Take Your SQL Skills to the Next Level by Understanding the Self Join

The last couple of blogs that I have written have been great for beginners ( Data Concepts Without Learning To Code or Developing A Data Scientist’s Mindset). But, I would really like to push myself to create content for other members of my audience as well. So today, we are going to take a step into more intermediate data analysis territory dun dun dun…. and discuss self joins: what they are and how to use them to take your analysis to the next level.

Disclaimer: This post assumes that you already understand how joins work in SQL. If you are not familiar with this concept yet, no worries at all! Save this article for later because I think it’ll definitely be useful as you master SQL in the future.

What is a “Self” Join?

A self join is actually as literal as it gets — it is joining a database table to itself. You can use any kind of join you want to perform a self join (left, right, inner, etc.) — what makes it the self join is that you use the same table on both sides. Just make sure that you select the correct join type for your specific scenario and desired outcome.

When Should I Use a Self Join?

If you’ve been working or studying in the field of data analytics and data science for more than, say, 5 minutes, you’ll know that there are always 27 ways to solve a problem. Some are better than others of course, but sometimes the differences are almost indiscernible.

That being said, there is probably never going to be one exact case where you MUST HAVE a self join or your analysis will shrivel up and die with nowhere to turn. drop me a scenario in the comments below if you’ve got one, of course

But, I do at least have some scenarios where I have used self joins to solve my analytics problems, at work or in personal analysis. Here’s my own spin on two of the best (AKA the ones that I remember and can think of a good example for).

#data-analysis #data-science #data #sql #database #data analysis

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Introduction to Structured Query Language SQL pdf

SQL stands for Structured Query Language. SQL is a scripting language expected to store, control, and inquiry information put away in social databases. The main manifestation of SQL showed up in 1974, when a gathering in IBM built up the principal model of a social database. The primary business social database was discharged by Relational Software later turning out to be Oracle.

Models for SQL exist. In any case, the SQL that can be utilized on every last one of the major RDBMS today is in various flavors. This is because of two reasons:

1. The SQL order standard is genuinely intricate, and it isn’t handy to actualize the whole standard.

2. Every database seller needs an approach to separate its item from others.

Right now, contrasts are noted where fitting.

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How to Bash Read Command

Bash has no built-in function to take the user’s input from the terminal. The read command of Bash is used to take the user’s input from the terminal. This command has different options to take an input from the user in different ways. Multiple inputs can be taken using the single read command. Different ways of using this command in the Bash script are described in this tutorial.

Syntax

read [options] [var1, var2, var3…]

The read command can be used without any argument or option. Many types of options can be used with this command to take the input of the particular data type. It can take more input from the user by defining the multiple variables with this command.

Some Useful Options of the Read Command

Some options of the read command require an additional parameter to use. The most commonly used options of the read command are mentioned in the following:

Option	Purpose
-d <delimiter>	It is used to take the input until the delimiter value is provided.
-n <number>	It is used to take the input of a particular number of characters from the terminal and stop taking the input earlier based on the delimiter.
-N <number>	It is used to take the input of the particular number of characters from the terminal, ignoring the delimiter.
-p <prompt>	It is used to print the output of the prompt message before taking the input.
-s	It is used to take the input without an echo. This option is mainly used to take the input for the password input.
-a	It is used to take the input for the indexed array.
-t <time>	It is used to set a time limit for taking the input.
-u <file descriptor>	It is used to take the input from the file.
-r	It is used to disable the backslashes.

 

Different Examples of the Read Command

The uses of read command with different options are shown in this part of this tutorial.

Example 1: Using Read Command without Any Option and variable

Create a Bash file with the following script that takes the input from the terminal using the read command without any option and variable. If no variable is used with the read command, the input value is stored in the $REPLY variable. The value of this variable is printed later after taking the input.

#!/bin/bash  
#Print the prompt message
echo "Enter your favorite color: "  
#Take the input
read  
#Print the input value
echo "Your favorite color is $REPLY"

Output:

The following output appears if the “Blue” value is taken as an input:

Example 2: Using Read Command with a Variable

Create a Bash file with the following script that takes the input from the terminal using the read command with a variable. The method of taking the single or multiple variables using a read command is shown in this example. The values of all variables are printed later.

#!/bin/bash  
#Print the prompt message
echo "Enter the product name: "  
#Take the input with a single variable
read item

#Print the prompt message
echo "Enter the color variations of the product: "  
#Take three input values in three variables
read color1 color2 color3

#Print the input value
echo "The product name is $item."  
#Print the input values
echo "Available colors are $color1, $color2, and $color3."

Output:

The following output appears after taking a single input first and three inputs later:

Example 3: Using Read Command with -p Option

Create a Bash file with the following script that takes the input from the terminal using the read command with a variable and the -p option. The input value is printed later.

#!/bin/bash  
#Take the input with the prompt message
read -p "Enter the book name: " book
#Print the input value
echo "Book name: $book"

Output:

The following output appears after taking the input:

Example 4: Using Read Command with -s Option

Create a Bash file with the following script that takes the input from the terminal using the read command with a variable and the -s option. The input value of the password will not be displayed for the -s option. The input values are checked later for authentication. A success or failure message is also printed.

#!/bin/bash  
#Take the input with the prompt message
read -p "Enter your email: " email
#Take the secret input with the prompt message
read -sp "Enter your password: " password

#Add newline
echo ""

#Check the email and password for authentication
if [[ $email == "admin@example.com" && $password == "secret" ]]
then
   #Print the success message
   echo "Authenticated."
else
   #Print the failure message
   echo "Not authenticated."
fi

Output:

The following output appears after taking the valid and invalid input values:

Example 5: Using Read Command with -a Option

Create a Bash file with the following script that takes the input from the terminal using the read command with a variable and the -a option. The array values are printed later after taking the input values from the terminal.

#!/bin/bash  
echo "Enter the country names: "  
#Take multiple inputs using an array  
read -a countries

echo "Country names are:"
#Read the array values
for country in ${countries[@]}
do
    echo $country
done

Output:

The following output appears after taking the array values:

Example 6: Using Read Command with -n Option

Create a Bash file with the following script that takes the input from the terminal using the read command with a variable and the -n option.

#!/bin/bash  
#Print the prompt message
echo "Enter the product code: "  
#Take the input of five characters
read -n 5 code
#Add newline
echo ""
#Print the input value
echo "The product code is $code"

Output:

The following output appears if the “78342” value is taken as input:

Example 7: Using Read Command with -t Option

Create a Bash file with the following script that takes the input from the terminal using the read command with a variable and the -t option.

#!/bin/bash  
#Print the prompt message
echo -n "Write the result of 10-6: "  
#Take the input of five characters
read -t 3 answer

#Check the input value
if [[ $answer == "4" ]]
then
   echo "Correct answer."
else
   echo "Incorrect answer."
fi

Output:

The following output appears after taking the correct and incorrect input values:

Conclusion

The uses of some useful options of the read command are explained in this tutorial using multiple examples to know the basic uses of the read command.

Original article source at: https://linuxhint.com/

#bash #command 

SQL Self Join Example | Self Join in SQL Tutorial

SQL Self Join is the kind of join in which a table is joined with itself means we are joining a table with that same table (which is also called Unary relationships), especially when the table has the FOREIGN KEY which references its PRIMARY KEY. If we want to join the table itself means that each row of the table is combined with itself and with every other row of the table.

The self join allows you to join the table to itself. It is useful for querying the hierarchical data or comparing rows within the same table. Till now, we have seen the  Outer Join,  Cross Join,  Left Join,  SQL Joins Overview and  Inner Join in this blog.

#SQL Self JOIN Key Points

1. The self JOIN occurs when a table takes a ‘selfie.’
2. The self JOIN is a regular join, but the table is joined with itself.
3. It can be useful when modeling hierarchies.
4. They are also useful for comparisons within the table.
5. You use the self join when the table references data in itself.
6. One most used example is where you wanted to get a list of employees and their immediate managers.

#sql #sql self join #sql joins overview

Your Ultimate Guide to SQL Join: CROSS JOIN

CROSS JOIN is in the spotlight. This article finishes our small series of SQL JOIN-related publications.

SQL Server CROSS JOIN is the simplest of all joins. It implements a combination of 2 tables without a join condition. If you have 5 rows in one table and 3 rows in another, you get 15 combinations. Another definition is a Cartesian Product.

Now, why would you want to combine tables without a join condition? Hang on a bit because we are getting there. First, let’s refer to the syntax.

#sql server #cross join #inner join #outer join #sql join #sql

Criar QProgressbar com QThread Exemplo Real | PyQt5 | Python

Neste tutorial python - PyQt5 aprenderemos como criar uma QProgressbar usando QThread Real Example | PyQt5 | Python. Uma barra de progresso é usada para dar ao usuário uma indicação do progresso de uma operação e para assegurar que o aplicativo ainda está em execução.

O que é PyQt5 QProgressbar?

O widget QProgressBar consiste em uma barra horizontal ou vertical que é preenchida gradualmente para indicar o andamento de uma tarefa. é frequentemente usado em aplicativos que envolvem operações demoradas, como uploads ou downloads de arquivos, instalações de software ou qualquer outro processo que pode demorar um pouco para ser concluído.

O widget QProgressBar pode ser personalizado para exibir diferentes cores, fontes e tamanhos. Ele também fornece várias propriedades e métodos que permitem aos desenvolvedores controlar seu comportamento, como os valores mínimo e máximo, o valor atual e a orientação da barra.

No geral, o widget QProgressBar é uma ferramenta útil para fornecer feedback visual aos usuários sobre o andamento de uma tarefa e pode ajudar a tornar os aplicativos mais fáceis de usar e intuitivos.

Estas são as importações que precisamos, por exemplo

from PyQt5 import QtGui
from PyQt5.QtWidgets import QApplication, QDialog, QProgressBar, QPushButton, QVBoxLayout
import sys
from PyQt5.QtCore import Qt, QThread, pyqtSignal
import time

Esta é nossa classe de encadeamento e esta classe se estende de QThread, um objeto QThread gerencia um encadeamento de controle dentro do programa. QThreads começam a executar em run(). Por padrão, run() inicia o loop de evento chamando exec() e executa um loop de evento Qt dentro do thread.

class MyThread(QThread):
    # Create a counter thread
    change_value = pyqtSignal(int)
    def run(self):
        cnt = 0
        while cnt < 100:
            cnt+=1
            time.sleep(0.3)
            self.change_value.emit(cnt)

Depois criamos nossa classe Window que estende de QDialog e nessa classe adicionamos os requisitos de nossa janela como título, geometria e ícone com QProgresBar e também um QPushButton. também usamos algum estilo e design para nossa barra de progresso.

class Window(QDialog):
    def __init__(self):
        super().__init__()
        self.title = "PyQt5 ProgressBar"
        self.top = 200
        self.left = 500
        self.width = 300
        self.height = 100
        self.setWindowIcon(QtGui.QIcon("icon.png"))
        self.setWindowTitle(self.title)
        self.setGeometry(self.left, self.top, self.width, self.height)
        vbox = QVBoxLayout()
        self.progressbar = QProgressBar()
        #self.progressbar.setOrientation(Qt.Vertical)
        self.progressbar.setMaximum(100)
        self.progressbar.setStyleSheet("QProgressBar {border: 2px solid grey;border-radius:8px;padding:1px}"
                                       "QProgressBar::chunk {background:yellow}")
        #qlineargradient(x1: 0, y1: 0.5, x2: 1, y2: 0.5, stop: 0 red, stop: 1 white);
        #self.progressbar.setStyleSheet("QProgressBar::chunk {background: qlineargradient(x1: 0, y1: 0.5, x2: 1, y2: 0.5, stop: 0 red, stop: 1 white); }")
        #self.progressbar.setTextVisible(False)
        vbox.addWidget(self.progressbar)
        self.button = QPushButton("Start Progressbar")
        self.button.clicked.connect(self.startProgressBar)
        self.button.setStyleSheet('background-color:yellow')
        vbox.addWidget(self.button)
        self.setLayout(vbox)
        self.show()

Esses são os métodos que usaremos para iniciar e definir o valor de QProgressBar.

  def startProgressBar(self):
        self.thread = MyThread()
        self.thread.change_value.connect(self.setProgressVal)
        self.thread.start()
 
    def setProgressVal(self, val):
        self.progressbar.setValue(val)

Além disso, todo aplicativo PyQt5 deve criar um objeto de aplicativo. 

App = QApplication(sys.argv)

Por fim, entramos no mainloop da aplicação. A manipulação do evento começa a partir deste ponto. 

window = Window()
sys.exit(App.exec_())

Código-fonte completo para QProgressbar com QThread

from PyQt5 import QtGui
from PyQt5.QtWidgets import QApplication, QDialog, QProgressBar, QPushButton, QVBoxLayout
import sys
from PyQt5.QtCore import Qt, QThread, pyqtSignal
import time
 
 
class MyThread(QThread):
    # Create a counter thread
    change_value = pyqtSignal(int)
    def run(self):
        cnt = 0
        while cnt < 100:
            cnt+=1
            time.sleep(0.3)
            self.change_value.emit(cnt)
class Window(QDialog):
    def __init__(self):
        super().__init__()
        self.title = "PyQt5 ProgressBar"
        self.top = 200
        self.left = 500
        self.width = 300
        self.height = 100
        self.setWindowIcon(QtGui.QIcon("icon.png"))
        self.setWindowTitle(self.title)
        self.setGeometry(self.left, self.top, self.width, self.height)
        vbox = QVBoxLayout()
        self.progressbar = QProgressBar()
        #self.progressbar.setOrientation(Qt.Vertical)
        self.progressbar.setMaximum(100)
        self.progressbar.setStyleSheet("QProgressBar {border: 2px solid grey;border-radius:8px;padding:1px}"
                                       "QProgressBar::chunk {background:yellow}")
        #qlineargradient(x1: 0, y1: 0.5, x2: 1, y2: 0.5, stop: 0 red, stop: 1 white);
        #self.progressbar.setStyleSheet("QProgressBar::
        # chunk {background:
        # qlineargradient(x1: 0, y1: 0.5, x2: 1, y2: 0.5, stop: 0 red, stop: 1 white); }")
        #self.progressbar.setTextVisible(False)
        vbox.addWidget(self.progressbar)
        self.button = QPushButton("Start Progressbar")
        self.button.clicked.connect(self.startProgressBar)
        self.button.setStyleSheet('background-color:yellow')
        vbox.addWidget(self.button)
        self.setLayout(vbox)
        self.show()
 
    def startProgressBar(self):
        self.thread = MyThread()
        self.thread.change_value.connect(self.setProgressVal)
        self.thread.start()
 
    def setProgressVal(self, val):
        self.progressbar.setValue(val)
 
 
 
App = QApplication(sys.argv)
window = Window()
sys.exit(App.exec_())

Este será o resultado do código para o PyQt5 QProgressBar.

PyQt5 QProgressbar com QThread Exemplo prático

Além disso, você pode assistir ao vídeo completo do exemplo prático PyQt5 QProgressbar com QThread.

fonte do artigo em: https://codeloop.org

#python  #pyqt5