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Technical Articles

Database

How to use the NOT operator in MySQL

In MySQL, the NOT operator is used to invert the result of logical expressions. 1. The basic syntax is SELECTcolumn1, column2FROMtable_nameWHERENOTcondition, such as querying non-German customers; 2. Use NOTIN to exclude values in the list, but it should be noted that if the list contains NULL, the expected result may not be returned; 3. Use NOTBETWEEN to exclude values in a certain range; 4. Use NOTLIKE to find records that do not match the specified pattern; 5. When combining AND/OR, brackets should be used to clarify the logical priority to avoid errors caused by operator priority. At the same time, beware of the impact of NULL value on NOT operations. Finally, you must be vigilant about the impact of NULL value on NOT operations.

Aug 11, 2025 pm 07:16 PM

How to describe a table in MySQL

To describe the MySQL table structure, you can use the DESCRIBE or DESC commands; 1. Use DESCRIBEtable_name or DESCtable_name to view the field name, data type, whether NULL, key type, default value and extra attributes are allowed; 2. Use SHOWCREATETABLEtable_name to obtain complete table building statements containing primary keys, unique keys, storage engines and character sets; 3. Query the INFORMATION_SCHEMA.COLUMNS table to obtain detailed metadata such as column annotations, permissions, etc., and select the appropriate method according to your needs to complete a comprehensive understanding of the table structure.

Aug 11, 2025 pm 07:05 PM

MySQL EXPLAIN Output Interpretation

To analyze MySQL query performance, it is key to use the EXPLAIN command to view the execution plan of the SELECT statement. 1. Check the type field to determine the access type, give priority to system/const, and avoid ALL or index; 2. Combine key and rows to confirm whether the index hits and the number of scanned rows; 3. Check whether there are any performance-affected problems such as Usingfilesort or Usingtemporary in the Extra column; 4. Analyze possible_keys and key_len to understand the available and actual used indexes, so as to determine whether the index is fully utilized. Mastering these key points can effectively locate and optimize SQL query performance problems.

Aug 11, 2025 pm 07:00 PM

mysql explain

How to revoke privileges from a user in MySQL

TorevokeprivilegesfromauserinMySQL,usetheREVOKEstatement;1.Specifytheprivilegetype(e.g.,SELECT,INSERT,ALLPRIVILEGES);2.Definethescopeusingdatabase_name.table_name(e.g.,mydb.usersormydb.*);3.Identifytheexact'username'@'host'account;4.OptionallyrevokeG

Aug 11, 2025 pm 06:34 PM

SQL Server Agent Jobs for Automation

To create a basic SQLServerAgent job, 1. Open SSMS, connect to the database instance, expand SQLServerAgent, right-click Jobs, and select New job; 2. Fill in the name and description on the "General" page; 3. Add T-SQL or CmdExec and other execution steps on the "Step" page; 4. Set the execution frequency on the "Scheduling" page, such as daily, weekly, or custom cycles; 5. Test the job logic to ensure there is no syntax and permissions problem; 6. Set the failure notification method, such as email, event log or alarm, and decide whether to continue to execute the next step after failure; at the same time, pay attention to details such as server time, job owner permissions, step type selection, and output log path.

Aug 11, 2025 pm 06:17 PM

How to get the first and last day of the year in SQL?

ThefirstdayoftheyearisobtainedbyconstructingortruncatingtoJanuary1stofthegivenyear,andthelastdayisDecember31stofthesameyear,withmethodsvaryingbydatabasesystem;2.Fordynamiccurrentyeardates,MySQLusesDATE_FORMATorMAKEDATE,PostgreSQLusesDATE_TRUNCorDATE_

Aug 11, 2025 pm 05:42 PM

sql date

What is the character set and collation in MySQL?

Acharactersetdefineswhichcharacterscanbestored,whileacollationdetermineshowtheyarecomparedandsorted.1.Charactersetslikeutf8mb4supportfullUnicode,includingemojisandinternationalcharacters.2.Collationssuchasutf8mb4_0900_ai_ciproviderulesforcase-insensi

Aug 11, 2025 pm 05:22 PM

mysql character set

How to find the intersection of two tables in SQL?

UsetheINTERSECToperatorifsupported(e.g.,PostgreSQL,SQLServer,Oracle)todirectlyreturnuniquerowspresentinbothtableswithmatchingcolumnsandautomaticduplicateremoval.2.SimulateINTERSECTusingINNERJOINbyexplicitlycomparingallrelevantcolumnsandwrappingwithDI

Aug 11, 2025 pm 05:16 PM

How to get the row count for all tables in a database in SQL?

ForSQLServer,usesys.dm_db_partition_statstogetaccuraterowcountsfrommetadatawithoutscanningtables.2.ForMySQL,queryinformation_schema.TABLESforestimatedrowcounts,butuseCOUNT()pertableforexactnumbers.3.ForPostgreSQL,usepg_stat_user_tablesforestimatedcou

Aug 11, 2025 pm 04:50 PM

sql Row count

How to check the execution plan of a query in Oracle

UseEXPLAINPLANFORtogenerateanestimatedexecutionplanwithoutrunningthequery,thenqueryDBMS_XPLAN.DISPLAYtoviewitinreadableformat.2.EnableAutotraceinSQL*PlusorSQLDeveloperwithSETAUTOTRACEONtoseeboththeexecutionplanandruntimestatisticsafterqueryexecution.

Aug 11, 2025 pm 04:37 PM

oracle Implementation plan

How to work with XML data in SQL

StoreXMLinXML-typecolumnsandquerywithXQuerymethods:use.value()toextractscalars,.query()forXMLfragments,.nodes()toshredintorows,and.exist()forconditionalfiltering;2.ModifyXMLusing.modify()inSQLServerwithXQueryDMLforinsertionsordeletions;3.GenerateXMLf

Aug 11, 2025 pm 04:28 PM

xml sql

How to migrate from SQL Server to MySQL

First, clear the answer: Migrating from SQLServer to MySQL requires evaluation, transformation, migration, testing and adjustment in steps. 1. Evaluate the source database, inventory the objects and identify dependency and incompatibility characteristics; 2. Map the data type differences, such as INTIDENTITY to INTAUTO_INCREMENT, NVARCHAR to VARCHAR and set the utf8mb4 character set, DATETIME maintains the corresponding, BIT to TINYINT(1), UNIQUEIDENTIFIER to CHAR(36) or BINARY(16); 3. Through the MySQLWorkbench migration wizard, manual export import or third-party tools such as AWSDMS

Aug 11, 2025 pm 04:09 PM

How to use the WHERE clause in MySQL

TheWHEREclauseinMySQLfiltersrecordsbasedonspecifiedconditions.1.ItisusedinSELECT,UPDATE,andDELETEstatementstoaffectonlymatchingrows.2.Comparisonoperatorslike=,!=,,=allowprecisefiltering.3.LogicaloperatorsAND,OR,andNOTcombinemultipleconditions,requiri

Aug 11, 2025 pm 03:54 PM

How to remove duplicate rows in SQL?

UseSELECTDISTINCTtoretrieveuniquerowswithoutmodifyingthetable.2.UseROW_NUMBER()withaCTEtoidentifyduplicatesanddeleterowswithrn>1,keepingthefirstrowbasedonaspecifiedorder.3.CreateatemporarytablewithDISTINCTdata,droptheoriginal,andrenamethecleantabl

Aug 11, 2025 pm 03:51 PM