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Home

Technical Articles

Database

SQL

What is a trigger in SQL

SQL triggers are special stored procedures that are automatically executed when specific events such as INSERT, UPDATE, or DELETE occur in the database. It is mainly used to maintain data consistency and reliability. Typical uses include recording data changes, implementing complex constraints, synchronizing data, and sending notifications. To create a trigger, you need to define the trigger timing (BEFORE/AFTER), event type (INSERT/UPDATE/DELETE), scope of action (FOREACHROW) and execution actions (SQL statements in BEGIN...END). When using it, you should pay attention to avoiding excessive dependence, preventing infinite loops, controlling performance impacts, and clarifying documentation.

Jul 06, 2025 am 12:51 AM

What are the main syntax differences between MySQL and PostgreSQL SQL?

MySQL and PostgreSQL are different in string concatenation. MySQL uses the CONCAT() function, while PostgreSQL uses the || operator; 2. In LIKE operations, MySQL is case-insensitive by default, while PostgreSQL needs to use ILIKE to achieve case-insensitiveness; 3. In terms of identifier reference, MySQL uses backticks (`), and PostgreSQL uses double quotes ("); 4. The syntax of restriction results is basically the same, but PostgreSQL supports a more complex OFFSETFETCH paging mechanism, while MySQL uses LIMIToffset, count; 5. In terms of data types, the time types of the two

Jul 06, 2025 am 12:47 AM

Creating and Using User-Defined Functions in SQL Queries

User-defined functions (UDFs) are user-defined reusable functions used to encapsulate common logic to enhance SQL query flexibility. 1. UDF can be used for data conversion, simplifying complex expressions, and improving code readability; 2. The creation method varies from database to database. For example, SQLServer uses CREATEFUNCTION, and PostgreSQL supports multiple languages; 3. When using it, avoid frequent access to tables and pay attention to performance impact; 4. Clear naming and add comments to facilitate maintenance and migration. The rational use of UDF can improve development efficiency, but it requires weighing performance and abstraction.

Jul 06, 2025 am 12:44 AM

How to connect SQL to Excel

There are three ways to connect SQL database to Excel, which are suitable for different user needs. 1. Using PowerQuery to connect to SQL database is the most recommended way. After opening Excel, select "From SQLServer" through the "Data" tab, enter server and database information and select authentication method. You can import the entire table or custom SQL query statements, which support timed refresh and data cleaning. 2. Use MicrosoftQuery directly to suit old version users. Select "From MicrosoftQuery" in the "Data" tab. After creating the data source, you can obtain data through a graphical interface or SQL query. The operation is simple but the function is weak. 3. Use ODBC data source to connect to high

Jul 06, 2025 am 12:41 AM

Aggregating data with multiple columns in a SQL GROUP BY clause.

In SQL queries, the correct way to use multiple columns of GROUPBY is to write all columns that need to be grouped units into the GROUPBY clause. 1. The non-aggregated fields that appear in SELECT must also be in GROUPBY; 2. The grouping is based on the combined values ??of multiple columns, for finer-grained data analysis; 3. The aggregate functions (such as SUM, COUNT) and HAVING can be filtered; 4. Pay attention to performance impact and avoid unnecessary columns being added to GROUPBY; 5. The NULL value is regarded as the same group in GROUPBY. Mastering these key points can effectively realize multi-dimensional data analysis.

Jul 06, 2025 am 12:38 AM

Performing UPSERT (Insert or Update) Operations in SQL.

There are several ways to implement UPSERT operations in SQL: 1. PostgreSQL uses INSERT...ONCONFLICT syntax to update existing records when a unique constraint conflict occurs; 2. MySQL uses INSERT...ONDUPLICATEKEYUPDATE to update when a duplicate key conflict is performed; 3. SQLServer and Oracle support MERGE statements, which can perform insertion or update based on matching conditions; 4. If the database does not support the above features, UPSERT operations can be simulated by manually using UPDATE first and then INSERT, but you need to pay attention to concurrency issues and ensure transaction consistency. The prerequisite for using these methods correctly is that appropriately defined

Jul 06, 2025 am 12:38 AM

Various Types of Indexes Available in SQL Databases.

TooptimizeSQLqueryperformance,choosetherightindextypebasedonyourusecase.1.B-Treeindexesareidealforequalityandrangequeriesandsorting,especiallywithhigh-cardinalitydata.2.Hashindexesprovidefastequalitylookupsbutdonotsupportrangequeriesandhavelimiteddat

Jul 06, 2025 am 12:34 AM

Can Pattern Matching Be Used Consistently Across Different SQL Databases?

Pattern matching cannot be used consistently in different SQL databases, but pattern matching across databases can be achieved through specific methods. 1) Use the LIKE operator for basic matching, but complex patterns need to consider database differences. 2) Use ORM tools such as Hibernate to abstract differences, but pay attention to performance and functional trade-offs. 3) Optimize according to database characteristics, such as PostgreSQL's regular expression engine. Through these methods, effective and consistent pattern matching can be achieved.

Jul 06, 2025 am 12:29 AM

Converting data types within a SQL query.

InSQL,useCAST()forsimpletypeconversionsliketurningstringsintodatesorintegersintodecimals,asit’swidelysupportedandstraightforward.UseCONVERT()whenformattingmatters,especiallyinSQLServerorMySQL,asitallowsstylecodesfordate/timedisplay.Becautiousofimplic

Jul 06, 2025 am 12:12 AM

How to use GROUP BY in SQL

GROUPBY is used for classification statistics, and the SELECT field must correspond. When using it, you need to clarify the grouping field, such as calculating the total amount by grouping by customer_id; multi-field grouping requires the same value to be grouped; errors often appear in redundant or missing grouping fields; non-aggregated fields in SELECT must appear in GROUPBY, otherwise an error will be reported; HAVING is used to filter the results after grouping, WHERE is used to filter before grouping; NULL values ??will be classified into a group and can be processed by COALESCE.

Jul 06, 2025 am 12:10 AM

Backing up and restoring a SQL database.

KnowinghowtobackupandrestoreSQLdatabasesisessentialforprotectingagainstdatalossduetohardwarefailure,accidentaldeletion,orapplicationbugs.1)Usefullbackupsasthefoundation,optionallycombiningwithdifferentialandtransactionlogbackupsforreducedrecoverytime

Jul 06, 2025 am 12:05 AM

Defining Database Schemas with SQL CREATE TABLE Statements

In database design, use the CREATETABLE statement to define table structures and constraints to ensure data integrity. 1. Each table needs to specify the field, data type and primary key, such as user_idINTPRIMARYKEY; 2. Add NOTNULL, UNIQUE, DEFAULT and other constraints to improve data consistency, such as emailVARCHAR(255)NOTNULLUNIQUE; 3. Use FOREIGNKEY to establish the relationship between tables, such as orders table references the primary key of the users table through user_id.

Jul 05, 2025 am 01:55 AM

How to create an auto-incrementing primary key column in SQL?

The method of creating an automatic incrementing primary key in SQL varies from database system to database system. There are three main implementation methods: 1. MySQL uses the AUTO_INCREMENT keyword, which needs to be defined as an integer primary key column, and automatically increments if no value is specified during insertion; 2. PostgreSQL uses the SERIAL type, and its essence is to create a sequence object that binds columns and automatically obtains the value when the ID is omitted; 3. SQLServer uses the IDENTITY attribute, which is defined through IDENTITY (start value, step size), and also supports automatic assignment. In addition, each system requires that the self-increment column must be part of the index, and a table can only have one self-increment column. After manually inserting the ID, MySQL and SQLServer will base it

Jul 05, 2025 am 01:53 AM

How to create a user and grant permissions in SQL

Create a user using the CREATEUSER command, for example, MySQL: CREATEUSER'new_user'@'host'IDENTIFIEDBY'password'; PostgreSQL: CREATEUSERnew_userWITHPASSWORD'password'; 2. Grant permission to use the GRANT command, such as GRANTSELECTONdatabase_name.TO'new_user'@'host'; 3. Revoke permission to use the REVOKE command, such as REVOKEDELETEONdatabase_name.FROM'new_user

Jul 05, 2025 am 01:51 AM