Programming VIP

Related sub query -- Related sub query: sub queries cannot be run independently, and external queries must be run before sub queries -- Advantages: simple and powerful -- Disadvantages: difficult to understand -- Query the employee with the highest salary in the Department SELECT deptno,ename,max(sal) from emp GROUP BY deptno -- The state ...

The original content of GreatSQL community cannot be used without authorization. Please contact Xiaobian and indicate the source for reprint.1. Problem descriptionOccasionally encountered gtid during MGR testing_ The executed transaction ID is discontinuous, but it does not affect the normal operation of the database. The phenomenon is as follo ...

catalogue 0 psql introduction 2.3 console commands 2.5 database import and export 2.6 problem solving 3.API usage 3.0 libpqxx compilation 3.2 connection examples 3.3 creating an example 3.4 added examples 3.5 selection examples 3.6 modification example 3.7 delete example 5. Reference: 0 psql introduction In d ...

I Basic introduction    regular expression describes a string matching rule. Regular expression itself is a string. This string is used to describe and define matching rules to match a series of strings that meet a certain syntactic rule. In development, regular expressions are often used to retrieve and replace text that conf ...

Redis Cluster is a Redis Cluster scheme launched by redis version 3.0. It distributes data in different service areas to reduce the system's dependence on a single master node and greatly improve the read-write performance of redis services. Redis divides all data into 16384 slots, and each node is responsible for a part of the slots. When a r ...

7: Database connection pool 7.1 necessity of JDBC database connection pool When developing web programs based on database, the traditional mode basically follows the following steps: Establish database connection in the main program (such as servlet and beans)Perform sql operationsDisconnect database Problems in the development of this m ...

Mybatis plus common annotations and plug-ins (II) 1, @ TableName value attribute The name of the entity class is User and the name of the database table is t_user @TableName(value = "t_user") public class User { //... } 2, @ TableId 1. Snowflake algorithm By default, the id column of the database is generated by the strategy based o ...

This note is the learning content of Alibaba cloud Tianchi Longzhu plan SQL training camp. The link is: https://tianchi.aliyun.com/specials/promotion/aicampsql 1, Addition and subtraction of tables 1.1 what is set operation A set represents "the sum of all kinds of things" in mathematics and a set of records in database Specific ...

1, Business Characteristics of transactions Atomicity: a transaction must be an atomic unit of work. All statements in a transaction must be done or none of them.Consistency: keep the data logically "reasonable". When A transfers to B, A must pay less and B must pay moreIsolation: if multiple transactions are executed concurrent ...

Today, I saw someone on the Internet asking me to delete PG by mistake_ filenode. How to restore or rebuild a map? Let's learn about PG before solving this problem_ filenode. Map file. For each table in PostgreSQL, there are files related to it on disk, and the name of these files is relfilenode. We can use pg_class to query the relfilenode fi ...