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Technical Articles

Database

How to use wildcards with the LIKE operator in SQL?

The LIKE operator in SQL is used to search for specified patterns in columns in WHERE clauses, mainly using % and _ wildcards. 1.% match zero or more characters, such as 'Jo%' matches "John" and "Jose"; 2._match a single character, such as 'A___' matches four letters and names starting with A; 3. Can be used in combination, such as 'J_h%' matches "John" or "Josh"; 4. When searching for literals containing % or _, you need to use ESCAPE keywords or brackets to escape; 5. Different databases have different case sensitivity to upper and lowercase and lowercase, and you can use UPPER() or LO

Aug 07, 2025 pm 10:21 PM

How to select the first row in each GROUP BY group in SQL?

ToselectthefirstrowineachGROUPBYgroupinSQL,youtypicallyneedtousewindowfunctions,asstandardGROUPBYdoesn'tguaranteeroworderorletyoudirectlypickafullrowbasedonrankingwithinthegroup.Themostreliableandwidelysupporte

Aug 07, 2025 pm 10:15 PM

sql group by

How to unpivot a table in MySQL

MySQL does not have a built-in UNPIVOT operator, but row-to-column conversion can be achieved through SQL technology; 1. Use UNIONALL method: query each quarter column and merge it, which is suitable for situations where the number of columns is small, simple and intuitive, but the code is repeated; 2. Use CROSSJOIN to CASE: generate a column name list through cross-connection, and then use CASE to match the value, logically concentrated but the column name needs to be maintained manually; both methods need to manually process the column, and cannot dynamically adapt to column changes. It is recommended to combine the application layer or stored procedure when there are many columns, and HAVINGvalueISNOTNULL can be added to filter the empty value, and finally select a suitable method based on the data scale and maintenance needs to complete the wide table to long table operation.

Aug 07, 2025 pm 10:05 PM

How do you update existing records in a SQL table?

To update existing records in SQL tables, you need to use the UPDATE statement; 1. Specify the table name, 2. Use the SET clause to set a new value, 3. Use the WHERE clause to define the conditions to avoid errors in all data. For example, UPDATEemployeesSETsalary=65000WHEREname='Bob'; can update Bob's salary, and be sure to use SELECT to test the conditions first, and use transactions to ensure safe operations if necessary.

Aug 07, 2025 pm 09:57 PM

How to handle duplicate values in MySQL

First, identify duplicate rows through GROUPBY and HAVING, then use ROW_NUMBER(), self-join or create a new table based on whether there is a primary key. Finally, use unique constraints, use INSERTIGNORE or ONDUPLICATEKEYUPDATE to prevent future duplication; 1. Use SELECTemail, COUNT()AScountFROMusersGROUPBYemailHAVINGCOUNT()>1 to identify duplicate mailboxes; 2. If there is a primary key and MySQL version is 8.0, use DELETEt1FROMusersst1INNERJOIN(SEL

Aug 07, 2025 pm 09:46 PM

How to get the first day of the month in SQL?

ForPostgreSQL,BigQuery,andSnowflake,useDATE_TRUNC('month',date)togetthefirstdayofthemonth.2.InSQLServer,useDATEFROMPARTS(YEAR(date),MONTH(date),1)orDATEADDandDATEDIFFwithabasedate.3.InMySQL,useDATE_FORMAT(date,'%Y-%m-01')orDATE_SUB(date,INTERVALDAY(d

Aug 07, 2025 pm 09:43 PM

How to perform conditional inserts in SQL?

UseINSERT...SELECTwithWHERENOTEXISTStoinsertonlyifnoduplicateexists,whichiswidelysupportedacrossdatabases;2.UseMERGE(upsert)inSQLServer,Oracle,orPostgreSQLtoinsertorupdatebasedonrowexistence;3.InMySQL,useINSERT...ONDUPLICATEKEYUPDATEtoconditionallyin

Aug 07, 2025 pm 09:34 PM

sql 條件插入

How to calculate the interquartile range (IQR) in SQL?

TocalculateIQRinSQL,usePERCENTILE_CONT(0.25)andPERCENTILE_CONT(0.75)inPostgreSQL,SQLServer,orOracletogetQ1andQ3,thensubtractQ1fromQ3toobtaintheIQR;2.InMySQL,sincePERCENTILE_CONTisnotavailable,approximateQ1andQ3usingROW_NUMBER()withpercentile-basedrow

Aug 07, 2025 pm 09:32 PM

How to work with geography and geometry data types in SQL?

Usegeographyforglobal,real-worldaccuratedataandgeometryforlocal,high-performanceapplications;2.StorespatialdatawithappropriatetypesandSRIDs;3.Insertdatausingdatabase-specificfunctionslikeST_GeogFromTextinPostGISorgeography::PointinSQLServer;4.Querywi

Aug 07, 2025 pm 09:28 PM

sql geographical data

Writing and Executing Stored Procedures in SQL Databases

Stored procedures are suitable for scenarios with high repetition, multi-table association, high security and performance requirements. For example: 1. Business logic with high repetition, such as timed statistical reports; 2. Multi-table correlation and complex logic; 3. Access is controlled through stored procedures when security requirements are high; 4. Performance optimization requirements. When writing basic stored procedures, taking MySQL as an example, you need to use DELIMITER to define the ending character, CREATEPROCEDURE to declare the process name and parameters, and BEGIN...END wrap the logic body. When calling, use CALL statements and pass parameters, and debugging is performed according to different database characteristics. Notes include: avoiding conflicts between parameters and field names, using transactions reasonably, ensuring execution permissions, controlling logic complexity to reduce dimensions

Aug 07, 2025 pm 08:49 PM

How to create a function that returns a table in SQL?

Yes, functions that return tables can be created in SQLServer and PostgreSQL. 1. Use RETURNSTABLE in SQLServer to create inline table value functions, such as CREATEFUNCTIONdbo.GetEmployeesByDepartment(@DeptIDINT)RETURNSTABLEASRETURN(SELECTEmployeeID,Name,Salary,DepartmentIDFROMEmployeesWHEREDepartmentID=@DeptID); 2. For multi-statement functions, use RETURNS@ta

Aug 07, 2025 pm 08:47 PM

sql function 表值函數(shù)

How to set a default value for a column in MySQL

Tosetadefaultvaluewhencreatingatable,usetheDEFAULTkeywordinthecolumndefinition,suchasDEFAULT'active'orDEFAULTCURRENT_TIMESTAMP.2.Toaddorchangeadefaultonanexistingcolumn,useALTERTABLEusersALTERCOLUMNstatusSETDEFAULT'inactive'inMySQL8.0 ,oruseMODIFYfor

Aug 07, 2025 pm 08:44 PM

How to Fix a Corrupted MySQL Table?

First, use the CHECKTABLE command or myisamchk tool to check whether the table is corrupted, and then select the repair method according to the storage engine after confirmation; 2. For MyISAM table, you can use the REPAIRTABLE command or run myisamchk-r-f offline for repair; 3. For InnoDB table, you should configure innodb_force_recovery to restart MySQL, export the data and rebuild the table; 4. If the repair fails, you need to restore data from backup or library; 5. To prevent future damage, you should give priority to using InnoDB engine, ensure normal shutdown, enable checksum, regularly backup and monitor disk health. All repair operations must be done before

Aug 07, 2025 pm 08:24 PM

How to update existing records in a table in SQL?

To update existing records in SQL tables, you need to use the UPDATE statement; 1. Use UPDATEtable_name to specify the target table; 2. Use SETcolumn1=value1,column2=value2 to set a new value; 3. The rows to be updated must be limited with the WHERE condition, otherwise all records will be affected; 4. The WHERE condition can be tested through SELECT to ensure accuracy; 5. It is recommended to perform critical updates in the transaction so that errors can be rolled back; 6. Multiple columns or rows can be updated at the same time, supporting calculations based on the current value; be sure to verify carefully before operation to avoid error updates.

Aug 07, 2025 pm 08:13 PM

sql Update records