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Technical Articles

Database

How to select a row with the maximum value in a column in SQL?

Use the subquery method to obtain all rows corresponding to the maximum value of a specific column, which is suitable for all SQL databases and can handle parallelism; 2. Use ORDERBY and LIMIT (or TOP, FETCHFIRST) to quickly return a single row, suitable for scenarios where only one result is needed and performance-oriented; 3. Use window functions such as ROW_NUMBER() and RANK() to achieve more complex logical control, such as partitioning or explicit processing of parallelism; 4. If explicit processing of parallelism is required, it is recommended to use a combination of subquery and MAX() to ensure that all maximum rows are returned. When selecting a method, it should be decided based on whether all parallel rows need to be returned and the database system used. The subquery method is the most common, and ORDERBY plus LIMI

Aug 12, 2025 am 09:04 AM

sql maximum value

What is a sequence in Oracle?

Oracle sequence is a database object used to generate unique numbers, mainly used for primary key value creation; its core mechanism is to obtain the next value through NEXTVAL and CURRVAL get the current value; common uses include primary key generation, audit trail and batch processing; when creating, use CREATESEQUENCE statements and set parameters such as STARTWITH, INCREMENTBY, MAXVALUE, MINVALUE, CYCLE and CACHE; the advantage is that it has high performance, supports concurrent access, and complies with SQL standards; it should be noted that the sequence does not guarantee gap-free, because rollback, cache or failure may cause number jumps. Each NEXTVAL call consumes the sequence value, and cache can improve performance but increase

Aug 12, 2025 am 08:30 AM

How to rename a table in MySQL

To rename MySQL table, you should use the RENAMETABLE statement; its basic syntax is RENAMETABLEold_table_nameTOnew_table_name; for example, RENAMETABLEusers_oldTOusers; 1. Single table renaming: one or more tables can be renamed at a time; 2. Atomic operation: During the renaming period, the table is locked and unavailable until it is completed; 3. Permission requirements: ALTER and DROP permissions are required for the original table, and CREATE and INSERT permissions are required for the database; 4. Storage engine compatibility: supports mainstream engines such as InnoDB and MyISAM; multiple tables can be renamed at once using RENAMETABLE.

Aug 12, 2025 am 08:06 AM

What are some common error codes in MongoDB, and how can they be interpreted?

Common problems with MongoDB error code include connection errors, write errors, authentication errors, and query errors. 1. Connection errors such as Error6 (host is not reached), 7 (host not found), 8 (network timeout), 47 (connection is rejected) are usually related to network configuration or service status. The server operation status, firewall settings and connection parameters need to be checked. 2. Write operation errors such as Error11000 (repeated key error) due to unique index conflicts, Error91 (write attention failed) is related to the replica set confirmation mechanism not satisfied, and the index definition and replica set member status should be verified. 3. Authentication errors such as Error18 (authentication failed) and 13 (unauthorized) originate from credentials or permissions. Username, password and role need to be checked.

Aug 12, 2025 am 07:47 AM

mongodb error code

What are the different types of joins in SQL?

INNERJOINreturnsonlyrowswithmatchingvaluesinbothtables,suchascustomerswhohaveplacedorders;2.LEFTJOINreturnsallrowsfromthelefttableandmatchedrowsfromtheright,includingunmatchedrowswithNULLsontheright,usefulforlistingallcustomersregardlessoforders;3.RI

Aug 12, 2025 am 07:37 AM

How to perform a RIGHT JOIN in SQL

ARIGHTJOINreturnsallrowsfromtherighttableandmatchingrowsfromthelefttable,withNULLsfornon-matchinglefttablecolumns,ensuringtherighttable'sdataisfullyincludedintheresultsetregardlessofmatches,asdemonstratedwhenlistingallcustomersincludingthosewithoutor

Aug 12, 2025 am 07:19 AM

sql

How to return a table from a stored procedure in SQL?

UseaSELECTstatementinsidethestoredproceduretoreturnaresultsetthatbehaveslikeatable,whichissupportedinSQLServer,MySQL,andPostgreSQL.2.InSQLServer,useOUTPUTparameterswithuser-definedtabletypestopasstabulardatabetweenprocedures.3.Utilizeglobaltemporaryt

Aug 12, 2025 am 06:32 AM

Building a REST API with MongoDB

To build a MongoDB-based RESTAPI, you need to follow the following steps: 1. Initialize the Node.js project and install Express, Mongoose, dotenv and other dependencies, create server files and configure middleware and MongoDB connections; 2. Design Mongoose model, such as defining a Book model containing title, author and year of publication; 3. Create a REST route to implement CRUD operations, including obtaining all books, querying according to ID, adding, updating and deleting books, and handling ID searches and errors through middleware; 4. Optionally add input verification, error handling, CORS and rate limiting to enhance security; 5. Use .env file to manage environment variables, match

Aug 12, 2025 am 06:16 AM

mongodb rest api

How to restore a database with RMAN in Oracle

StartthedatabaseinNOMOUNTmodeusingSTARTUPNOMOUNTinRMAN,creatingaPFILEmanuallyiftheSPFILEislost;2.RestorethecontrolfilefrombackuporautobackupusingRESTORECONTROLFILE,thenmountthedatabasewithALTERDATABASEMOUNT;3.RestoredatafileswithRESTOREDATABASE,using

Aug 12, 2025 am 06:07 AM

How to query the size of a table in an Oracle database

To query the size of a table in an Oracle database, you need to summarize the footprint of the table and its related segments (such as indexes, LOBs). The most accurate method is to query the USER_SEGMENTS or DBA_SEGMENTS view; 1. Use USER_SEGMENTS to query the total size of a specific table: SELECTsegment_nameAStable_name,ROUND(SUM(bytes)/1024/1024,2)ASsize_mbFROMuser_segmentsWHEREsegment_name='YOUR_TABLE_NAME'GROUPBYsegment_name; 2.

Aug 12, 2025 am 06:05 AM

oracle database

How to monitor query cache performance in MySQL

CheckQcachestatusvariablesusingSHOWSTATUSLIKE'Qcache%'tomonitorhits,inserts,prunes,andmemoryusage;2.Calculatehitratewith(Qcache_hits/(Qcache_hits Qcache_inserts))*100,aimingfor70–80%orhigher;3.AssessmemoryefficiencybyanalyzingQcache_free_memoryandfra

Aug 12, 2025 am 05:55 AM

How to list all users in MySQL

TolistallusersinMySQL,executeSELECTUser,HostFROMmysql.user;afterlogginginwithsufficientprivileges.2.Fordetailedinformationsuchasauthenticationpluginandaccountstatus,useSELECTUser,Host,authentication_string,plugin,account_locked,password_expiredFROMmy

Aug 12, 2025 am 05:52 AM

What is a transaction in Redis and what are the MULTI/EXEC commands?

Redis transactions implement atomic operations through MULTI and EXEC commands, ensuring that multiple commands are executed in sequence and are not interfered by other clients. When using it, send MULTI to start the transaction first, then the sent command enters the queue without executing it immediately, and finally sends EXEC to execute all commands. If a command error occurs in a transaction, it will not roll back, and all commands will still be executed in turn. Suitable for scenarios where multiple keys need to be updated synchronously or ensure operational continuity, such as inventory deduction and sales count updates. Complex rollback logic is not supported, but more complex functions can be implemented through Lua scripts. Common precautions include: no pre-check, errors are exposed only during execution, and retry may be caused by WATCH conflicts under high concurrency.

Aug 12, 2025 am 05:18 AM

How to export results with column headers?

Exporting results with column headers requires adjustments to the settings according to different tools to ensure that the title is included. The specific operations are as follows: 1. When exporting CSV in Excel, the title is included by default, but you need to confirm the title of the first behavior in the data area, and check the option "Use the first line as the title"; 2. Databases such as MySQL command line export automatically has titles, PostgreSQL needs to add HEADER keyword, and the graphical interface tool needs to check "Includeheaders"; 3. Pandas in Python exports the title by default, and when reading, you need to specify header=0 to avoid losing the title; 4. BI tools are usually automatically included according to the view title, and you can choose whether to include the title when exporting. Just make sure the source data has a title and is correctly configured to export

Aug 12, 2025 am 05:05 AM

Column header 導(dǎo)出結(jié)果