Joins in Python – Ultimate Guide on Joins Functions

In this article, we are going to discuss what are the Joins functionality in Python and how they work in detail using examples.

Python Join function join() used to join all the elements from the iterable create a string and return it as an output to the user.

The Python join() function is used to join all the elements from the iterable create a string and return it as an output to the user.

Joins Functions in Python

Joins Functions play a vital role in Database management systems (DBMS). There are three types of joins i.e

  1. Inner Join
  2. Outer Join
  3. Full Join.

These joins are used to retrieve data from a table based on specified fields. Joins can be classified as CROSS JOIN, OUTER JOIN, and INNER JOIN.

In CROSS JOIN query rows are joined with all rows of other tables whereas INNER JOIN query rows are joined with some rows from another table.

A full join returns only those records that meet both criteria. An inner join returns only those records that meet both criteria; outer joins return records even if they fail one criterion.

An outer join also combines two tables and has access to one or more columns of each table.

The right-hand side arguments can be any iterable objects, not just sequences (lists, tuples) or list-like objects.

If a sequence is given as a left argument, its elements must appear in order from first to last.

If an iterator is given instead of an argument, it will yield its elements one by one until exhausted.

Elements are matched according to their position in keys. For example, ‘abc’ will match against ‘bc’ but not against ‘abcdef’. ‘*’ matches anything but maybe a lot slower than element matching because of the continued reopening of files.

As opposed to using the python join () function you can use Structural Operators which is much faster.

Joins Methods Usability

Join() function of python is very easy to use. All you need to do is call a function, pass an iterable object or sequence from which you want to take elements, and let python handle the rest.

You can also join a string with a list by using the + operator. Join methods in python will be useful when we have a list of numbers and want to put commas between them.

For example, if your user provides values 1, 2, 3 then according to those values we need a comma as the join method.

We can easily write code as follows:

result = '1 , 2 , 3' 


#Output: 1 , 2 , 3 

Joins Methods Usability: Join() function of python is very easy to use.

Joining Multiple Lists Together

In python join() function can be used to join two or more lists and return a single list from all of them.

The function can accept multiple arguments as well and combine them into a single string value, which is returned to users.

This can come in very handy when you have some data stored separately and need to bring it together for later use.

Join() will help you do that with ease by turning each element within each iterable into a single string value.

Each new element is placed directly after another element with no spaces or other breakable spaces between them.

Every element is joined together to create one large output of text. You’ll find several scenarios where join() might come in handy; here are three common examples:

1) Joining Newlines,

2) Joining Strings and

3) Joining Lists Together with Join().

For join(), we need the following:

1) A source Iterable,

2) An optional separator string, and

3) An optional extension (join.extend).

The Join Method has been added to strings since Python version 2.0. It lets us join any number of strings or even sequences together without needing to call join() repeatedly.

Joins Operators in SQL

Joins are used to combine strings together, Inner joins and left outer joins can be achieved using python’s built-in function sep(). If you would like a full list of all available joins, check out Join Operators in SQL.

The most basic join operation is called an inner join or equi-join, which connects rows from two data sets based on matching values between each pair.

In simple terms, we want to combine rows that have identical values for all of their attributes.

To create a left join you simply add a + sign to your query where you would normally place your equals sign (=).

This will tell SQL to return all of your results and include any unmatched records from the second table as well.

For example, if I had two tables named employees (EMPLOYEE_ID and NAME) and locations (LOCATION_ID and NAME), I could do an inner join to get only those employees who work at a particular location like so:

SELECT employee_id FROM employees LEFT JOIN locations ON employee_id = location_id; 

This leaves me with just those employees whose names matched up with their respective locations.

Different Joins Examples in Python

There are two join types available in python which includes inner and outer joins.

Inner join: It is used to select all rows from the left table even if there is no match found from the right table.

Outer join: It selects all columns from the left table even if there is no match found from the right table.

The other types of joins include natural, left, right, self join, and anti join. For example, inner_join(A, B), means that when A has a column called ID it will be joined with all columns of B in order to create new records using values from both tables.

Left join: Left join returns tuple only if the row exists on left otherwise tuple with null value or blank string will be returned for each position where row does not exist on the right side.

Right Join: Right Join returns tuple only if the row exists on the right otherwise tuple with a null value or blank string will be returned for each position where the row does not exist on the left side.

Natural join: A natural join is a special type of relational database operation that combines rows from two tables, Natural join returns all tuples common to both tables but row otherwise.

This operation is commonly used to associate data from one table with data from another table when there is no other explicit relationship defined between them.

The result of a natural join (or equi-join) is that all rows from both tables are joined together to create one larger table containing all possible combinations of pairs of rows.

Join is used to join all elements from iterable separated by a string operator, The join function is used to join all elements from an iterable create a string and return it as an output to the user.

1. Concatenation Join: Joins two strings together using either + or.

2. Substring Join: Joins every element with another element of the same length but at a specified start index and ending index.

Python Code for Joins

join() performs a concatenation of elements from an iterable separated by another element.

When you need to join all your elements into one, pass your list to join() and use it as an output. Join is also available under python dictionary keys but that is not considered a good practice.

Here’s some sample code using join():

list_of_numbers = [1,2,3] 


## 1 2 3 

This code joins all its elements into one string, One might have desired to know that it is actually a shortcut for

str(list_of_numbers) + ', ' + str(list_of_numbers) 

## 1 2 3 

Join provides you with similar functionality if you’re working with python lists or other iterable as well.

Let us take a look at an example that uses another iterable:

list_of_tuples = [(1,2), (3,4)]


## (1, 2, 3, 4)

This will return you a string containing all elements from list of tuples and join them using a comma separator.

Join also comes handy if you’re working with strings themselves, for example:

one, two.join([three, four]) 

## 'onetwothreefour'

Here I’ve combined my two strings with a comma separator. Join can even be used to join arrays:

one, three, five.join([six, seven]) 

## 'onetwothreesixseven'

The python join() function takes as input an iterable and joins all its elements using a string operator as shown above and returns an output which can then be used where ever you may need to combine multiple values into one string or one iterable into another.

Related Article: Top 11 Data Visualization Libraries in Python.


Join() is a built-in function in python that returns a string created by joining all elements from an iterable to it and separating them using a specified separator.

The returned output of join() function is always a string that can be converted into various data types as required by the user.

Join() function can be used to join lines from files, fields from records etc.

Join() function will return a tuple containing concatenated values for given set of elements if there is no argument passed.

Join(separator,iterable) in python creates a tuple filled with values taken from iterable separated by ‘separator’ string.

Join(separator, iterable[,maxlen]) takes more than one parameter but syntax remains the same for both two parameters except maxlen parameter where 3rd parameter indicates the maximum length of joined strings to return back.