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Home

Technical Articles

Database

SQL

Implementing Primary Keys for Data Integrity in SQL Tables

Each table should have a primary key, because the primary key not only ensures record uniqueness and non-emptyness, but also serves as a clustered index to improve query performance. 1. The primary key forces non-empty to avoid data ambiguity; 2. The primary key serves as the basis for physical storage order to improve search and connection efficiency; 3. The absence of primary keys may lead to duplicate data, erroneous operations and foreign key failure. When selecting a primary key, you can use auto-increment ID, UUID or stable natural keys, but frequent modifications should be avoided. The primary key is different from a unique index. The former emphasizes entity integrity, while the latter is used for business verification and allows null values. When setting primary keys, you need to be named uniformly, use compound primary keys with caution, avoid frequent updates, and select data types reasonably.

Jul 13, 2025 am 02:08 AM

SQL CASE WHEN statement example

SQL's CASEWHEN statement is used for conditional judgment and can be classified or converted. 1. The basic usage is to classify the field value, such as classifying the order amount; 2. Support multi-field combination judgment, such as grouping according to gender and age, suitable for complex classification scenarios; 3. Used in conjunction with aggregate functions to realize statistical functions, such as counting order counts in different time periods. Pay attention to the order of conditions, add ELSE, alias the result list, and recommend that when the logic is complex, disassemble the molecular query to improve readability.

Jul 13, 2025 am 02:05 AM

How to perform a case-insensitive search in a SQL query?

There are three main ways to implement case-insensitive search in SQL queries. 1. Use the ILIKE operator (only applicable to PostgreSQL), such as SELECTFROMusersWHEREnameILIKE'alice'; 2. Use the LOWER() or UPPER() function, which is suitable for all mainstream databases, but may affect performance. It is recommended to establish a function index, such as SELECTFROMusersWHERELOWER(name)=LOWER('Alice'); 3. Use regular expressions (such as PostgreSQL's ~ or MySQL's REGEXP) to be suitable for complex pattern matching, such as SELECTFROMu

Jul 13, 2025 am 02:04 AM

How do you declare and use a variable in a SQL script?

The methods of declaring and using variables in SQL are as follows: 1. Declaring variables requires specifying the name and data type, such as DECLARE@ageINT; 2. Assign values using SET or SELECTINTO, such as SET@age=30; or SELECTageINTO@ageFROMusersWHEREid=1; 3. Use variables to filter or calculate in queries, such as SELECT*FROMusersWHEREage>@age; 4. Variables are usually session scope and are not applicable to all contexts, such as views or triggers.

Jul 13, 2025 am 01:59 AM

SQL LAG and LEAD function examples

SQL's LAG and LEAD functions are used to access row data with specified offsets before and after the current row. 1. LAG (column,offset,default) gets the value of the offset line before the current line. The default offset is 1. If it does not exist, it will return NULL or specify the default value; 2. LEAD (column,offset,default) gets the value of the offset line after the current line. The usage is similar to LAG; 3. The basic syntax is to calculate adjacent records by sorting or grouping in combination with OVER clauses, such as viewing the sales of the previous month and the next month by date; 4. Group calculation can be implemented through PARTITIONBY, such as analyzing trends by region and product classification; 5. It can be combined with C

Jul 13, 2025 am 01:54 AM

What is the purpose of the WITH clause (Common Table Expression) in a SQL statement?

TheWITHclauseinSQL,orCommonTableExpression(CTE),simplifiescomplexqueriesbycreatingreusabletemporaryresultsets.1.Itimprovesreadabilitybybreakingdownnestedsubqueriesintomodularcomponents,asseenwiththetop_customersexamplethatcleansupthemainquery.2.Itena

Jul 13, 2025 am 01:46 AM

What is a SQL cursor and when should you use it?

SQL cursor is a database object used to process data line by line, usually including steps such as declaring a query, opening a cursor, obtaining a cursor line by line, processing data and closing a cursor. The main scenarios for using cursors include: 1. The lines need to be processed in order, especially the next line depends on the previous line; 2. Execute complex business logic that is difficult to express with a single query; 3. Iterate through the result set to execute dynamic SQL or process code; 4. Generate reports that need to be formatted line by line. However, cursors should be avoided as they are: 1. consume more memory and server resources; 2. May cause blocking and locking problems; 3. It is harder to maintain and debug than standard queries; 4. It often indicates that the correct SQL method is not adopted. In most cases, it can be done through JOIN, CTE, window function or base

Jul 13, 2025 am 01:44 AM

How to handle NULL values in SQL

When dealing with NULL values in SQL, you need to pay attention to the following methods: 1. Use ISNULL and ISNOTNULL to judge, and cannot use =NULL or !=NULL; 2. Use COALESCE function to replace the NULL value and return the first non-NULL parameter, which is suitable for display and calculation scenarios; 3. Use CASEWHEN to implement more complex logical processing, such as classification according to different situations; 4. Avoid accidental NULL generation when inserting and updating. It is recommended to add NOTNULL constraints to the key fields, set default values, and check data integrity before insertion. Correct handling of NULL can reduce query errors and potential bugs.

Jul 13, 2025 am 01:39 AM

Common Aggregate Functions Used in SQL Queries.

SQL aggregation function is used to extract key information from data. Common ones include: 1. SUM() calculates the sum of numerical values, which is suitable for stating total sales, etc.; 2. COUNT() counts the number of rows, which can be used to obtain the number of records that meet the conditions; 3. AVG() calculates the average value, which is suitable for analyzing data such as scores or prices; 4. MIN() and MAX() find the minimum and maximum values respectively, which can be used for numeric or string comparisons. These functions are often used in conjunction with GROUPBY or WHERE to complete complex data analysis tasks.

Jul 13, 2025 am 01:38 AM

How to delete data from a table in SQL

To delete data in SQL tables, select DELETE or TRUNCATE according to your needs. 1.DELETE is used to delete part of data according to conditions, supports WHERE clause, can be rolled back, triggered and logged, but pay attention to foreign key constraints; 2.TRUNCATE is used to quickly clear the entire table, does not record single-row operations, and is usually not rolled back, the speed is fast and the self-increment column is reset, but the trigger will not be triggered, which may be restricted by foreign keys. There are differences in logs, performance, triggers and foreign key processing between the two. When selecting, consider whether to delete all or part of the data, transaction log requirements, and performance factors.

Jul 13, 2025 am 01:35 AM

Implementing Error Handling Mechanisms in SQL Code

In SQL development, an effective error handling mechanism can be implemented through the following methods: 1. Use TRY...CATCH to catch exceptions to prevent errors from being exposed to the caller, and record logs for easy troubleshooting; 2. Use THROW or RAISERROR to actively throw errors to ensure that information is specific and unified; 3. Combined with transaction control, roll back operations when errors occur, and maintain data consistency; 4. Pay attention to avoid silent failure, pre-judgment, boundary testing and environment-level error information control. Together, these methods improve the stability and reliability of database applications.

Jul 13, 2025 am 01:30 AM

Using SQL CHECK constraints to enforce data validation rules.

CHECK constraints prevent invalid data from being inserted or updated by setting rules in table definitions. For example, if you ensure that the price is not negative, the employee is between 18-65 years old, and the salary is greater than zero, you can use CHECK(price>=0) and CHECK(age>=18ANDage

Jul 13, 2025 am 01:28 AM

Working with Date and Time Data Using SQL Functions

The key to processing date and time data is to master common functions and their differences in different databases. 1. Get the current time: MySQL uses NOW(), PostgreSQL supports NOW() and CURRENT_TIME, SQLServer uses GETDATE() or SYSDATETIME(); 2. Extract the date part: MySQL uses YEAR(), MONTH() and other functions, PostgreSQL recommends EXTRACT(), SQLServer uses DATEPART(); 3. Date operation: MySQL is implemented through DATE_ADD() or INTERVAL, PostgreSQL uses INTER

Jul 13, 2025 am 01:23 AM

How to create a function in SQL

To create a function in SQL, you need to use the CREATEORREPLACEFUNCTION statement. Taking PostgreSQL as an example, the basic structure includes function name, parameters, return type and function body. 1. Use DECLARE to declare variables when defining functions, 2. Assign values through SELECTINTO in the function body, 3. Use RETURN to return the result. For example, the function get_employee_name that returns a name based on the employee ID, contains the parameter emp_id, the variable emp_name and query logic. The call method is the SELECT function name (parameter), and it can also be embedded in complex queries. Notes include permissions, performance impact, debugging difficulties and naming conflicts, different database systems

Jul 13, 2025 am 01:20 AM