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Technical Articles

Database

SQL

Using the SQL CASE statement for conditional logic in queries.

TheCASEstatementinSQLisusedtoimplementconditionallogicwithinqueries,allowingdifferentvaluestobereturnedbasedonspecifiedconditions.1.Itcomesintwoforms:simpleCASE,whichcomparesasingleexpressionagainstmultiplevalues,andsearchedCASE,whichevaluatesseparat

Jul 13, 2025 am 01:10 AM

Understanding the various SQL JOIN types and their applications.

INNERJOIN only retains two table matching rows, LEFTJOIN retains all data on the left table, RIGHTJOIN retains all data on the right table, FULLOUTERJOIN retains all records, and CROSSJOIN generates Cartesian product. INNERJOIN is suitable for situations where both table data exist; LEFTJOIN is used to list all records in the left table and supplement the right table information; RIGHTJOIN is used similar to LEFTJOIN but focuses on the right table; FULLOUTERJOIN is used to compare and analyze data that are inconsistent on both sides; CROSSJOIN is used to generate all combinations but should be used with caution. Each JOIN has a clear scenario, and correct use can improve query efficiency and accuracy.

Jul 12, 2025 am 02:54 AM

Common reasons for slow SQL queries

The main reasons for slowing SQL queries include lack of appropriate indexes, inefficient query statements, unreasonable table structure design and improper database configuration. 1. The lack of a suitable index will lead to full table scanning. Indexes for commonly used query fields should be established and useless indexes should be cleaned regularly; 2. The query statement is poorly written, such as SELECT*, field function use, excessive nesting, etc., should be optimized to only check the necessary fields, overwrite the function conditions for range query, and reduce subquery nesting; 3. Table structure design problems such as improper selection of field types, large tables not split, historical data accumulation should be selected according to query requirements, horizontal/vertical splitting, and cold and hot separation archives; 4. Database configuration problems such as connection pool restrictions, insufficient cache settings, hardware bottlenecks should adjust connection and cache configuration, monitoring load,

Jul 12, 2025 am 02:49 AM

What is the BETWEEN operator in SQL

The BETWEEN operator in SQL is used to filter data within a specified range and contains boundary values. Its basic syntax is: SELECTcolumn_nameFROMtable_nameWHEREcolumn_nameBETWEENvalue1ANDvalue2; where value1 and value2 define ranges, the order must be arranged from small to large, otherwise there may be no results; it is suitable for data types such as numbers, dates and strings, but string comparisons are affected by database sorting rules; common errors include misuse of order, non-range data or forgetting the characteristics of containing boundary values; in addition, NOTBETWEEN can be used to filter data outside the range; pay attention to data classes when using it

Jul 12, 2025 am 02:38 AM

Explain the Role of Primary Keys in SQL Database Design.

The primary key is a field or field combination in a database table that uniquely identifies each row of data. Its core function is to ensure data integrity, improve query efficiency and support foreign key constraints. The primary key must satisfy uniqueness and non-emptyness. Each table can only have one primary key, but it can consist of multiple columns to form a composite primary key. It maintains data consistency by preventing duplicate records and providing a basis for foreign key references, while automatically creating indexes to speed up queries. When selecting primary keys, the principles of stability, simplicity and business-free meaning should be followed. Common methods include self-incremental integers, GUID/UUID and natural keys if necessary. Composite primary keys are suitable for situations where a single field cannot uniquely identify data. For example, student ID and course ID form the primary key of the course selection schedule, but due to its complexity and performance impacts.

Jul 12, 2025 am 02:33 AM

Understanding Transaction Isolation Levels in SQL.

The transaction isolation level determines the data visibility of concurrent transactions. The SQL standard defines four levels: read not submitted, read submitted, read repeatable, and serialized. The higher the level, the stronger the consistency, but the worse the concurrency; different levels allow or prohibit dirty reading, non-repeatable, phantom reading, or missing updates; when choosing, you should weigh the business. If the report system can be read and has been submitted, the financial system recommends repetitive reading or serialization, and the high concurrency scenarios can be appropriately relaxed; MySQL defaults to repetitive reading, PostgreSQL and Oracle defaults to read and submitted; the setting method includes database commands or ORM framework configuration, but modifications should be cautious.

Jul 12, 2025 am 02:32 AM

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