But IN makes this much cleaner:

SELECT *
FROM customers
WHERE city IN ('Pune', 'Mumbai', 'Delhi');

IN checks whether a value belongs to a specified list.

🔹 23. NOT IN

You can also exclude multiple values.

SELECT *
FROM customers
WHERE city NOT IN ('Pune', 'Mumbai');

This returns customers whose city isn't Pune or Mumbai.

🔹 24. LIKE

LIKE is used for pattern matching.

Suppose we want names beginning with A.

SELECT *
FROM customers
WHERE name LIKE 'A%';

Here:

% → Any sequence of characters

So this could match:

Alice

Amit

Ananya

🔹 25. LIKE with %

Example:

SELECT *
FROM customers
WHERE name LIKE '%an%';

This searches for names containing the sequence an.

The exact behavior can depend on database collation and case-sensitivity settings.

🔹 26. LIKE with _

The underscore _ generally represents exactly one character.

Example:

SELECT *
FROM products
WHERE product_code LIKE 'A_1';

This could match:

A11

AB1

AX1

But not:

A123

A1

because _ represents one character.

🔹 27. NULL Values

One of the most important concepts in SQL filtering is NULL.

NULL generally means:

Missing, unknown, or unavailable value.

It does not mean:

Zero

Empty string

False

For example:

customer_id name phone

101 Alice 9999999999

102 Bob NULL

Bob's phone number is missing or unknown.

🔹 28. Checking for NULL

You should not normally write:

WHERE phone = NULL

Instead, use:

SELECT *
FROM customers
WHERE phone IS NULL;

To find records where the value exists:

SELECT *
FROM customers
WHERE phone IS NOT NULL;

This is extremely important in Data Analytics.

🔹 29. WHERE and NULL Logic

Suppose:

WHERE salary > 50000

What happens when salary is NULL?

The condition isn't considered true.

The row won't be returned.

SQL uses three-valued logic involving:

TRUE

FALSE

UNKNOWN

This is one reason NULL handling requires special attention.

🔹 30. WHERE with SELECT

WHERE works together with SELECT.

Example:

SELECT
    customer_id,
    name,
    city
FROM customers
WHERE city = 'Pune';

The query:

1.

Retrieves selected columns

2.

From the customers table

3.

Keeps only rows satisfying the condition

🔹 31. WHERE in Real-World Data Science

Imagine a transaction database containing millions of records.

A Data Scientist needs:

Successful transactions above ₹10,000 from January 2026 onward.

A query might look like:

SELECT
    transaction_id,
    customer_id,
    transaction_date,
    amount
FROM transactions
WHERE status = 'Success'
AND amount > 10000
AND transaction_date >= '2026-01-01';

This is much more efficient for analysis than extracting the entire table and filtering everything later in Python.

🔹 32. WHERE Before Python

A common Data Science workflow is:

Database → SQL → Filter/Transform → Python → Analysis → Model

For example:

SELECT
    customer_id,
    amount,
    transaction_date
FROM transactions
WHERE status = 'Success';

Then load the result into Pandas:

import pandas as pd

df = pd.read_sql(query, connection)

SQL handles the database-side filtering, while Python can then handle deeper analysis.

🔹 33. Common Mistakes

❌ Mistake 1: Using = with NULL

Incorrect:

WHERE phone = NULL;

Correct:

WHERE phone IS NULL;

❌ Mistake 2: Forgetting quotes around text

Incorrect:

WHERE city = Pune;

Correct:

WHERE city = 'Pune';

❌ Mistake 3: Using AND when you mean OR

Incorrect if you want either city:

WHERE city = 'Pune'
AND city = 'Mumbai';

A single city value cannot normally be both at the same time.

Correct:

WHERE city = 'Pune'
OR city = 'Mumbai';

Or:

WHERE city IN ('Pune', 'Mumbai');

❌ Mistake 4: Forgetting parentheses

For complex conditions, use parentheses:

WHERE
    (city = 'Pune' OR city = 'Mumbai')
    AND age > 30;

❌ Mistake 5: Assuming BETWEEN excludes the boundaries

BETWEEN is generally inclusive.

🔹 34. Interview Questions

💡 What is the purpose of WHERE?

WHERE filters rows based on a condition.

💡 What is the difference between WHERE and SELECT?

SELECT → Determines what columns/expressions appear in the result.

WHERE → Determines which rows are included.

💡 How do you check for NULL?

Use:

IS NULL

or:

IS NOT NULL

💡 What is the difference between IN and OR?

IN provides a concise way to test whether a value matches any value in a list.

💡 Is BETWEEN inclusive?

Yes, BETWEEN generally includes both boundary values.

🎯 Practice Questions

Q1. Write a query to retrieve employees whose salary is greater than 50,000.

Q2. Write a query to retrieve customers from Pune or Mumbai.

Q3. Write a query to retrieve products priced between 1,000 and 5,000.

Q4. Write a query to retrieve customers whose phone number is missing.

Q5. Write a query to retrieve orders where the status is Success and the amount is greater than 10,000.

🎯 Key Takeaways

✅ WHERE is used to filter rows.

✅ = checks equality.

✅ <> and != can be used for not equal.

✅ AND requires all specified conditions to be true.

✅ OR requires at least one condition to be true.

✅ IN is useful for matching multiple values.

✅ BETWEEN is useful for ranges and is generally inclusive.

✅ LIKE is used for pattern matching.

✅ % represents a sequence of characters.

✅ _ represents one character.

✅ Use IS NULL and IS NOT NULL for NULL values.

✅ Parentheses make complex AND/OR logic clearer and safer.

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Important Excel, Tableau, Statistics, SQL related Questions with answers

1. What are the common problems that data analysts encounter during analysis?

The common problems steps involved in any analytics project are:

Handling duplicate data
Collecting the meaningful right data at the right time
Handling data purging and storage problems
Making data secure and dealing with compliance issues

2. Explain the Type I and Type II errors in Statistics?

In Hypothesis testing, a Type I error occurs when the null hypothesis is rejected even if it is true. It is also known as a false positive.

A Type II error occurs when the null hypothesis is not rejected, even if it is false. It is also known as a false negative.

3. How do you make a dropdown list in MS Excel?

First, click on the Data tab that is present in the ribbon.
Under the Data Tools group, select Data Validation.
Then navigate to Settings > Allow > List.
Select the source you want to provide as a list array.

4. How do you subset or filter data in SQL?

To subset or filter data in SQL, we use WHERE and HAVING clauses which give us an option of including only the data matching certain conditions.

5. What is a Gantt Chart in Tableau?

A Gantt chart in Tableau depicts the progress of value over the period, i.e., it shows the duration of events. It consists of bars along with the time axis. The Gantt chart is mostly used as a project management tool where each bar is a measure of a task in the project

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Complete Data Analytics Mastery: From Basics to Advanced 🚀

Begin your Data Analytics journey by mastering the fundamentals:
- Understanding Data Types and Formats
- Basics of Exploratory Data Analysis (EDA)
- Introduction to Data Cleaning Techniques
- Statistical Foundations for Data Analytics
- Data Visualization Essentials

Grasp these essentials in just a week to build a solid foundation in data analytics.

Once you're comfortable, dive into intermediate topics:
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- SQL for Data Analytics

Take another week to solidify these skills and enhance your ability to draw meaningful insights from data.

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These advanced concepts can be mastered in a couple of weeks with focused study and practice.

Remember, mastery comes with hands-on experience:
- Work on a simple data analytics project
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Consistent practice and application of analytics techniques are the keys to becoming a data analytics pro.

Best platforms to learn:
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