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Home

Technical Articles

Database

SQL

How to create a new table in SQL?

The key to creating a new SQL table is to master the basic syntax and field definitions. 1. Use the CREATETABLE statement and specify the table name, such as CREATETABLEusers(idINT, nameVARCHAR(50)); 2. Fields need to define appropriate data types, note that different database system types may be named differently; 3. It is recommended to use lowercase underscore style to name tables and fields; 4. In most cases, the primary key should be set, MySQL uses AUTO_INCREMENT, and PostgreSQL uses SERIAL; 5. NOTNULL, DEFAULT, UNIQUE and other constraints can be added to prevent dirty data; 6. Reasonably design fields and complete constraints for subsequent management and query

Jun 30, 2025 am 12:50 AM

How to find the length of a string in a SQL column?

A common way to query the length of a string field in SQL is to use the LENGTH() or LEN() function, depending on the database system. 1.MySQL and PostgreSQL use the LENGTH() function to return byte length. If the number of characters is required, CHAR_LENGTH() can be used; 2.SQLServer uses LEN() to return the number of characters (excluding the tail space), if the space is required, DATALENGTH()/2 can be used; 3. Oracle's LENGTH() returns the number of characters, but depends on the character set settings. In addition, specific length data can be filtered in combination with WHERE, CASEWHEN classification lengths, and NULL values ??can be processed with COALESCE(). No

Jun 30, 2025 am 12:38 AM

sql string length

How to handle division by zero errors in SQL?

There are many ways to deal with the problem of dividing by zero in SQL queries, and choose the appropriate solution according to the database type and requirements. 1. Use the CASE statement to determine whether the denominator is zero and return NULL or default value; 2. Use the NULLIF function to simplify the judgment logic, and return NULL when the denominator is zero; 3. Use the COALESCE function to set the default value when the division result is NULL; 4. Filter out invalid data in advance through the WHERE clause in the view or report to improve performance. These methods can be used individually or in combination to ensure safe and stable execution of queries.

Jun 30, 2025 am 12:26 AM

How do you implement pagination in a SQL query using LIMIT and OFFSET?

ToimplementpaginationinSQLusingLIMITandOFFSET,useLIMITtodefinethenumberofrecordsperpageandOFFSETtospecifythestartingpoint.1.CalculateOFFSETas(page_number-1)*page_sizefordynamicqueries.2.AlwaysincludeORDERBYonindexedcolumnstomaintainconsistentandeffic

Jun 30, 2025 am 12:14 AM

What does it mean to normalize a database in SQL?

NormalizingadatabaseinSQLmeansorganizingdatatoreduceredundancyandimproveintegritybystructuringtablessoeachpieceofinformationisstoredonlyonce.1)Eliminateredundantdatabyseparatingrepeatedinformationintotheirowntables,suchasmovingproductdetailsfromanord

Jun 29, 2025 am 12:55 AM

How to find the number of days between two dates in SQL?

Calculating the number of days between two dates in SQL can be achieved through built-in functions of different databases. 1. MySQL and SQLServer use DATEDIFF(end_date, start_date) function to obtain the difference in the number of days, and the result is end-decreasing start. If the positive value is required, you can use ABS() to wrap it; 2. PostgreSQL directly subtracts the two date fields into date types and directly subtracts (end_date::date-start_date::date) to avoid partial interference of time; 3. Oracle can use TRUNC(end_date)-TRUNC(start_date) to remove partial shadow of time

Jun 29, 2025 am 12:45 AM

What is a Common Table Expression (CTE) in SQL?

CTE (public table expression) is a temporary result set that improves the readability of SQL queries and supports recursive queries. The difference between it and subqueries is better readability, reusability and recursive support. CTE can be referenced multiple times in the same query through the WITH keyword definition, which is suitable for scenarios such as splitting complex queries, creating recursive queries, and avoiding temporary tables. When using it, you should pay attention to clear naming, avoid excessive chain structures, and make reasonable use of commentary logic. Although CTE does not have cross-session reuse capabilities and its performance is similar to subqueries, it has obvious advantages in organizational logic and improving code maintenance.

Jun 29, 2025 am 12:27 AM

sql CTE

What is a self join in SQL?

AselfjoininSQLisusedtojoinatabletoitself,enablingthecomparisonorcombinationofrowswithinthesametable.Thistechniqueisparticularlyusefulforhandlinghierarchicaldatasuchasemployee-managerrelationships,threadedcomments,orcategorytrees.1.Itinvolvesassigning

Jun 29, 2025 am 12:21 AM

How to handle special characters like single quotes in a SQL string?

The most efficient way to deal with single quotes in SQL strings is to use parameterized queries or escape single quotes correctly. 1. Parameterized query is a best practice. It separates data from SQL code through placeholders (such as %s), automatically handles special characters and prevents SQL injection; 2. When it must be processed manually, add a single quote before each single quote in the string for escape; 3. When manual escape is implemented in different programming languages ??or scripts, the corresponding method should be used to replace single quotes with double single quotes, and ensure the logic is correct to avoid syntax errors and security vulnerabilities.

Jun 28, 2025 am 12:28 AM

What is the SQL LIKE Operator, and How Do I Use It Effectively in My Queries?

SQLLIKEoperatorisusedforpatternmatchinginstrings.1)Itallowssearchingforpatternsusing'%'foranynumberofcharactersand'_'forasinglecharacter.2)ItcanbecombinedwithotherSQLclauseslikeORDERBYforsortingresults.3)Forexactmatches,'='ismoreefficientthanLIKE.4)P

Jun 28, 2025 am 12:27 AM

How does the ROW_NUMBER() window function work in SQL?

ROW_NUMBER() is used in SQL to assign a unique sequence number to each row of the result set, and is often used for sorting, paging, and deduplication. Its basic syntax is ROW_NUMBER()OVER(ORDERBYcolumn_name), which can be combined with PARTITIONBY to realize group sorting, such as grouping by department and sorting in descending order of salary. Common uses include pagination query, deletion of duplicate records, and Top-N analysis. Note when using: 1. ORDERBY must be specified to ensure that the results are predictable; 2. ROW_NUMBER() cannot be directly referenced in the WHERE clause, subquery or CTE should be used; 3. No parallel ranking is processed, each row has a unique number; 4. Obtain the first row of each group and needs to be concluded.

Jun 28, 2025 am 12:06 AM

SQL window functions

What is the difference between a function and a stored procedure in SQL?

FunctionsreturnavalueandareusedinSQLstatementsforcalculations,whilestoredproceduresperformactions,canmodifythedatabase,andareexecutedseparately.Functionsareidealforcomputingvalueswithinqueries,whereasstoredprocedureshandlecomplexlogicanddatamanipulat

Jun 28, 2025 am 12:03 AM

stored procedure sql function

How to change the data type of a column in a SQL table?

Modifying the data type of a column in SQL requires selecting the corresponding method according to the database system. 1. SQLServer uses the ALTERCOLUMN statement to modify it directly, such as: ALTERTABLEemployeesALTERCOLUMNsalaryDECIMAL(10,2); 2. PostgreSQL needs to add USING clause to deal with incompatible types, such as: ALTERTABLEusersALTERCOLUMNageTYPEVARCHAR(10)USINGage::VARCHAR; 3. The general solution is to add new columns and migrate data before replacing them, such as adding amount_new columns, updating data, deleting old columns and re-replacing them.

Jun 28, 2025 am 12:01 AM

What are the differences between OLTP and OLAP?

OLTPfocusesonreal-timetransactions,whileOLAPisdesignedfordataanalysis.1)OLTPensuresfastandreliableoperationsinsystemslikebankingande-commerce.2)OLAPusescomplexqueriestouncovertrendsandinsightsinbusinessintelligencetools.

Jun 27, 2025 am 01:33 AM

OLTP OLAP