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Home

Technical Articles

Database

How to rename a table in SQL

To rename SQL tables, different syntaxes need to be used according to the database system: 1. In MySQL, PostgreSQL and SQLite, use ALTERTABLEold_table_nameRENAMETOnew_table_name; 2. In SQLServer, use EXECsp_rename'old_table_name','new_table_name'; before the operation, you need to ensure that you have appropriate permissions, check and update dependent objects such as views or stored procedures. It is recommended to test and back up the data in the development environment before executing to avoid production environment problems.

Sep 08, 2025 am 04:37 AM

What is the purpose of the HAVING clause in MySQL?

HAVING is used to filter grouped data after GROUPBY, especially when the conditions involve aggregation functions such as COUNT and SUM; for example, if you look for departments with more than 5 employees, you need to use HAVINGCOUNT(*)>5; unlike WHERE, WHERE filters a single row before grouping and cannot use an aggregate function, HAVING filters after grouping and supports an aggregate function; the two can be used in combination, such as first using WHERE to screen employees with a salary of more than 30,000, then group them by department, and finally use HAVING to screen departments with an average salary of more than 50,000, so as to achieve effective filtering of the aggregated data and fully support complex query needs.

Sep 08, 2025 am 04:26 AM

mysql HAVING子句

How to create an index in MySQL

Creating indexes can improve query performance. It should be created on columns frequently used in WHERE, JOIN, ORDERBY or GROUPBY; 2. Use CREATEINDEXindex_nameONtable_name(column_name) to create a single column index; 3. Use composite index for multi-column joint queries, with the syntax CREATEINDEXindex_nameONtable_name(column1,column2), note that the column order affects index usage; 4. The index can be directly defined when CREATETABLE; 5. Use CREATEUNIQUEINDEX to ensure that the column values ??are unique and prevent duplication; 6. Pass A

Sep 08, 2025 am 04:19 AM

How to check tablespace size in Oracle

To check the Oracle tablespace size, SQL query must be performed using the data dictionary view; 1. Pass SELECTdf.tablespace_name,ROUND(SUM(df.bytes)/1024/1024/1024,2)AStotal_size_gb,ROUND(SUM(NVL(fs.bytes,0))/1024/1024/1024,2)ASfree_space_gb,ROUND((SUM(df.bytes)-SUM(NVL(fs.bytes,0)))/1024/1024/1024,2)ASused_space_gb,

Sep 08, 2025 am 03:54 AM

How does the LATENCY DOCTOR command work?

TheLATENCYDOCTORcommandisadiagnostictoolusedtoidentifyandanalyzesourcesoflatencyinreal-timesystems.1)ItcheckssubsystemslikeCPUscheduling,memoryaccess,networkstack,andhardwaredriversforbottlenecks.2)Theoutputincludessectionswithlatencyscoresindicating

Sep 08, 2025 am 03:24 AM

How to add a column to a table in MySQL?

To add columns to an existing table, you need to use the ALTERTABLE statement to the ADDCOLUMN clause; for example, ALTERTABLEusersADDCOLUMNemailVARCHAR(100) can add email columns to the users table; you can add constraints such as NOTNULL and DEFAULT at the same time, such as automatically recording time when creating a time column; you can specify the location of the column through FIRST or AFTER; support adding multiple columns at a time to improve efficiency; be careful when operating large tables to avoid locking tables affecting performance, and backup data should be backed up before modification. If you add non-empty columns, the default value must be set to prevent the operation from failing.

Sep 08, 2025 am 03:23 AM

mysql database

How to calculate the distance between two points using GEODIST?

GEODIST is a function used to calculate the straight line distance between two points on the earth. Before using it in Stata, it is required to install: sscinstallgeodist. 1. When using it, you need to specify the latitude and longitude parameters of the two points, such as geodistlat1lon1lat2lon2,gen(distance_km) to generate a unit distance of kilometers; 2. If you need a unit of miles, add the mile option; 3. Pay attention to ensuring that the latitude and longitude is in the decimal system format, the range is legal and the order of variables is correct; 4. During batch calculation, you can copy the reference point to all lines and execute the command. This method is suitable for store location selection, traffic analysis and other scenarios.

Sep 08, 2025 am 03:06 AM

distance calculation GEODIST

What is a NATURAL JOIN in MySQL?

ANATURALJOINinMySQLautomaticallyjoinstablesbasedoncolumnswiththesamenameandcompatibledatatypes,returningonlyonecopyofeachcommoncolumn;itrequiresnoexplicitONorUSINGclause,makingitconcisebutriskyduetoimplicitbehaviorthatcanleadtounexpectedresultsifsche

Sep 08, 2025 am 03:04 AM

mysql join

How to reconnect to a database?

The database connection suddenly disconnects can be checked according to the following steps: 1. Check whether the database service is running, use the systemctl or pg_ctl command for Linux, use the systemctl or pg_ctl command for Windows, and view the service manager for cloud database to view the console status of the cloud database; 2. Make sure that the connection parameters are correct, including the host address, port, user name, password and database name; 3. Check the network and permissions to ensure that the firewall releases the port, the user has remote access rights and correctly configure SSL; 4. Use the telnet or nc command for testing network connectivity; 5. Restart the application or reset the connection pool to clear the old connection.

Sep 08, 2025 am 02:56 AM

How to Use Partial Indexes to Optimize Queries in MongoDB

PartialindexesinMongoDBimproveperformancebyindexingonlydocumentsthatmatchaspecifiedcondition,reducingstorageandenhancingqueryspeed.TheyarecreatedusingthepartialFilterExpressionoption,suchasindexingemailsonlyforactiveusers:db.users.createIndex({email:

Sep 08, 2025 am 02:25 AM

mongodb

How to use user-defined variables in MySQL

User-defined variables start with @, and are assigned through SET or :=, which are only valid in the current session. They can be used to store values, simulate line numbers, etc. 1. Set variables using SET or SELECT; 2. Reference variables in subsequent statements; 3. Pay attention to the session scope and initialization, avoid undefined use, and finally end with a complete statement.

Sep 08, 2025 am 02:19 AM

How to connect to MySQL using Node.js

Install the mysql2 package: Run npminstallmysql2 to obtain a MySQL client with better performance and support Promise; 2. Create a connection: Use the mysql.createConnection() method and pass in the correct host, user name, password and database name to establish a connection; 3. Execute query: execute SQL statements through the connection.query() method and process the results; 4. Close the connection: Call the connection.end() method to safely close the connection; 5. Use Promise: Introduce the 'mysql2/promise' module to support async/await syntax to improve the readability of asynchronous operations

Sep 08, 2025 am 02:10 AM

How to Create and Manage Indexes in MongoDB

Indexesimprovequeryperformancebyreducingdocumentscans;createthemonfrequentlyqueriedfieldsusingcreateIndex(),monitorwithgetIndexes()andexplain(),andremoveunusedonestomaintainefficiency.

Sep 08, 2025 am 01:46 AM

How to trim whitespace from a string in SQL

UseTRIM()toremovebothleadingandtrailingwhitespace,LTRIM()forleadingonly,andRTRIM()fortrailingonly;thesefunctionsarewidelysupportedinPostgreSQL,MySQL,Oracle,SQLServer(2017 ),andrecentSQLite,withSQLServerbefore2017requiringLTRIM(RTRIM())andsomedatabase

Sep 08, 2025 am 01:36 AM

sql String processing