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Technical Articles

Database

How to force a query to use a specific index in SQL?

InMySQL,useUSEINDEXorFORCEINDEXtosuggestorenforceanindex.2.InSQLServer,useWITH(INDEX(...))tospecifyanindex.3.PostgreSQLlacksnativeindexhints;settingenable_seqscan=OFFcanforceindexusebutisnotproduction-safe.4.InOracle,use/ INDEX(tableindex_name)/optim

Aug 11, 2025 am 09:11 AM

sql index

How to Master SQL for Data Science: A Comprehensive Learning Path

Master the basic syntax of SQL, including SELECT, FROM, WHERE, ORDERBY, LIMIT, DISTINCT and the use of aggregate functions and GROUPBY and HAVING; 2. Learn table connection operations, focusing on mastering INNERJOIN and LEFTJOIN to achieve multi-table data integration; 3. Proficient in using subqueries and CTE (WITH statements) to process complex logic; 4. Master window functions such as ROW_NUMBER, RANK, LAG/LEAD and SUM/AVGOVER to implement analysis functions; 5. Respond to real data challenges, including NULL value processing, string and date operations, and CASE condition judgment; 6. Optimize query performance and understand indexes

Aug 08, 2025 pm 05:59 PM

How to find the sum of a column in SQL?

TofindthesumofacolumninSQL,usetheSUM()function,whichreturnsthetotalofallnumericvaluesinaspecifiedcolumnwhileignoringNULLs;1.Usebasicsyntax:SELECTSUM(column_name)ASaliasFROMtable_name;2.Ensurethecolumnhasnumericdatatoavoiderrors;3.ApplyWHEREtofilterro

Aug 08, 2025 pm 05:54 PM

sql Sum

What is recursion in SQL and how can you use it with CTEs?

RecursiveSQLqueriesuserecursiveCommonTableExpressions(CTEs)tohandlehierarchicalorrepetitivedata.1.ArecursiveCTEconsistsofananchormemberthatdefinesthebasecase.2.ItincludesarecursivememberthatreferencestheCTEitselftoiterativelyprocessdata.3.Therecursio

Aug 08, 2025 pm 05:53 PM

Stress Testing SQL Applications

The core of stress testing SQL applications is to discover performance bottlenecks in advance and optimize them to ensure that the database runs stably under high loads. 1. It is necessary to simulate real concurrent access, use tools to simulate multiple SQL operations, and discover performance problems under high concurrency; 2. Pay attention to slow query and index use, analyze and optimize inefficient SQL through execution plan and monitoring tools; 3. Check the connection pool configuration and resource release to ensure reasonable allocation and timely release of connections; 4. Simulate fault scenarios to test fault tolerance capabilities, verify exception handling and system stability.

Aug 08, 2025 pm 05:47 PM

How do you limit the number of rows returned in a SQL query?

TolimitrowsinaSQLquery,usetheappropriateclausebasedonyourdatabasesystem:1.ForMySQL,PostgreSQL,andSQLite,useLIMIT10;2.ForSQLServerandMSAccess,useSELECTTOP10;3.ForstandardSQL,IBMDb2,Oracle12 ,andnewerPostgreSQL,useFETCHFIRST10ROWSONLY;4.ForolderOraclev

Aug 08, 2025 pm 05:46 PM

sql 行數(shù)限制

How to use the NTILE window function to create quartiles in SQL?

To create quartiles, use NTILE(4) and sort by target column. This function divides the data into four roughly equal groups, each accounting for 25%; 1. Use NTILE(4)OVER(ORDERBYcolumn_name) to allocate quartiles; 2. You can ensure that the results of the parallel values are consistent by adding secondary sorting keys; 3. Use PARTITIONBY to calculate quartiles separately in different groups; 4. Each group is as large as possible. If the number of rows cannot be divisible by 4, the previous group will have one more row; this method is suitable for databases such as PostgreSQL, SQLServer, Snowflake, and BigQuery, and must contain ORDERBY to ensure that the results are clear.

Aug 08, 2025 pm 05:44 PM

How to schedule events in MySQL to run at specific times?

To use the MySQL event scheduler to schedule events at a specific time, you must first enable the event scheduler, then create one-time or periodic events, and finally manage the events through relevant commands; the specific steps are: 1. Execute SETGLOBALevent_scheduler=ON to enable the event scheduling function; 2. Use CREATEEVENT to specify the specific time to perform SQL operations, such as DELETEFROMlogsWHEREcreated_at

Aug 08, 2025 pm 05:43 PM

scheduled tasks

How to perform string manipulations in SQL?

StringmanipulationinSQLisessentialforcleaning,formatting,andextractingdata,andcommonoperationsinclude:1.ConcatenatingstringsusingCONCAT(), ,orCONCAT_WS();2.ExtractingsubstringswithSUBSTRING()orSUBSTR();3.FindingsubstringpositionsusingPOSITION(),INSTR

Aug 08, 2025 pm 05:42 PM

How to delete rows from a table with a self-referencing foreign key

To safely delete rows in a table with self-referenced foreign keys, the dependency must be processed first. The following three methods can be adopted: 1. If the table structure can be modified, use ONDELETECASCADE to automatically delete all child rows when deleting the parent row, but be careful to prevent accidental deletion of large amounts of data; 2. Manually control the deletion order, first delete the leaf nodes through recursive CTE, and gradually delete upwards, for example, use the WITHRECURSIVE clause to locate the specified node and all subordinates and delete them in batches; 3. Only temporarily disable foreign key checking (SETFOREIGN_KEY_CHECKS=0) in a controlled environment, and re-enable it after operation, but this method has high risk, which may lead to inconsistent data. Best practice is to combine recursive CTE with precise deletion

Aug 08, 2025 pm 05:40 PM

How to execute a query in Python using MySQL Connector

To use MySQLConnector to execute queries in Python, you first need to install and import the mysql-connector-python library, then establish a database connection, then create a cursor object to execute SQL statements, obtain the results or submit transactions according to the query type, and finally close the cursor and connection. 1. Install the library: pipinstallmysql-connector-python; 2. Import the library: importmysql.connector; 3. Establish the connection: conn=mysql.connector.connect(host='localhost',user='your_usern

Aug 08, 2025 pm 05:39 PM

SQL Server Stretch Database for Archiving Cold Data

StretchDatabase is suitable for archiving cold data with low access frequency and long-term retention. 1. The judgment criteria include low access frequency, large data volume, and high retention requirements; 2. The enablement steps are to configure server parameters first, and then enable stretching for databases and tables through SSMS, and optional filtering functions to define cold data; 3. Notes cover query delay, index maintenance, status monitoring, cost control and security management; 4. The comparison plan includes partition tables, AzureArc and ETL tools combined with data lakes, etc., each with applicable scenarios.

Aug 08, 2025 pm 05:35 PM

Data archiving

How to use MySQL Workbench for database design

Create a new EER model: Start MySQLWorkbench, create a new model and add a named pattern (such as myapp_db) to start the design. 2. Add a table: Use the toolbar to add a table on the canvas, double-click to configure the table name, column name, data type, primary key, non-empty and self-increment properties, and set the index and foreign keys. 3. Define table relationships: Use foreign key tools to connect child tables with parent tables, set reference columns and cascade operations, and identify one-to-many relationships through visual symbols. 4. Forward-generation database: Generate SQL scripts and execute the model through "ForwardEngineer", thereby creating the actual database on the MySQL server. 5. Reverse generation model (optional): Connect to existing databases and reversely generate EER graphs for convenience

Aug 08, 2025 pm 05:32 PM

Database Design

How to create a filtered index in SQL?

AfilteredindexinSQLServerimprovesperformancebyindexingonlyasubsetofrowsusingaWHEREclause.2.Itmustbeanonclusteredindexcreatedonatable,suchasindexingactiveuserswithCREATENONCLUSTEREDINDEXIX_Users_ActiveEmailONUsers(Email)WHEREIsActive=1.3.Bestusedforsp

Aug 08, 2025 pm 05:31 PM

sql