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Technical Articles

Database

SQL

How to use the CAST function in SQL?

CAST function is used to convert values from one data type to another. When using it, you need to ensure data compatibility. 1. You can convert a string to an integer. For example, CAST('123'ASINT) result is 123, but you need to verify the data first to avoid errors; 2. You can convert numbers to strings for splicing, such as 'OrderID:' CAST(order_idASVARCHAR(10)); 3. You can convert a string to a date, such as CAST('2024-05-20'ASDATE); 4. When converting a decimal to an integer, it will be truncated instead of rounding, and you need to use the ROUND function to achieve rounding; 5. You can convert between numeric types to adjust the accuracy, such as CAST(123.456ASDEC

Aug 18, 2025 am 02:04 AM

How to use the HAVING clause in SQL

HAVINGisusedtofiltergroupsafteraggregation,unlikeWHEREwhichfiltersrowsbeforegrouping;1.UseHAVINGwithGROUPBYtoapplyconditionsonaggregatefunctionslikeSUM,COUNT,orAVG;2.PlaceHAVINGafterGROUPBY,anduseWHEREbeforeGROUPBYfornon-aggregatefilters;3.Avoidusing

Aug 18, 2025 am 01:32 AM

sql

How to fetch a specific range of rows in SQL

TofetchaspecificrangeofrowsinSQL,useLIMITandOFFSETinMySQL,PostgreSQL,andSQLite,orOFFSETFETCHinSQLServerandOracle;forexample,toretrieverows6to10,applyORDERBYemployee_idLIMIT5OFFSET5inMySQLorOFFSET5ROWSFETCHNEXT5ROWSONLYinSQLServer,alwaysensuringconsis

Aug 18, 2025 am 01:29 AM

sql 行范围

How to format numbers into currency in SQL?

UseFORMAT()inSQLServerandMySQL8.0 with'C'specifierorCONCATforcurrencysymbol;2.UseTO_CHAR()inPostgreSQLandOraclewith'L999G999D99'formatmodelforlocale-awarecurrencyformatting;3.InolderSQLServer,combineCONVERT()withMONEYtypeandstylecode1,prefixedby'$';4

Aug 18, 2025 am 01:19 AM

How to use the DENSE_RANK function in SQL

DENSE_RANK()assignsrankswithoutgapswhentiesoccur,meaningtiedrowsreceivethesamerankandthenextrankfollowsimmediately;forexample,withvalues100,100,90,DENSE_RANK()outputs1,1,2,makingitidealforcontinuousrankingsinleaderboards,top-Npergroup,ordetectingties

Aug 18, 2025 am 12:42 AM

How to disable and enable constraints in SQL?

InSQLServer,useALTERTABLETableNameNOCHECKCONSTRAINTtodisableandALTERTABLETableNameWITHCHECKCHECKCONSTRAINTtoenableconstraints,withALLtoapplytoallconstraints.2.InOracle,useALTERTABLETableNameDISABLECONSTRAINTandALTERTABLETableNameENABLECONSTRAINT,opti

Aug 17, 2025 am 09:13 AM

How to find the average value of a column in SQL?

TofindtheaveragevalueofacolumninSQL,usetheAVG()function.1.UseSELECTAVG(column_name)FROMtable_nametocalculatethemeanofnon-NULLvalues.2.UseanaliaslikeASaverage_amountforaclearerresultlabel.3.AVG()automaticallyignoresNULLvalues,ensuringonlyvalidnumerice

Aug 17, 2025 am 09:00 AM

sql average value

How to prevent SQL injection in your queries

Useparameterizedqueries(preparedstatements)toseparateSQLlogicfromdata,ensuringuserinputistreatedasdata,notexecutablecode.2.Validateandsanitizeinputbycheckingdatatypes,limitinglength,usingallowlists,andrejectingsuspiciouspatterns.3.Usestoredprocedures

Aug 17, 2025 am 08:49 AM

Safety sql injection

How to use the CONVERT function to convert data types in SQL

TheCONVERTfunctioninSQLServerisusedtochangeanexpressionfromonedatatypetoanother,withsupportforformattingthroughoptionalstylecodes;itiscommonlyappliedforconvertingstringstodates(e.g.,CONVERT(DATE,'2024-03-15')),formattingdatesasstrings(e.g.,CONVERT(VA

Aug 17, 2025 am 08:47 AM

sql

How to import and export data in SQL?

Use SQL commands such as INSERT and SELECT to process the import and export of small datasets, but cannot be directly exported to files; 2. Most databases support exporting as CSV or text files, MySQL uses INTOOUTFILE and LOADDATAINFILE, PostgreSQL uses COPY, SQLite uses .output and .import in shell; 3. Command line tools such as mysqldump, pg_dump and bcp are suitable for large data volumes, efficient and flexible; 4. GUI tools such as phpMyAdmin, pgAdmin and SSMS provide visual operations, suitable for manual tasks but are not easy to automate; 5. Best practices include guidance

Aug 17, 2025 am 08:38 AM

How to convert an IP address to a number and back in SQL?

To convert the IP address to a number and convert it back in SQL, 1. Use INET_ATON() to convert the IP to a number, such as SELECTINET_ATON('192.168.1.1') to get 3232235777; 2. Use INET_NTOA() to convert the number back to the IP, such as SELECTINET_NTOA(3232235777) to get '192.168.1.1'; For unsupported databases, it can be implemented through manual arithmetic or bit operations, and BIGINT storage should be used to ensure capacity. In the end, it is recommended to use built-in database functions and be compatible with manual methods.

Aug 17, 2025 am 08:21 AM

Unpivoting columns into rows using SQL techniques.

A common method for column-to-line conversion in SQL is to use UNIONALL or CROSSAPPLY. 1. When using UNIONALL, it is suitable for scenarios with small columns and fixed structures. Each column of data is extracted separately through multiple SELECT statements and merged results with UNIONALL. However, it is necessary to note that the number of fields and types are consistent, and the statements are lengthy when there are many columns; 2. Use CROSSAPPLY to be suitable for databases that support this syntax (such as SQLServer). Dynamically construct row data through VALUES clauses, and the code is more concise and easy to expand; precautions include performance testing, unified data type, and preservation of related information such as original ID, which is common in data analysis scenarios such as converting wide tables into long tables or generating dynamic reports.

Aug 17, 2025 am 08:12 AM

How to use the EXISTS operator in SQL

EXISTSinSQLchecksifasubqueryreturnsanyrows,returningTRUEorFALSEaccordingly;itisusedinaWHEREclausetofilterbasedontheexistenceofrelateddata,withthebasicsyntax:SELECTcolumn_name(s)FROMtable1WHEREEXISTS(SELECT1FROMtable2WHEREcondition);itisefficientbecau

Aug 17, 2025 am 05:56 AM

sql exists

How to select rows with a NULL value in a specific column in SQL?

To find rows with a column value of NULL, the ISNULL condition must be used; because NULL means unknown or missing values, the standard comparison operator such as =NULL cannot be judged correctly, so the SELECTcolumn1, column2FROMtable_nameWHEREcolumn_nameISNULL syntax should be used; for example, SELECT*FROMemployeesWHEREdepartmentISNULL can return employees whose department is NULL; note that =NULL cannot be used, because its result in SQL is unknown rather than true value. The correct way is to use ISNULL or ISNOTNULL to judge.

Aug 17, 2025 am 04:15 AM

sql null value