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Technical Articles

Database

SQL

How to use Common Table Expressions (CTEs) in SQL?

CommonTableExpressions(CTEs)improveSQLqueryreadabilitybycreatingtemporaryresultsetsusingtheWITHkeyword,whichcanbereferencedinsubsequentSELECT,INSERT,UPDATE,orDELETEstatements.1.ThebasicsyntaxstartswithWITH,followedbyaname,optionalcolumns,AS,andaquery

Aug 31, 2025 am 06:15 AM

How do you handle NULL values in SQL?

To correctly handle NULL values ??in SQL, you must use ISNULL or ISNOTNULL to judge, because NULL is different from normal values, you cannot compare with = or!=; 1. Use ISNULL or ISNOTNULL to check NULL values, such as SELECTFROMemployeesWHEREphoneISNULL; 2. Use COALESCE or ISNULL to replace NULL, such as COALESCE(email,'N/A') to return the first non-NULL value; 3. Note that NULL is ignored in the aggregate function, COUNT() counts all rows, while COUNT(column) only counts non-NULL values; 4. Use CASE

Aug 31, 2025 am 06:04 AM

How to create a clustered and non-clustered index in SQL?

Aclusteredindexdefinesthephysicalorderoftabledataandallowsonlyonepertable,typicallycreatedautomaticallyviaaPRIMARYKEYconstraintormanuallyusingCREATECLUSTEREDINDEX,whileanon-clusteredindexcreatesaseparatestructurepointingtodatarowsandallowsupto999pert

Aug 31, 2025 am 05:51 AM

How to rethrow an error in SQL

The correct way to re-throw an error in SQLServer is to use the THROW statement. 1. Use THROW (without parameters) in the CATCH block to re-throw the original error, retaining the error number, message, severity, status and call stack. 2. Optionally, first record the error message, such as using PRINTERROR_MESSAGE(). 3. To throw a custom error, use THROWerror_number,message,state, where the user-defined error number should be greater than or equal to 50000. 4. Avoid using RAISERROR because it truncates messages, does not retain the original error number and is not recommended for re-throwing errors. 5.THROW can only be used in CAT

Aug 31, 2025 am 05:27 AM

SQL Bulk Insert Techniques for Large Datasets

ToefficientlyhandlelargedatasetsusingSQL'sBULKINSERTcommand,firstensureproperdatafileformattingwithconsistentdelimiters,correctdatatypes,andoptionalpre-sortingforclusteredindexes.1.Useaformatfileifdatastructuredoesn'talignwiththetable.2.ApplyBULKINSE

Aug 31, 2025 am 05:12 AM

How to delete duplicate rows from a table in SQL

The method of deleting duplicate rows depends on the database system. Use CTE and ROW_NUMBER() to identify and retain the first row in each group, which is suitable for SQLServer, PostgreSQL and MySQL8.0; if CTE is not supported, you can create a temporary table to store deduplicate data and replace the original table; in MySQL or SQLite, you can delete duplicate records through self-connection, and keep the rows with the smallest primary key; if there is no primary key, you should first add a unique identity and then perform a deduplication operation. Before all operations, you should back up the data and preview the results with SELECT, and finally select a method suitable for the specific database and table structure to complete deduplication.

Aug 31, 2025 am 02:27 AM

sql Delete duplicate lines

SQL CLR Integration: Extending Database Functionality

SQLServer's CLR integration refers to the technology of writing database objects in C# or VB.NET by loading .NET assembly. The advantages are handling complex calculations, string operations, and accessing external resources. The steps to enable: 1. Use the sp_configure command to enable CLR; 2. Set security policy to restrict permissions. The deployment process includes: 1. Create a SQLCLR project and compile and generate DLLs; 2. Load the assembly in SQLServer and create SQL object calling methods. Notes include: 1. Pay attention to performance impact; 2. Strengthen safety control; 3. Pay attention to maintenance complexity; 4. Check version compatibility.

Aug 30, 2025 am 07:11 AM

How to understand a query execution plan in SQL?

TointerpretaSQLqueryexecutionplaneffectively,startbyunderstandingthatthephysicalplanrevealsactualdatabaseoperationslikescans,seeks,andjoins;readtheplanbottom-upinmosttools,whereleafoperationsretrievedataandthetopmostoperatorreturnsthefinalresult;focu

Aug 30, 2025 am 06:57 AM

How to delete rows from a table in SQL?

TodeleterowsinSQL,usetheDELETEstatementwithaWHEREclausetospecifywhichrowstoremove;2.AlwaystesttheconditionwithaSELECTqueryfirsttoensureaccuracy;3.Usetransactionstoallowrollbackincaseoferrors;4.UseTRUNCATETABLEonlywhenremovingallrows,asitisfasterbutla

Aug 30, 2025 am 05:56 AM

How to drop a view if it exists in SQL

To delete a view only if it exists, you should use the DROPVIEWIFEXISTS command, which is supported in most modern databases; 1. PostgreSQL: Execute DROPVIEWIFEXISTSmy_view; add CASCADE to delete dependent objects; 2. MySQL: syntax is the same as above, supports CASCADE option; 3. SQLServer2016 and above: Use DROPVIEWIFEXISTSmy_view directly, and old versions need to use IFOBJECT_ID to check; 4.SQLite3.7.11: Support DROPVIEWIFEXISTS; 5.Oracle: does not support IFEXI

Aug 30, 2025 am 03:49 AM

sql view

How to update multiple columns in a single statement in SQL?

Yes, multiple columns can be updated in one SQL statement. The method is to list the columns to be modified in the SET clause of the UPDATE statement and separate them with commas; 1. The basic syntax is: UPDATEtable_nameSETcolumn1=value1,column2=value2WHEREcondition; 2. Any number of columns can be updated, and the values ??can be literals, expressions or subqueries; 3. Use the WHERE clause to accurately define the target rows to avoid mistakenly updating all records; 4. In PostgreSQL and Oracle, support grouping multiple columns in parentheses for subquery updates, such as SET(col1,col2)=(subquery); 5

Aug 30, 2025 am 02:55 AM

How to find the longest and shortest string in a column in SQL?

Tofindthelongestandshorteststringlengths,useMAX(LENGTH(column_name))andMIN(LENGTH(column_name))inSQL,orMAX(LEN())andMIN(LEN())inSQLServer.2.Toretrievetheactuallongestandshorteststrings,useORDERBYLENGTH(column_name)DESCLIMIT1forthelongestandORDERBYLEN

Aug 30, 2025 am 01:39 AM

How to merge two rows into one in SQL?

To merge two rows of data, you need to select methods according to the specific scenario: 1. If you merge rows according to the same ID, use GROUPBY to combine SUM, STRING_AGG and other aggregate functions to merge multiple row values ??into one row; 2. If you need to merge two specific rows (such as login/logout records), you can combine MAX and GROUPBY through CASE statements to realize row conversion; 3. If there is an association relationship between the two rows (such as employee and manager), use self-connection (SELF-JOIN) to merge the two rows into one row; 4. If you need to merge multiple rows of text into a single column string, you can use functions such as STRING_AGG (PostgreSQL), GROUP_CONCAT (MySQL) and other functions. The syntax of different databases is slightly different, so

Aug 30, 2025 am 12:49 AM

SQL Optimistic vs. Pessimistic Concurrency Control

Pessimistic concurrency control is suitable for scenarios with frequent write operations and high conflict probability, such as bank transfer systems, which avoid conflicts by adding locks; optimistic concurrency control is suitable for scenarios with more reads, fewer writes and fewer conflicts, such as e-commerce systems browsing products and detecting conflicts through version numbers. The choice of the two depends on the business scenario: 1. Pessimistic locks are preferred for high-conflict scenarios, pay attention to lock granularity and deadlock issues; 2. Optimistic locks are used in low-conflict scenarios to improve concurrency performance; 3. It can be mixed according to the business module, with pessimistic locks used for critical paths, and optimistic locks used for non-critical paths.

Aug 30, 2025 am 12:39 AM