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SQL Query: 3 Methods for Calculating Cumulative SUM

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SQL provides various constructs for calculating cumulative sums, offering flexibility and efficiency in data analysis. In this article, we explore three distinct SQL queries that facilitate the computation of cumulative sums. Each query leverages different SQL constructs to achieve the desired outcome, catering to diverse analytical needs and preferences. Using Window Functions (e.g., PostgreSQL, SQL Server, Oracle) SELECT id, value, SUM(value) OVER (ORDER BY id) AS cumulative_sum  FROM your_table; This query uses the SUM() window function with the OVER clause to calculate the cumulative sum of the value column ordered by the id column. Using Subqueries (e.g., MySQL, SQLite): SELECT t1.id, t1.value, SUM(t2.value) AS cumulative_sum FROM your_table t1 JOIN your_table t2 ON t1.id >= t2.id GROUP BY t1.id, t1.value ORDER BY t1.id; This query uses a self-join to calculate the cumulative sum. It joins the table with itself, matching rows where the id in the first table is greater than or

All About Init and Delete Constructors Python

Python init Vs. del Constructors


Python class has two constructors. One is the init, and the other one is del. Why do you need these two and their real purpose explained?


The initialization method is called __init__ while the finalization or destructor method is called __del__. Python methods with a double underscore character are for internal (not intended for direct access by the outside world) use.

There are no true private methods in Python classes, but convention says that a method that begins with a single underscore is considered private, and a double underscore indicates it is internal (only to be used by the system.)

Python Constructors 

Init Constructor


object.__init__(self[, ...])

Called after the instance has been created (by __new__()), but before it is returned to the caller. The arguments are those passed to the class constructor expression. 


If a base class has an __init__() method, the derived class’s __init__() method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example, super().__init__([args...]).


Because __new__() and __init__() work together in constructing objects (__new__() to create it, and __init__() to customize it), no non-None value may be returned by __init__(); doing so will cause a TypeError to be raised at runtime.

Del Constructor

object.__del__(self)

Called when the instance is about to be destroyed. This is also called a finalizer or (improperly) a destructor. If a base class has a __del__() method, the derived class’s __del__() method, if any, must explicitly call it to ensure proper deletion of the base class part of the instance.

It is possible (though not recommended!) for the __del__() method to postpone the destruction of the instance by creating a new reference to it.

This is called object resurrection. It is implementation-dependent whether __del__() is called a second time when a resurrected object is about to be destroyed; the current CPython implementation only calls it once.

It is not guaranteed that __del__() methods are called for objects that still exist when the interpreter exits. Note: del x doesn’t directly call x.__del__() — the former decrements the reference count for x by one, and the latter is only called when the x’s reference count reaches zero.


Summary

  • A method that starts with a single underscore is PRIVATE
  • A method that starts with a double underscore is INTERNAL (not to use by the outside world).

Self Argument

In the python class methods, we supply an argument called SELF. After the self, you can pass on other arguments.

Example

def my_method(self, , , , , , ,)

The purpose of SELF is to access varibles that assigned in Class level.

Example


class Foo: 
wheels = 3 
max_speed_mph = 75.0 
def __init__(self): 
pass 
def printme(self): 
print(self.wheels)
print(self.max_speed_mph)


Here, the method printime has an argument of self. Also, here I've added a init method. The purpose of it as I said to use the system.

So, the self method and self as an argument in the method, useful to access class-level variables.


Class & Instance

Here, the main class name is 'Foo'. Since it's an object-oriented approach, you can create an 'n' number of instances.


foo = Foo()


Here, I have assigned the class 'Foo' to 'foo'. I can now access variables of the 'Foo' class.


foo.wheels 

==> 3

foo.max_speed_mph

==> 75.0


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