LINQ standard query operators - Join, GroupJoin, GroupBy, Concat

4, Join operator Join refers to the operation of associating or associating one data source object with another data source object. The two data source objects are associated by a common value or attribute. LINQ has two Join operators: Join and GroupJoin. 1. Join The Join operator is similar to the inner join in T-SQL. It joins two data sou ...

Posted on Fri, 01 Oct 2021 17:58:03 -0400 by jake8

Use of mysql-index

select * from tb_item where title = "Goods No. 25" Indexing is one of the most common and important means of database optimization, and it usually helps users solve most of MySQL performance optimization problems. Validating indexes improves query efficiency In the table structure tb_item we prepared, a total of 3 million records were stored ...

Posted on Fri, 01 Oct 2021 12:07:22 -0400 by geroid

JDBC Java datebase connectivity

JDBC Java datebase connectivity Concept: Java language operation database JDBC essence: it is a set of rules defined by the official (sun company) to operate all relational databases, that is, interfaces. Each database manufacturer implements this interface and provides database driver jar package. We can use this set of interfaces (JDBC) ...

Posted on Tue, 28 Sep 2021 13:57:36 -0400 by localhost1

MyBatis study notes

1, Introduction to MyBatis MyBatis is a semi automated persistence layer framework. MyBatis is an excellent persistence layer framework that supports customized SQL, stored procedures, and advanced mapping.MyBatis avoids almost all JDBC code and manually setting parameters and getting result sets.MyBatis can use xml or annotations to configur ...

Posted on Tue, 28 Sep 2021 13:55:09 -0400 by robmarston

Five common time crossing and continuous login problems in HIVE

This article is intended to record learning experience and facilitate communication. The blogger's level is limited. If there are any errors in the article, you are welcome to point out. The blogger will be grateful. (Note: please indicate the source for reprint and quotation) Continuous questions: The following data are the carbon emission r ...

Posted on Mon, 27 Sep 2021 03:55:42 -0400 by mandukar

Autumn move script A

Exercise 1: the highest paid employees in each department Create an Employee table containing all Employee information. Each Employee has its corresponding Id, salary and department Id. create table Employee ( Id integer(4) not null, employee_name varchar(128) not null, salary integer(128) not null, departmentid integer(4) not null, ...

Posted on Sun, 26 Sep 2021 22:22:18 -0400 by cuvaibhav

Chapter 26 SQL command DECLARE

Chapter 26 SQL command DECLARE declare cursor outline DECLARE cursor-name CURSOR FOR query parameter Cursor name - the name of the cursor, which must start with a letter and contain only letters and numbers. (the cursor name does not follow the SQL identifier Convention). Cursor names are case sensitive. They are subject to o ...

Posted on Sun, 26 Sep 2021 06:31:11 -0400 by Chips

Sum of sincerity: how to use JSON arrays in PL/SQL?

Oracle Database 12c version 2 is built based on the 12.1 SQL/JSON feature, adding many built-in object types (similar to classes in object-oriented language) for manipulating JSON data in PL/SQL blocks. In this article, I explored some ways through JSON_ ARRAY_ Array oriented JSON features provided by type T and its methods. Like classes, obj ...

Posted on Sun, 26 Sep 2021 00:35:05 -0400 by azsolo

Getting started with MyBatis: real-world additions, deletions, and changes

A concept 1.What is MyBatis? MyBatis is an excellent persistence layer framework that supports custom SQL, stored procedures, and advanced mapping. MyBatis eliminates almost all JDBC code, setting parameters, and getting result sets. MyBatis can configure and map raw types, interfaces, and Java POJO s (Plain Old Java Objects, plain old-fa ...

Posted on Sat, 25 Sep 2021 12:44:12 -0400 by ewillms

The most detailed introduction of the character set and sorting rules in MySQL. Why do you have to use utf8mb4 under Chinese character coding (Unicode)?

Two different character sets cannot have the same collation. And each character set has a default collation. Most of the content of this article comes from the official website documents View currently supported character sets INFORMATION_SCHEMA.CHARACTER_SETS INFORMATION_ SCHEMA.CHARACTER_ Available character sets can be viewed in the ...

Posted on Sat, 25 Sep 2021 07:49:38 -0400 by justbane