Python MySQL Tutorial – Using the WHERE Clause to Filter Data

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Tags:- MySQL Python

The WHERE clause in SQL allows you to filter rows returned by a query based on specific conditions. In this tutorial, you'll learn how to use the WHERE clause in MySQL queries through Python using the mysql-connector-python package.


Table of Contents

  1. What is the WHERE Clause?

  2. Prerequisites

  3. Install MySQL Connector

  4. Connect to MySQL with Python

  5. Use WHERE Clause with a Single Condition

  6. Use Multiple Conditions (AND, OR)

  7. Use Wildcards (LIKE)

  8. Use Placeholders to Prevent SQL Injection

  9. Full Working Example

  10. Tips and Common Pitfalls


✅ 1. What is the WHERE Clause?

The WHERE clause is used in SQL to filter records that meet a certain condition.

Example (SQL):

SELECT * FROM users WHERE age > 25;

⚙️ 2. Prerequisites

You need the following:

  • Python installed

  • MySQL Server with a database and table

  • The mysql-connector-python library installed


3. Install MySQL Connector

Use pip to install the MySQL connector:

pip install mysql-connector-python

4. Connect to MySQL with Python

import mysql.connector

db = mysql.connector.connect(
    host="localhost",
    user="your_username",
    password="your_password",
    database="mydatabase"
)

cursor = db.cursor()

Replace with your actual credentials.


5. Use WHERE Clause with a Single Condition

sql = "SELECT * FROM users WHERE age > %s"
val = (25,)

cursor.execute(sql, val)
results = cursor.fetchall()

for row in results:
    print(row)

Explanation:

  • %s is a placeholder that prevents SQL injection.

  • val is a tuple with values to substitute into the query.


6. Use Multiple Conditions (AND / OR)

AND Condition:

sql = "SELECT * FROM users WHERE age > %s AND city = %s"
val = (25, "New York")

OR Condition:

sql = "SELECT * FROM users WHERE age < %s OR city = %s"
val = (20, "Chicago")

7. Use Wildcards with LIKE

Use LIKE to perform pattern matching.

sql = "SELECT * FROM users WHERE name LIKE %s"
val = ("%J%",)  # Names containing 'J'

cursor.execute(sql, val)
results = cursor.fetchall()

for row in results:
    print(row)

8. Use Placeholders to Prevent SQL Injection

Always use %s placeholders and avoid string concatenation in SQL queries:

Don’t do this:

sql = "SELECT * FROM users WHERE name = '" + name + "'"

Do this:

sql = "SELECT * FROM users WHERE name = %s"
val = (name,)

9. Full Working Example

import mysql.connector
from mysql.connector import Error

def filter_users():
    try:
        db = mysql.connector.connect(
            host="localhost",
            user="root",
            password="your_password",
            database="mydatabase"
        )

        cursor = db.cursor()

        # Filter users older than 25 in New York
        sql = "SELECT id, name, email FROM users WHERE age > %s AND city = %s"
        val = (25, "New York")

        cursor.execute(sql, val)
        results = cursor.fetchall()

        for row in results:
            print(row)

    except Error as e:
        print("Error:", e)

    finally:
        if db.is_connected():
            db.close()
            print("Connection closed.")

filter_users()

⚠️ 10. Common Pitfalls and Tips

Pitfall Solution
SQL injection vulnerability Always use %s placeholders
Wrong number of parameters Ensure the number of %s matches values
Forgetting comma in single-value tuple Use a comma: ("New York",)
Empty results Make sure the data actually matches the condition

✅ Tips:

  • Use fetchone() for a single result, fetchall() for multiple.

  • Debug by printing the query and values.

  • Use LIKE '%value%' for substring searches.


Summary Table

Task Query Example
Filter by age "WHERE age > %s"
Multiple conditions "WHERE age > %s AND city = %s"
Pattern match "WHERE name LIKE %s"
Safe query Use parameterized queries (%s)

Final Thoughts

Using the WHERE clause in MySQL with Python helps you query and filter records effectively. By writing safe, parameterized queries, you protect your application from SQL injection and ensure reliable database operations.

 

Tips and Tricks


What is pass in Python?

Python | Pass Statement

The pass statement is used as a placeholder for future code. It represents a null operation in Python. It is generally used for the purpose of filling up empty blocks of code which may execute during runtime but has yet to be written.

 

def myfunction():
    pass

 


How can you generate random numbers?

Python | Generate random numbers

Python provides a module called random using which we can generate random numbers. e.g: print(random.random())

 

 

We have to import a random module and call the random() method as shown below:

 import random

 print(random.random())

The random() method generates float values lying between 0 and 1 randomly.


To generate customized random numbers between specified ranges, we can use the randrange() method
Syntax: randrange(beginning, end, step)
 

import random

print(random.randrange(5,100,2))

 


What is lambda in Python?

Python | Lambda function

A lambda function is a small anonymous function. This function can have any number of parameters but, can have just one statement.
 

 

Syntex: 
lambda arguments : expression
 

a = lambda x,y : x+y

print(a(5, 6))

It also provides a nice way to write closures. With that power, you can do things like this.

def adder(x):
    return lambda y: x + y

add5 = adder(5)

add5(1)    #6

As you can see from the snippet of Python, the function adder takes in an argument x and returns an anonymous function, or lambda, that takes another argument y. That anonymous function allows you to create functions from functions. This is a simple example, but it should convey the power lambdas and closures have.
 


What is swapcase() function in the Python?

Python | swapcase() Function

It is a string's function that converts all uppercase characters into lowercase and vice versa. It automatically ignores all the non-alphabetic characters.
 

string = "IT IS IN LOWERCASE."  

print(string.swapcase())  

 


How to remove whitespaces from a string in Python?

Python | strip() Function | Remove whitespaces from a string 

To remove the whitespaces and trailing spaces from the string, Python provides a strip([str]) built-in function. This function returns a copy of the string after removing whitespaces if present. Otherwise returns the original string.
 

string = "  Python " 
 
print(string.strip())  

 


What is the usage of enumerate() function in Python?

Python | enumerate() Function

The enumerate() function is used to iterate through the sequence and retrieve the index position and its corresponding value at the same time.
 

lst = ["A","B","C"] 
 
print (list(enumerate(lst)))

#[(0, 'A'), (1, 'B'), (2, 'C')]

 


Can you explain the filter(), map(), and reduce() functions?

Python | filter(), map(), and reduce() Functions

  • filter()  function accepts two arguments, a function and an iterable, where each element of the iterable is filtered through the function to test if the item is accepted or not.
    >>> set(filter(lambda x:x>4, range(7)))
    
    # {5, 6}
    
    

     

  • map() function calls the specified function for each item of an iterable and returns a list of result

    >>> set(map(lambda x:x**3, range(7)))
    
    # {0, 1, 64, 8, 216, 27, 125}

     

  • reduce() function reduces a sequence pair-wise, repeatedly until we arrive at a single value..
     

    >>> reduce(lambda x,y:y-x, [1,2,3,4,5])
    
    # 3
    

    Let’s understand this:

    2-1=1
    3-1=2
    4-2=2
    5-2=3

    Hence, 3.

 


What is a namedtuple?

Python | namedtuple

A namedtuple will let us access a tuple’s elements using a name/label. We use the function namedtuple() for this, and import it from collections.

>>> from collections import namedtuple

#format
>>> result=namedtuple('result','Physics Chemistry Maths') 

#declaring the tuple
>>> Chris=result(Physics=86,Chemistry=92,Maths=80) 

>>> Chris.Chemistry
# 92

 


Write a code to add the values of same keys in two different dictionaries and return a new dictionary.

We can use the Counter method from the collections module

from collections import Counter

dict1 = {'a': 5, 'b': 3, 'c': 2}
dict2 = {'a': 2, 'b': 4, 'c': 3}

new_dict = Counter(dict1) + Counter(dict2)


print(new_dict)
# Print: Counter({'a': 7, 'b': 7, 'c': 5})


 


Python In-place swapping of two numbers

 Python | In-place swapping of two numbers

>>> a, b = 10, 20
>>> print(a, b)
10 20

>>> a, b = b, a
>>> print(a, b)
20 10

 


Reversing a String in Python

Python | Reversing a String

>>> x = 'PythonWorld'
>>> print(x[: : -1])
dlroWnohtyP

 


Python join all items of a list to convert into a single string

Python | Join all items of a list to convert into a single string

>>> x = ["Python", "Online", "Training"]
>>> print(" ".join(x))
Python Online Training

 


python return multiple values from functions

Python | Return multiple values from functions

>>> def A():
	return 2, 3, 4

>>> a, b, c = A()

>>> print(a, b, c)
2 3 4

 


Python Print String N times

Python | Print String N times

>>> s = 'Python'
>>> n = 5

>>> print(s * n)
PythonPythonPythonPythonPython

 


Python check the memory usage of an object

Python | Check the memory usage of  an object

>>> import sys
>>> x = 100

>>> print(sys.getsizeof(x))
28