Python PIP

PIP is a package manager for Python packages, or modules if you like.

Note: If you have Python version 3.4 or later, PIP is included by default.

What is a Package?

A package contains all the files you need for a module. Modules are Python code libraries you can include in your project.

Check if PIP is Installed

Navigate your command line to the location of Python’s script directory, and type the following:

Example

Check PIP version:

C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip –version
[mai mult...]

Python Scope

A variable is only available from inside the region it is created. This is called scope.

Local Scope

A variable created inside a function belongs to the local scope of that function, and can only be used inside that function.

Example

A variable created inside a function is available inside that function:

def myfunc():
x = 300
print(x)myfunc()

Function Inside Function

As explained in the example above, the variable x is not available outside the function, but it is available for any function inside the function:

Example

The local variable can be accessed from a function within the function:

def myfunc():
x = 300
def myinnerfunc():
print(x)
myinnerfunc()myfunc()

Global Scope

A variable created in the main body of the Python code is a global variable and belongs to the global scope. Global variables are available from within any scope, global and local.

Example

A variable created outside of a function is global and can be used by anyone:

x = 300

def myfunc():
print(x)

myfunc()

print(x)

[mai mult...]

Python Inheritance

Inheritance allows us to define a class that inherits all the methods and properties from another class.

  • Parent class is the class being inherited from, also called base class.
  • Child class is the class that inherits from another class, also called derived class.

Any class can be a parent class, so the syntax is the same as creating any other class:

  • Example

Create a class named Person, with firstname and lastname properties, and a printname method:

class Person:
def __init__(self, fname, lname):
self.firstname = fname
self.lastname = lnamedef printname(self):
print(self.firstname, self.lastname)

#Use the Person class to create an object, and then execute the printname method:

x = Person(“John”, “Doe”)
x.printname()

  • Create a Child Class

To create a class that inherits the functionality from another class, send the parent class as a parameter when creating the child class:

Create a class named Student, which will inherit the properties and methods from the Person class:

class Student(Person):
pass
[mai mult...]

Python Lambda

A lambda function is a small anonymous function. A lambda function can take any number of arguments, but can only have one expression.

Syntax
lambda arguments : expression

The expression is executed and the result is returned:

Example

Add 10 to argument a, and return the result:

x = lambda a : a + 10
print(x(5
[mai mult...]

SQL EXISTS Operator

The SQL EXISTS Operator

The EXISTS operator is used to test for the existence of any record in a subquery.

The EXISTS operator returns TRUE if the subquery returns one or more records.

EXISTS Syntax
SELECT column_name(s)
FROM table_name
WHERE EXISTS
(SELECT column_name FROM table_name WHERE condition);
Demo Database

Below is a selection from the “Products” table in the Northwind sample database:

ProductID ProductName SupplierID CategoryID Unit Price
1 Chais 1 1 10 boxes x 20 bags 18
2 Chang 1 1 24 – 12 oz bottles 19
3 Aniseed Syrup 1 2 12 – 550 ml bottles 10
4 Chef Anton’s Cajun Seasoning 2 2 48 – 6 oz jars 22
5 Chef Anton’s Gumbo Mix 2 2 36 boxes 21.35

And a selection from the “Suppliers” table:

SupplierID SupplierName ContactName Address City PostalCode Country
1 Exotic Liquid Charlotte Cooper 49 Gilbert St. London EC1 4SD UK
2 New Orleans Cajun Delights Shelley Burke P.O. Box 78934 New Orleans 70117 USA
3 Grandma Kelly’s Homestead Regina Murphy 707 Oxford Rd. Ann Arbor 48104 USA
4 Tokyo Traders Yoshi Nagase 9-8 Sekimai Musashino-shi Tokyo 100 Japan
[mai mult...]

SQL HAVING Clause

The SQL HAVING Clause

The HAVING clause was added to SQL because the WHERE keyword cannot be used with aggregate functions.

HAVING Syntax
SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
HAVING condition
ORDER BY column_name(s);
Demo Database

Below is a selection from the “Customers” table in the Northwind sample database:

CustomerID CustomerName ContactName Address City PostalCode Country
1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany
2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico
3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 México D.F. 05023 Mexico
4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK
5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S-958 22 Sweden
SQL HAVING Examples

The following SQL statement lists the number of customers in each country. Only include countries with more than 5 customers:

Example
SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
HAVING COUNT(CustomerID) > 5;

The following SQL statement lists the number of customers in each country, sorted high to low (Only include countries with more than 5 customers):

Example
SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
HAVING COUNT(CustomerID) > 5
ORDER BY COUNT(CustomerID) DESC;The following SQL statement lists the number of customers in each country, sorted high to low (Only include countries with more than 5 customers):
[mai mult...]

SQL GROUP BY Statement

The SQL GROUP BY Statement

The GROUP BY statement groups rows that have the same values into summary rows, like “find the number of customers in each country”.

The GROUP BY statement is often used with aggregate functions (COUNT(), MAX(), MIN(), SUM(), AVG()) to group the result-set by one or more columns.

GROUP BY Syntax
SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);
Demo Database

Below is a selection from the “Customers” table in the Northwind sample database:

CustomerID CustomerName ContactName Address City PostalCode Country
1 Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany
2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico
3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 México D.F. 05023 Mexico
4 Around the Horn Thomas Hardy 120 Hanover Sq. London WA1 1DP UK
5 Berglunds snabbköp Christina Berglund Berguvsvägen 8 Luleå S-958 22 Sweden

 

[mai mult...]

SQL UNION Operator

The SQL UNION Operator

The UNION operator is used to combine the result-set of two or more SELECT statements.

  • Every SELECT statement within UNION must have the same number of columns
  • The columns must also have similar data types
  • The columns in every SELECT statement must also be in the same order
UNION Syntax
SELECTcolumn_name(s)FROMtable1
UNION
SELECTcolumn_name(s)FROMtable2;
UNION ALL Syntax

The UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL:

SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;

Note: The column names in the result-set are usually equal to the column names in the first SELECT statement.

[mai mult...]

React JSX

What is JSX?
  • JSX stands for JavaScript XML.
  • JSX allows us to write HTML in React.
  • JSX makes it easier to write and add HTML in React.
Coding JSX

JSX allows us to write HTML elements in JavaScript and place them in the DOM without any createElement()  and/or appendChild() methods.

  • JSX converts HTML tags into react elements.
  • You are not required to use JSX, but JSX makes it easier to write React applications.
  • Here are two examples. The first uses JSX and the second does not:
Example 1

JSX:

const myelement = <h1>I Love JSX!</h1>;

ReactDOM.render(myelement, document.getElementById('root'));
[mai mult...]

Styling React Using Sass

What is Sass

Sass is a CSS pre-processor.

Sass files are executed on the server and sends CSS to the browser.

Can I use Sass?

If you use the create-react-app in your project, you can easily install and use Sass in your React projects.

Install Sass by running this command in your terminal:

>npm i sass

Now you are ready to include Sass files in your project!

Create a Sass file

Create a Sass file the same way as you create CSS files, but Sass files have the file extension .scss

In Sass files you can use variables and other Sass functions:

my-sass.scss:

Create a variable to define the color of the text:

$myColor: red;

h1 {
  color: $myColor;
}
[mai mult...]