTsuitableforlargernumbersincaseslikedistributedsystemIDsorlargecounters,whereasINTissuffi

Aug 23, 2025 am 03:35 AM

How to use the JSON_EXTRACT function in MySQL

The JSON_EXTRACT function is used to extract data of a specified path from a JSON document. The answer is to use JSON path syntax to locate and get the value; 1. The path represents the root with $, $.key obtains the object key value, $[index] obtains the array elements, and [] represent all values ??of the object and array respectively; 2. When extracting the JSON object value, return the quoted JSON type, which can be obtained without quotes; 3. When processing the array, a single element can be extracted through indexing or all elements can be extracted using [*]; 4. For nested objects, the path continuously specifies key names at each level to extract deep data; 5. In the WHERE clause, JSON_EXTRACT or ->, -&g

Aug 23, 2025 am 02:48 AM

How to delete duplicate rows from a table in MySQL?

TodeleteduplicaterowsinMySQL,useatemporarytableforcompatibility,ROW_NUMBER()inMySQL8.0 tokeepthesmallestID,oraself-joinonolderversionstoremoveduplicateswhilepreservingoneinstance,andaddauniqueconstrainttopreventfutureduplicates;alwaysbackupdataandtes

Aug 23, 2025 am 12:11 AM

How to replace a substring in MySQL

Use the REPLACE() function to replace the substring in the string, with the syntax of REPLACE (original string, substring to be found, and new substring to be replaced); 2. You can preview the replacement effect in a SELECT query, such as SELECTREPLACE('HelloWorld','World','MySQL') to return 'HelloMySQL'; 3. When updating table data, you can use the UPDATE statement combined with REPLACE(), such as replacing the 'colour' of the description field in the products table with 'color'; 4. The REPLACE() function is case sensitive and only replaces the exact matching substring, and additional case variations need to be processed.

Aug 22, 2025 pm 02:09 PM

How to use IFNULL() in MySQL?

IFNULL()inMySQLreturnsthefirstexpressionifitisnotNULL,otherwisereturnsthesecondexpression,makingitidealforreplacingNULLvalueswithdefaults;forexample,IFNULL(middle_name,'N/A')displays'N/A'whenmiddle_nameisNULL,IFNULL(discount,0)ensurescalculationslike

Aug 22, 2025 pm 02:00 PM

mysql IFNULL()

How to find the size of a specific database in MySQL?

To view the size of a specific database in MySQL, you need to execute a SQL query to obtain data from information_schema.TABLES; calculate the total number of bytes of the specified database through SUM (data_length index_length) and convert it into MB units. For example, querying the size of the sales_db database will return a result similar to 100.00MB, indicating that the database occupies 100MB disk space. This method is safe and accurate and contains the total size of the data and index, and finally ends with a complete sentence.

Aug 22, 2025 pm 12:41 PM

mysql Database size

How to clone a table structure without data in MySQL

TocloneatablestructurewithoutcopyinganydatainMySQL,youcanusetheCREATETABLE...LIKEstatement.Thismethodissimple,efficient,andpreservestheschemaincludingindexes,constraints,andcolumndefaults.UseCREATETABLE...LIKECREATET

Aug 22, 2025 am 11:25 AM

How to find the Nth highest value in a MySQL table?

To find the N-th highest value in the MySQL table, if you use MySQL8.0 and above, it is recommended to use the DENSE_RANK() function to correctly handle duplicate values; otherwise, you can use LIMITOFFSET (no duplication processing) or related subqueries (compatible with old versions and deal with duplication). The DENSE_RANK() method ensures that the N-th highest value is returned accurately by assigning the same ranking to the same value and not skipping the subsequent ranking. The final choice should be based on the MySQL version and whether duplicate data needs to be processed. The specific method depends on the scenario.

Aug 22, 2025 am 11:03 AM

How to change the GROUP_CONCAT separator in MySQL

You can customize the separator by using the SEPARATOR keyword in the GROUP_CONCAT() function; 1. Use SEPARATOR to specify a custom separator, such as SEPARATOR'; 'The separator can be changed to a semicolon and plus space; 2. Common examples include using the pipe character '|', space'', line break character '\n' or custom string '->' as the separator; 3. Note that the separator must be a string literal or expression, and the result length is limited by the group_concat_max_len variable, which can be adjusted by SETSESSIONgroup_concat_max_len=10000; 4. SEPARATOR is optional

Aug 22, 2025 am 10:58 AM

mysql

How to enable or disable the event scheduler in MySQL

Tochecktheeventschedulerstatus,useSHOWVARIABLESLIKE'event_scheduler';.2.Toenableordisabletemporarily,useSETGLOBALevent_scheduler=ONorOFF,whichremainseffectiveuntilthenextrestart.3.Toenableordisablepermanently,addevent_scheduler=ONorOFFunder[mysqld]in

Aug 22, 2025 am 09:59 AM

mysql event dispatcher

How to get a substring in MySQL

To get substrings in MySQL, you should use the SUBSTRING() function; 1. Start extracting from the specified location: SELECTSUBSTRING('HelloWorld',5) returns 'oWorld'; 2. Extracting the specified length: SELECTSUBSTRING('HelloWorld',7,5) returns 'World'; 3. Combining POSITION() to extract data from the table: SELECTemail, SUBSTRING(email,POSITION('@'INemail) 1) ASdomainFROMusers can extract the mailbox domain name; 4. Support negative number location: SELECTemail: SELECTemail

Aug 22, 2025 am 07:32 AM

What is the EXPLAIN statement and how to use it in MySQL?

EXPLAINinMySQLisusedtoanalyzequeryexecutionplanswithoutrunningthequery,helpingidentifyperformanceissues.1.UseEXPLAINbeforeaSELECT,INSERT,UPDATE,DELETE,orREPLACEstatement,mostcommonlywithSELECT.2.Keyoutputcolumnsincludeid,select_type,table,type,possib

Aug 22, 2025 am 07:27 AM

mysql explain