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Technical Articles

Database

SQL

How Database Locking Works in Concurrent SQL Operations.

Database locks are tools that control concurrent access to ensure data consistency. Its core role is to avoid data confusion when multiple transactions access the same resource at the same time. Common locks include shared locks (allowing multiple transactions to read), exclusive locks (restricting only one transaction to read and write), row-level locks (small granularity, high concurrency), and table-level locks (large granularity, low concurrency). Locks are usually automatically triggered when SQL is executed, such as SELECT plus shared locks, UPDATE plus exclusive locks. Deadlocks may occur due to transactions waiting for each other and can be avoided by fixed access order and shortening transaction time.

Jul 12, 2025 am 02:31 AM

Finding the median value in SQL

ThemedianinSQLcanbecalculatedusingwindowfunctionswhennobuilt-infunctionisavailable.1)UseROW_NUMBER()andCOUNT()inaCTEtoassignrownumbersandcounttotalrows.2)Selectmiddlerow(s):ifodd,pickthecentralrow;ifeven,selecttwomiddlerows.3)Averageselectedvaluestog

Jul 12, 2025 am 02:29 AM

What Are the Practical Limits of Pattern Matching Capabilities in SQL?

SQL'spatternmatchinghaspracticallimitsthatimpactperformanceandfunctionality.1)LIKEiswidelysupportedbutlimitedtosimplepatterns.2)REGEXPofferscomplexpatternmatchingbutcanbecomputationallyexpensiveandunsupportedinsomesystems.Tooptimize,useLIKEwhenpossib

Jul 12, 2025 am 02:29 AM

database SQL Pattern Matching

Understanding and Implementing Database Indexes in SQL

Indexes should be added to query conditions, join fields, and sorting and grouping fields; common index types include B-Tree, hash, full text and combined indexes; redundancy should be avoided when creating, write more and read less tables with caution, and analyze usage regularly; judge whether the index takes effect through execution plans, and use it reasonably to improve performance.

Jul 12, 2025 am 02:25 AM

Transforming Rows to Columns Using SQL PIVOT Operations

TorotatedatafromrowsintocolumnsinSQL,usethePIVOToperation.1.PIVOTtransformsuniquerowvaluesintocolumnnames,commonlyusedforsummarizingdatainreports.2.Itrequiresanaggregationfunctionandistypicallyappliedwhenconvertingcategorieslikemonthsorproductsintose

Jul 12, 2025 am 02:21 AM

Creating and calling stored procedures in SQL.

Stored procedures are reusable SQL code blocks in the database. The creation steps are: 1. Use DELIMITER to define the statement ending character; 2. Declare parameters and logic bodies through CREATEPROCEDURE; 3. Write operation statements between BEGIN...END. Use the CALL command when calling and pass parameters, such as CALLcalculate_bonus(5000,@bonus). Advantages include reduced network transmission, improved performance and maintenance. The applicable scenarios are high-frequency operations, performance optimization and complex data logic, while small projects are more suitable for direct splicing of SQL.

Jul 12, 2025 am 02:16 AM

How to find the Nth highest value in a SQL column? (e.g., second highest salary)

There are three common methods to find the Nth highest value of a column in SQL. 1. Use subquery and LIMIT/OFFSET: First sort the target column in descending order, skip the first N-1 record and then take one. It is suitable for simple scenarios but may affect performance; 2. Exclude maximum values ??layer by layer through nested subqueries: the logic is clear but the structure is complex when the hierarchy increases; 3. Use DENSE_RANK or ROW_NUMBER window function (recommended): Flexible processing of duplicate values, supports precise ranking, suitable for database environments that support window functions. Which method to choose depends on the specific database type, data volume and structural requirements.

Jul 12, 2025 am 01:58 AM

sql

Writing Efficient SQL Subqueries for Complex Data Retrieval

The key to writing a good SQL subquery is to clarify its purpose, master the result type and optimize performance. 1. Make it clear that subqueries are to generate intermediate results for external query use, not universal tools, multi-layer nesting should be avoided, JOIN should be used first and alias or CTE should be improved readability; 2. Pay attention to the type of result returned by subqueries, use comparison operators for a single value, directly compare one row and multiple columns or wrap multiple fields, use IN, EXISTS, etc. for one column and multiple rows, and use multiple columns and multiple rows as temporary tables; 3. In terms of performance optimization, avoid repeated calculations caused by related subqueries, and can improve efficiency by rewriting it to JOIN group statistics, pre-calculate the subquery results or establishing an index.

Jul 12, 2025 am 01:39 AM

Analyzing SQL Query Performance with Execution Plans

The key to SQL query performance analysis is to understand the execution plan. First, through the execution plan, you can view how the database executes queries and determine whether to use indexes or full table scans; second, different databases such as MySQL, EXPLAIN, PostgreSQL, EXPLAINALYZE to view execution plans; then, key indicators include type (connection type), key (index used), rows (estimated row count), and Extra (additional information); finally, common optimization suggestions include adding appropriate indexes, avoiding wrong indexes, reducing unnecessary sorting and grouping, and avoiding SELECT*. Mastering these can effectively improve SQL efficiency.

Jul 12, 2025 am 01:37 AM

Implementing optimistic vs. pessimistic locking in SQL.

TochoosebetweenoptimisticandpessimisticlockinginSQL,assessyourapplication'sneeds:1.Usepessimisticlockingwhenconflictsarecommon,dataaccuracyiscritical,andtransactionsneedexclusiveaccess,typicallyviaSELECT...FORUPDATE;2.Optforoptimisticlockingwhenconfl

Jul 12, 2025 am 01:31 AM

What is referential integrity in a SQL database?

ReferentialintegrityinSQLensuresconsistencybetweenrelatedtablesbyenforcingrelationshipsusingforeignkeys.1)Itpreventsinvaliddataentriesbyensuringforeignkeysreferencevalidprimarykeys.2)Itblocksdeletionsofreferencedrecordsifdependentdataexists.3)Itallow

Jul 12, 2025 am 01:29 AM

How to Improve SQL Query Performance Using Indexes?

To make SQL queries faster, the key is to use indexes reasonably. 1. Understand different types of indexes: B-Tree is suitable for equal value and range queries, hash indexes only support equal value matching, full-text indexes are used for fuzzy matching, and combined indexes must follow the principle of leftmost prefix. 2. Avoid over-index and incorrect indexing: low cardinality sequences, improper field order, and unreasonable prefix length of large fields will affect performance, and unused indexes should be cleaned regularly. 3. Use EXPLAIN to analyze the execution plan: judge whether the index takes effect and optimize the structure through the type, key, rows and Extra fields. 4. Selection strategy according to business scenarios: high-frequency reading systems can appropriately build coverage indexes, while high-concurrent writing systems need to control the number of indexes to improve efficiency and balance the search.

Jul 12, 2025 am 01:19 AM

OLTP vs OLAP: Key Concepts

OLTPfocusesonreal-timetransactionprocessing,whileOLAPisdesignedforcomplexdataanalysis.1)OLTPhandleshigh-volumetransactionswithACIDpropertiesfordataintegrity.2)OLAPoptimizesforread-heavyoperationsandusesdatawarehousingforhistoricalanalysis.

Jul 12, 2025 am 01:10 AM