There are 8 SQL misuses that are often ignored. Have you ever stepped on a pit?

1. LIMIT statement Paging query is one of the most commonly used scenarios, but it is also the most prone to problems. For example, for the following simple statements, the general way DBA s think of is to add a composite index to the fields of type, name, create "time. In this way, conditional sorting can effectively utilize index and i ...

Posted on Sun, 17 Nov 2019 14:14:06 -0500 by cornelombaard

Java Serial 51-super Keyword

1. super keywords Let's start with an example package com.bjpowernode.java_learning; ​ public class D51_ { public static void main(String[] args) { Manager51 m1 = new Manager51(); m1.m1(); } } ​ class Employee51{ //Member function public void work() { System.out.print("Employees at work"); } } class Mana ...

Posted on Sat, 16 Nov 2019 13:31:32 -0500 by samdennis

4. Principle and use of spark--sparkSQL

[TOC] 1. Overview of spark SQL 1.1 What is spark SQL Spark SQL is a module Spark uses to process structured data. It provides a programming abstraction called DataFrame and functions as a distributed SQL query engine.Similar to what hive does. Features of 1.2 spark SQL 1. Easy to integrate: When you installed Spark, it was already integrated.N ...

Posted on Sat, 16 Nov 2019 01:24:56 -0500 by HokieTracks

5. Principles and uses of spark--spark streaming

1. Overview of spark-streaming 1.1 Common real-time computing engines Real-time computing engines, also known as streaming computing engines, currently have three commonly used:1. Apache Storm: True Streaming2. Spark Streaming: Strictly speaking, it's not really streaming (real-time computing)Processing continuous streaming data as discrete RD ...

Posted on Sat, 16 Nov 2019 01:20:27 -0500 by Snart

Record the irresponsible way of filing and supporting explosion in the first guide warehouse

There is another problem in the test data verification, but the card owner is still in the process of importing. Then I look at the logs and find that a large number of archive logs are generated during the import of impdp, so the storage and archive space is full, so it gets stuck Tail / u01 / APP / Oracle / diag / RDBMS / zhanky / zhanky / ...

Posted on Fri, 15 Nov 2019 16:21:29 -0500 by bo0

while loop statement

while loop statement Format: while (judgment condition statement){ Loop body statement; } Extended format: Initialization statement; while (judgment condition statement){ Loop body statement; Control condition statement; } Example 1 Guessing game Train of thought: While loop: Guess the number: int num = 456; Condition: int gu ...

Posted on Fri, 15 Nov 2019 12:51:19 -0500 by grimz

Deep understanding of PHP's seven predefined interfaces

Deep understanding of predefined interfaces Scenario: business modules are written in normal work, rarely to implement such interfaces, but there are many in the framework.   1. Traversable interface This interface cannot be directly implemented by a class. If a normal class is written directly to implement this traversal interface, it will ...

Posted on Fri, 15 Nov 2019 04:10:09 -0500 by imnu

I. Basic Introduction to scala

[TOC] I. overview of scala 1.1 INTRODUCTION Scala is a multi paradigm programming language. The original intention of its design is to integrate various features of object-oriented programming and functional programming. Scala runs on the Java platform (Java virtual machine) and is compatible with existing Java programs. It can also run in Jav ...

Posted on Sun, 10 Nov 2019 08:11:37 -0500 by nayone

8 common SQL error usage

Common SQL error usage 1. LIMIT statement Paging query is one of the most commonly used scenarios, but it is also the most prone to problems. For example, for the following simple statements, the general way DBA s think of is to add a composite index to the fields of type, name, create "time. In this way, conditional sor ...

Posted on Sun, 03 Nov 2019 08:04:24 -0500 by harsh00008

How to wake up H5 webpage in wechat and open external browser to open specified link

When using wechat to open the URL link, we often encounter the situation that H5 webpage cannot be opened and app cannot be downloaded. Then a mask will pop up to prompt the user to open it in the mobile browser window.In this way, we can judge the ua of wechat directly, and then pop up a mask to prompt the user to open the download in the bro ...

Posted on Sun, 03 Nov 2019 07:09:22 -0500 by spxdcz