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Technical Articles

Database

How to escape single quotes in SQL?

To correctly handle single quotes in SQL, you need to represent single quotes in strings with two single quotes, such as 'O''Connor', which is the standard method for most databases; 1. Use two single quotes in strings to represent a literal single quote; 2. Avoid relying on backslash escapes because their behavior depends on database schema settings; 3. The best practice is to use parameterized queries, and the database driver automatically and safely handle escapes, prevent SQL injection and simplify code.

Aug 07, 2025 pm 06:53 PM

Troubleshooting MySQL Storage Space Issues

If MySQL storage space is insufficient, you need to confirm the disk usage first and then handle it in a targeted manner. 1. Use df-h to check whether the /var/lib/mysql partition is fully loaded; 2. Locate a database that takes up a large space through SQL query; 3. Further query specific large tables and consider archiving or cleaning; 4. View and clean log files, such as binary logs; 5. Configure expire_logs_days to achieve automatic cleaning; 6. Execute OPTIMIZETABLE to release the free space after deleting data; 7. Regularly clean temporary tables, historical logs and useless data; 8. Use partition tables and slice them by time; 9. Reasonably set up automatic cleaning strategies; 10. Regularly check storage status and plan expansion in advance.

Aug 07, 2025 pm 06:14 PM

mysql storage

What is the difference between a primary key and a unique key in SQL?

PrimarykeydoesnotallowNULLvalues,whileuniquekeyallowsoneNULL;2.Atablecanhaveonlyoneprimarykeybutmultipleuniquekeys;3.Primarykeyautomaticallycreatesaclusteredindex,whereasuniquekeycreatesanon-clusteredindex;4.Primarykeyisusedforidentifyingrowsandinfor

Aug 07, 2025 pm 06:01 PM

How to calculate percentile in SQL?

UsePERCENTILE_CONT(p)forexactpercentilesinPostgreSQL,SQLServer,Oracle,andMySQL8.0 ;2.UseAPPROX_PERCENTILE(col,p)forapproximateresultsinBigQueryandSpark;3.Fordatabaseswithoutbuilt-insupport,manuallycalculatepercentilesusingROW_NUMBER(),COUNT(),andinte

Aug 07, 2025 pm 05:46 PM

How do you build and execute a dynamic SQL statement safely?

Dynamic SQL can be safely executed using parameterized queries, validating whitelists, the principle of minimum permissions, and avoiding string splicing. 1. Always use parameterized execution (such as sp_executesql) to pass user data; 2. Use whitelist verification for table names, column names, etc. that cannot be parameterized and safely referenced with functions such as QUOTENAME(); 3. Limit database account permissions to reduce the impact of potential attacks; 4. Prohibit directly splicing users to input into SQL strings; 5. Priority is given to using static queries or views instead of dynamic SQL. All user input should be considered untrusted and secured through defense in depth.

Aug 07, 2025 pm 05:41 PM

How can you audit database activity in SQL?

Usebuilt-indatabaseauditfeatureslikeSQLServerAuditforSQLServer,pgAuditforPostgreSQL,andtheAuditLogPluginforMySQLtotrackuseractionsandstorelogssecurely.2.EnableSQLServerordatabaselogssuchaserrorlogsorgeneralquerylogstomonitoractivity,butlimituseofhigh

Aug 07, 2025 pm 05:28 PM

sql 數(shù)據(jù)庫審計(jì)

phpMyAdmin manage relations between tables

EnsuretablesusetheInnoDBstorageenginebycheckingtheOperationstabandselectingInnoDB.2.CreateforeignkeyconstraintsviatheRelationviewintheStructuretabbylinkingacolumn(e.g.,user_id)toareferencedcolumninanothertable(e.g.,users.id),settingONUPDATE/ONDELETEr

Aug 07, 2025 pm 05:19 PM

Can you explain the role of Oracle background processes like PMON, SMON, DBWn, LGWR, and CKPT?

Key background processes of Oracle database include PMON, SMON, DBWn, LGWR, and CKpt. PMON cleans up failed user processes, rolls back uncommitted transactions, frees resources and registers instance information; SMON is responsible for instance recovery, applies redo logs, and merges free space; DBWn writes dirty data in the buffer to data files, batch processing to reduce I/O; LGWR writes the redo log buffer to disk to ensure that transaction security records are submitted; CKpt coordinates checkpoints, notifies DBWn to write data and updates checkpoint information. These processes jointly ensure the efficient operation of the database and the consistency of data.

Aug 07, 2025 pm 04:59 PM

oracle Background process

What are gap locks and next-key locks in InnoDB for MySQL?

Gaplockslockindexgapstopreventinsertions,next-keylockscombinerecordandgaplockstoblockinsertsandmodifications;1.GaplocksapplytorangesbetweenindexvaluesandpreventphantomreadsbyblockinginsertsinREPEATABLEREAD;2.Next-keylockslockbotharecordandthegapbefor

Aug 07, 2025 pm 04:54 PM

How to grant privileges to a user in Oracle

UseGRANTprivilege_nameTOusernameforsystemprivilegeslikeCREATESESSIONorCREATETABLE.2.UseGRANTobject_privilegeONschema.objectTOusernameforobjectprivilegessuchasSELECTorINSERTonspecifictables.3.GrantroleswithGRANTrole_nameTOusernametosimplifyprivilegema

Aug 07, 2025 pm 04:39 PM

What are the BLOB and CLOB data types in SQL?

BLOBstoresbinarydatalikeimages,audio,orPDFsasrawbyteswithoutcharacterencoding,whileCLOBstoreslargetextsuchasarticlesorJSONusingcharacterencodinglikeUTF-8andsupportsstringoperations;2.Bothcanhandleuptogigabytesofdatadependingonthedatabase,butperforman

Aug 07, 2025 pm 04:22 PM

sql

How to get a random row from a table in MySQL efficiently?

ORDERBYRAND() should be avoided for large tables, 1. Small table (=(SELECTFLOOR(RAND()(MAX(id)-MIN(id) 1)) MIN(id)FROMyour_table)ORDERBYidLIMIT1; 3. The better solution is to generate a random ID and find the first record greater than or equal to the ID, and cooperate with the retry mechanism to deal with the ID gap; 4. Pre-computed random columns with indexes can be added in high-performance scenarios, such as ALTERTABLEyour_tableADDCOLUMNrand_colFLOAT, CREATEINDEXidx_randONyour_table(rand)

Aug 07, 2025 pm 04:00 PM

How to empty a table in phpMyAdmin

ToemptyatableinphpMyAdmin,usethe"Empty"(TRUNCATE)optionbyselectingthetable,goingtothe"Operations"tab,andclicking"Go"inthe"Emptythetable(TRUNCATE)"section;2.Alternatively,runacommandintheSQLtabbyenteringTRUNCATE

Aug 07, 2025 pm 03:23 PM

How to increase the max_connections value in MySQL

Tosafelyincreasemax_connectionsinMySQL,firstcheckthecurrentvalueusingSHOWVARIABLESLIKE'max_connections';thentemporarilysetanewvaluewithSETGLOBALmax_connections=500;fortesting.Tomakeitpermanent,edittheMySQLconfigfile(e.g.,/etc/mysql/my.cnformy.ini)und

Aug 07, 2025 pm 01:31 PM