Born to vibe. Forced to survive.

Learn SQL from basic to advanced level in 30 days

Week 1: SQL Basics

Day 1: Introduction to SQL and Relational Databases

Overview of SQL Syntax

Setting up a Database (MySQL, PostgreSQL, or SQL Server)


Day 2: Data Types (Numeric, String, Date, etc.)

Writing Basic SQL Queries:

SELECT, FROM

Day 3: WHERE Clause for Filtering Data

Using Logical Operators:

AND, OR, NOT

Day 4: Sorting Data: ORDER BY

Limiting Results: LIMIT and OFFSET

Understanding DISTINCT

Day 5: Aggregate Functions:

COUNT, SUM, AVG, MIN, MAX


Day 6: Grouping Data: GROUP BY and HAVING

Combining Filters with Aggregations


Day 7: Review Week 1 Topics with Hands-On Practice

Solve SQL Exercises on platforms like HackerRank, LeetCode, or W3Schools


Week 2: Intermediate SQL

Day 8: SQL JOINS:

INNER JOIN, LEFT JOIN

Day 9: SQL JOINS Continued: RIGHT JOIN, FULL OUTER JOIN, SELF JOIN

Day 10: Working with NULL Values

Using Conditional Logic with CASE Statements

Day 11: Subqueries: Simple Subqueries (Single-row and Multi-row)

Correlated Subqueries

Day 12: String Functions:

CONCAT, SUBSTRING, LENGTH, REPLACE

Day 13: Date and Time Functions: NOW, CURDATE, DATEDIFF, DATEADD

Day 14: Combining Results: UNION, UNION ALL, INTERSECT, EXCEPT

Review Week 2 Topics and Practice

Week 3: Advanced SQL

Day 15: Common Table Expressions (CTEs)

WITH Clauses and Recursive Queries

Day 16: Window Functions:

ROW_NUMBER, RANK, DENSE_RANK, NTILE

Day 17: More Window Functions:

LEAD, LAG, FIRST_VALUE, LAST_VALUE


Day 18: Creating and Managing Views

Temporary Tables and Table Variables

Day 19: Transactions and ACID Properties

Working with Indexes for Query Optimization

Day 20: Error Handling in SQL

Writing Dynamic SQL Queries


Day 21: Review Week 3 Topics with Complex Query Practice

Solve Intermediate to Advanced SQL Challenges



Week 4: Database Management and Advanced Applications

Day 22: Database Design and Normalization:

1NF, 2NF, 3NF


Day 23: Constraints in SQL:
PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK, DEFAULT


Day 24: Creating and Managing Indexes

Understanding Query Execution Plans

Day 25: Backup and Restore Strategies in SQL

Role-Based Permissions

Day 26: Pivoting and Unpivoting Data

Working with JSON and XML in SQL

Day 27: Writing Stored Procedures and Functions

Automating Processes with Triggers

Day 28: Integrating SQL with Other Tools (e.g., Python, Power BI, Tableau)

SQL in Big Data: Introduction to NoSQL

Day 29: Query Performance Tuning:

Tips and Tricks to Optimize SQL Queries


Day 30: Final Review of All Topics

Attempt SQL Projects or Case Studies (e.g., analyzing sales data, building a reporting dashboard)

Since SQL is one of the most essential skill for data analysts, I have decided to teach each topic daily in this channel for free. Like this post if you want me to continue this SQL series 👍♥️

Share with credits: https://t.me/sqlspecialist

Hope it helps :)

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🚀 Data Analyst Roadmap — Part 29

POWER BI LEVEL 8 — ADVANCED DAX: FILTER(), VALUES(), SELECTEDVALUE() & DYNAMIC CALCULATIONS

Now let's move into DAX functions that help you build more dynamic Power BI reports.

These functions are especially useful when your calculation needs to react to slicers, selections, or the current report context.

🔹 1. FILTER()

You already know that FILTER() can create a filtered table.

Example:

High Value Sales =
CALCULATE(
    [Total Sales],
    FILTER(
        Sales,
        Sales[SalesAmount] > 10000
    )
)

This keeps only transactions where SalesAmount is greater than 10,000.

The important thing to understand:

• "FILTER()" works with a table and evaluates a condition for each row.

• Use it when your filtering requirement is more complex than a simple condition.

🔹 2. VALUES()

"VALUES()" returns the unique values from a column based on the current filter context.

Example:

Customer Count =
COUNTROWS(
    VALUES(Sales[CustomerID])
)

This counts the unique customers visible in the current context.

For example:

• Without filters → 1,000 customers

• Region = West → 250 customers

• Region = South → 300 customers

The result changes according to the report filters.

🔹 3. VALUES() vs DISTINCT()

Both can return unique values, but they aren't identical in every situation.

A useful beginner-level rule:

• "DISTINCT()" → returns unique values from a column.

• "VALUES()" → returns unique values while also being sensitive to the current DAX context and can include a blank value when appropriate.

In advanced DAX, "VALUES()" is extremely useful for understanding what values are currently available in the filter context.

🔹 4. SELECTEDVALUE()

This is one of the most useful functions for interactive reports.

Suppose you have a Region slicer.

You can write:

Selected Region =
SELECTEDVALUE(
    Sales[Region],
    "Multiple Regions"
)

If the user selects:

• West → Result: West

• West + South → Result: Multiple Regions

If nothing is selected, the result can also return the alternate value depending on the filter context.

🔹 5. SELECTEDVALUE() with Dynamic Titles

You can use SELECTEDVALUE() to make report titles dynamic.

Example:

Sales Title =
"Sales Performance - "
&
SELECTEDVALUE(
    Sales[Region],
    "All Regions"
)

If the user selects West:

• Sales Performance - West

If multiple regions are selected:

• Sales Performance - All Regions

This makes dashboards much more interactive.

🔹 6. HASONEVALUE()

"HASONEVALUE()" checks whether exactly one unique value exists in the current filter context.

Example:

Single Region Selected =
IF(
    HASONEVALUE(Sales[Region]),
    "One Region",
    "Multiple Regions"
)

If exactly one region is selected:

• One Region

Otherwise:

• Multiple Regions

🔹 7. SELECTEDVALUE() vs HASONEVALUE()

They are related but serve different purposes.

"HASONEVALUE()" asks:

• "Is exactly one value selected?"

"SELECTEDVALUE()" asks:

• "What is that selected value?"

For example:

SELECTEDVALUE(Sales[Region]) returns the actual region.

HASONEVALUE(Sales[Region]) returns TRUE or FALSE.

🔹 8. Dynamic KPI Calculation

Suppose you want a KPI to change based on a slicer containing:

• Sales

• Profit

• Orders

A measure can use the selected value to determine what should be displayed.

Conceptually:

Selected KPI =
SWITCH(
    SELECTEDVALUE(KPI[KPI Name]),
    "Sales", [Total Sales],
    "Profit", [Total Profit],
    "Orders", [Total Orders]
)

Now one visual can display different KPIs based on the user's selection.

This is called a:

• 👉 Dynamic Measure

🔹 9. SWITCH()

"SWITCH()" is extremely useful for dynamic DAX.

Instead of writing many nested IF statements:

Performance =
SWITCH(
    TRUE(),
    [Profit Margin] >= 0.30, "Excellent",
    [Profit Margin] >= 0.15, "Good",
    [Profit Margin] >= 0, "Needs Improvement",
    "Loss"
)

It evaluates conditions and returns the corresponding result.

This is useful for:

✔ KPI categories

✔ Business rules

✔ Dynamic labels

✔ Conditional calculations

✔ Performance classification

🔹 10. Building a Dynamic Customer Message

You can combine these functions to create business-friendly messages.

Example:

Customer Message =
"Selected Customers: "
&
COUNTROWS(VALUES(Sales[CustomerID]))

If the current filter context contains 125 unique customers:

• Selected Customers: 125

This can be displayed inside a Card or used in a report title.

🔹 11. Why These Functions Matter

Real dashboards rarely show the same calculation under every situation.

Users interact with:

• Slicers

• Filters

• Drill-downs

• Cross-highlighting

• Page filters

Your DAX measures should respond appropriately.

Functions such as:

• "FILTER()"

• "VALUES()"

• "SELECTEDVALUE()"

• "HASONEVALUE()"

• "SWITCH()"

help you build that dynamic behavior.

🎯 Interview Questions

1️⃣ What does FILTER() do?

• It returns a filtered table based on a specified condition.

2️⃣ What does SELECTEDVALUE() return?

• The single value in the current context, or an alternate result when there isn't exactly one value.

3️⃣ What is HASONEVALUE() used for?

• To check whether exactly one unique value exists in the current filter context.

4️⃣ How can SELECTEDVALUE() be used in a dashboard?

• It can create dynamic titles, labels, messages, and calculations based on slicer selections.

5️⃣ Why is SWITCH() useful in DAX?

• It allows multiple conditions or selections to determine which result should be returned.

🧪 PRACTICE

Create a Region slicer.

Then create:

✔ Selected Region

✔ Customer Count

✔ Dynamic Sales Title

✔ Dynamic KPI using SWITCH()

✔ One Region / Multiple Regions indicator

Select different regions and observe how every measure responds.

💡 Key lesson:

Advanced DAX is largely about making calculations respond intelligently to the user's current context.

Once you understand:

• FILTER()

• VALUES()

• SELECTEDVALUE()

• HASONEVALUE()

• SWITCH()

you can start building genuinely interactive Power BI reports.

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✅ SQL Interview Questions with Answers

1. What is a window function? 
A window function computes results over a group ("window") of rows related to the current row, without collapsing them (like GROUP BY).

Examples: ROW_NUMBER(), RANK(), SUM() OVER(...) for running totals, rankings, or moving averages.

2. What is the difference between RANK() and ROW_NUMBER()? 
• ROW_NUMBER(): assigns unique sequential numbers to all rows, even if values are equal.
• RANK(): gives same rank to tied values, then skips the next rank (e.g., 1, 1, 3).

3. How do you find the second highest salary? 
SELECT salary 
FROM ( 
  SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) as rnk 
  FROM employees 
) t 
WHERE rnk = 2; 
This avoids ties if you want exactly the second‑highest value.

4. What is a recursive CTE? 
A recursive CTE refers to itself in its WITH definition, usually in the form "anchor + UNION ALL recursive step". It is used for hierarchical data like managers‑employees, org charts, or tree structures.

5. What is the difference between correlated and non-correlated subquery? 
• Non‑correlated: runs once, independent of the outer query.
• Correlated: references columns from the outer query and runs once per outer row (e.g., SELECT ... FROM t1 WHERE col > (SELECT AVG(col) FROM t2 WHERE t2.id = t1.id)).

6. How do you remove duplicates without DISTINCT? 
Use window functions: 
DELETE FROM ( 
  SELECT ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY id) as rn 
  FROM table 
) t 
WHERE rn > 1; 
Or use GROUP BY and keep one row per group.

7. What is an INDEX and when do you use it? 
An index speeds up data retrieval on specified columns (used in WHERE, JOIN, ORDER BY). Use it on columns that are frequently filtered or joined; avoid on very small tables or columns updated often.

8. Explain self-join with example. 
A self‑join joins a table to itself using aliases. Example: 
SELECT e1.name as employee, e2.name as manager 
FROM employees e1 
LEFT JOIN employees e2 ON e1.manager_id = e2.id; 
Useful for parent‑child relationships.

9. What is the difference between DELETE, DROP, and TRUNCATE? 
• DELETE: removes rows (can be filtered by WHERE), can be rolled back.
• TRUNCATE: removes all rows quickly, resets storage; often not logged per row.
• DROP: removes entire table (structure + data); cannot be rolled back.

10. How do you pivot/unpivot data in SQL? 
• Pivot: turns rows into columns (e.g., sales per month as columns) using PIVOT or conditional aggregation (MAX(CASE WHEN ... END)).
• Unpivot: turns columns into rows (e.g., multiple month columns → one month column) using UNPIVOT or UNION ALL/VALUES.

11. What is LAG() and LEAD()? 
• LAG(col, n): value of col from n rows before current row.
• LEAD(col, n): value from n rows after. Used for time‑series analysis (MoM change, prior/next values).

12. How do you handle NULL in aggregates? 
Most aggregates (SUM, AVG, MAX, MIN) ignore NULL. 
• COUNT(col) ignores NULL; COUNT(*) counts all rows.
• Use COALESCE() or ISNULL() to replace NULL before aggregating.

13. What is the difference between VIEW and MATERIALIZED VIEW? 
• VIEW: virtual table; query runs every time you select.
• MATERIALIZED VIEW: stores result physically and refreshes periodically; faster reads, slower updates.

14. Explain ACID properties. 
• Atomicity: transaction is "all or nothing".
• Consistency: valid state before and after.
• Isolation: concurrent transactions don't interfere.
• Durability: committed changes survive crashes.

15. How do you optimize a slow query? 
• Add proper indexes on WHERE, JOIN, ORDER BY columns.
• Remove unnecessary SELECT *, DISTINCT, or functions on indexed columns.
• Check execution plan and avoid large scans; use LIMIT or partitioning if possible.

16. What is the difference between INNER JOIN and EXISTS? 
• INNER JOIN: returns combined columns from both tables where keys match.
• EXISTS: checks if a subquery returns any rows; usually faster when you only care about existence (e.g., filtering with WHERE EXISTS).

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🚀 Data Analyst Roadmap — Part 30

POWER BI LEVEL 9 — DAX VARIABLES: VAR, RETURN & CLEANER DAX

As DAX calculations become more complex, writing everything in one expression can make your measures difficult to understand and maintain.

That's where VAR and RETURN become extremely useful.

🔹 1. What is VAR?
VAR allows you to store the result of a calculation in a variable.

Example:
Profit =
VAR Revenue = [Total Sales]
VAR Cost = [Total Cost]
RETURN
Revenue - Cost

Instead of repeating [Total Sales] and [Total Cost], we give them meaningful names. The calculation becomes easier to read.

🔹 2. What does RETURN do?
RETURN tells DAX which final result should be returned.

VAR → Create temporary values
RETURN → Give me the final result

Example:
Profit Margin =
VAR Profit = [Total Profit]
VAR Sales = [Total Sales]
RETURN
DIVIDE(Profit, Sales)

🔹 3. Why use Variables?

Without variables:
Profit Margin =
DIVIDE(
[Total Sales] - [Total Cost],
[Total Sales]
)

With variables:
Profit Margin =
VAR Sales = [Total Sales]
VAR Cost = [Total Cost]
VAR Profit = Sales - Cost
RETURN
DIVIDE(Profit, Sales)

The second version is easier to understand. You can immediately see: Sales, Cost, Profit, Profit Margin

🔹 4. Variables Can Store Numbers

Example:
Sales Target Status =
VAR Sales = [Total Sales]
VAR Target = 1000000
RETURN
IF(
Sales >= Target,
"Target Achieved",
"Below Target"
)

Now the business rule is much easier to read.

🔹 5. Variables Can Store Text
Variables don't have to contain numbers.

Example:
Region Message =
VAR Region =
SELECTEDVALUE(
Sales[Region],
"Multiple Regions"
)
RETURN
"Current Region: " & Region

If West is selected: "Current Region: West"

🔹 6. Variables Can Store Tables
This is where DAX starts becoming more powerful. A variable can also contain a table expression.

Example:
High Value Customers =
VAR Customers =
FILTER(
VALUES(Sales[CustomerID]),
[Total Sales] > 100000
)
RETURN
COUNTROWS(Customers)

Here: "Customers" stores a temporary table. Then COUNTROWS() counts how many customers are in that table.

🔹 7. Variables and FILTER()
Variables make complex filtering easier to understand.

Example:
High Value Sales =
VAR FilteredSales =
FILTER(
Sales,
Sales[SalesAmount] > 10000
)
RETURN
SUMX(
FilteredSales,
Sales[SalesAmount]
)

Instead of putting everything into one long expression, we separate the logic into meaningful steps.

🔹 8. Variables Are Evaluated Once
A useful performance benefit is that variables can avoid repeatedly evaluating the same expression.

For example, instead of repeatedly calculating [Total Sales] you can store it:
VAR Sales = [Total Sales]
and reuse Sales. This can make complex measures cleaner and, in some cases, more efficient.

🔹 9. Variables Improve Debugging

Suppose you have:
Profit Analysis =
VAR Sales = [Total Sales]
VAR Cost = [Total Cost]
VAR Profit = Sales - Cost
VAR Margin = DIVIDE(Profit, Sales)
RETURN
Margin

If the final result looks incorrect, you can temporarily change the RETURN statement to:
RETURN Profit
or
RETURN Cost

This makes it easier to understand where the calculation is going wrong.

🔹 10. Variables Don't Create Model Columns
This is important. A variable inside a measure VAR Sales = [Total Sales] does NOT create a permanent column in your Power BI model. It exists only while that measure is being evaluated.

So:
Calculated Column → stored in the model
Measure Variable → temporary value during calculation

🔹 11. Real Business Example

Suppose management wants to classify performance:
Sales ≥ ₹10M → Excellent
Sales ≥ ₹5M → Good
Sales ≥ ₹2M → Average
Below ₹2M → Needs Attention

You can write:
Sales Performance =
VAR Sales = [Total Sales]
RETURN
SWITCH(
TRUE(),
Sales >= 10000000, "Excellent",
        Sales >= 5000000, "Good",
        Sales >= 2000000, "Average",
        "Needs Attention"
    )

This is much easier to maintain than repeatedly writing [Total Sales].

🔹 12. Best Practices

When writing complex DAX:
✔ Give variables meaningful names
✔ Break complicated calculations into logical steps
✔ Avoid repeating the same expression
✔ Use RETURN for the final result
✔ Keep business logic readable
✔ Use variables to make debugging easier

Avoid meaningless names such as VAR X =...
Prefer VAR TotalSales =...

Clear names make your DAX easier for another analyst to understand.

🎯 Interview Questions

1️⃣ What is VAR in DAX?
VAR creates a temporary variable that stores a value or table expression during calculation.

2️⃣ What does RETURN do?
It specifies the final expression that the measure should return.

3️⃣ Are DAX variables stored permanently in the model?
No. Variables exist only during the evaluation of the expression.

4️⃣ Why should you use variables?
They improve readability, reduce repeated calculations, and make complex DAX easier to debug.

5️⃣ Can a DAX variable contain a table?
Yes. A variable can store either a scalar value or a table expression.

🧪 PRACTICE

Create these measures using VAR:
✔ Total Profit
✔ Profit Margin
✔ Sales Target Status
✔ Sales Performance
✔ Selected Region Message

Then try to rewrite one of your older complex DAX measures using variables.

💡 Double Tap ❤️ For More

📊 Data Analyst Interview Series — Part 1

Guys, let's start a Data Analyst Interview Series where I'll cover the most important questions that are commonly asked in Data Analyst interviews.

I'll cover SQL, Excel, Power BI, Python, statistics, data cleaning, case studies, business questions, and scenario-based questions.

Let's start with the basics 👇

1️⃣ Tell me about yourself.

Sample Answer:

"I'm a Data Analyst with experience working with SQL, Excel, Power BI, Python, and data visualization. My work involves extracting and transforming data, analyzing business problems, building dashboards, and automating repetitive reporting processes. I focus not just on creating reports, but on understanding the business requirement and converting data into actionable insights."

2️⃣ What does a Data Analyst do?

Sample Answer:

"A Data Analyst collects, cleans, transforms, and analyzes data to help businesses make informed decisions. A typical workflow involves understanding the business requirement, collecting relevant data, cleaning it, performing analysis, identifying trends or patterns, and presenting the findings through reports or dashboards."

3️⃣ What is the difference between Data Analysis and Data Analytics?

Sample Answer:

"Data analysis generally focuses on examining data to understand what happened and why. Data analytics is a broader concept that includes data analysis along with processes such as data collection, preparation, visualization, statistical analysis, and sometimes predictive modeling. In practice, the terms are often used interchangeably depending on the organization."

4️⃣ What is the difference between structured and unstructured data?

Sample Answer:

"Structured data has a predefined format or schema, such as rows and columns in a relational database. Examples include customer IDs, transaction amounts, and dates.

Unstructured data does not follow a predefined tabular structure. Examples include emails, images, videos, documents, and social media posts.

Semi-structured data sits between the two, such as JSON and XML, where the data has some organizational structure but doesn't necessarily follow a relational table format."

5️⃣ What is data cleaning and why is it important?

Sample Answer:

"Data cleaning is the process of identifying and correcting problems in a dataset, such as missing values, duplicates, inconsistent formats, incorrect data types, and invalid values.

It is important because analysis performed on poor-quality data can produce misleading results. Before analyzing data, I would first understand the data quality issues and determine how each issue should be handled based on the business context."

6️⃣ How do you handle missing values?

Sample Answer:

"I first investigate why the values are missing and how much data is affected. The appropriate treatment depends on the business context.

For example, I might remove records if only a very small number are affected and they aren't important to the analysis. For numerical fields, I might use an appropriate statistical value such as median or mean when justified. For categorical fields, I might use a meaningful category such as 'Unknown.'

I avoid blindly replacing missing values because missingness itself can sometimes contain useful information."

7️⃣ How do you identify duplicate records?

Sample Answer:

"I first determine what defines a unique record. Then I compare the relevant columns or business key to identify duplicates.

For example, if Customer_ID and Transaction_ID together uniquely identify a transaction, I can use those fields to identify duplicate combinations.

In SQL, I could use GROUP BY with HAVING COUNT(**) > 1 to identify duplicated keys."

SELECT Customer_ID, Transaction_ID, COUNT(**) AS duplicate_count
FROM transactions
GROUP BY Customer_ID, Transaction_ID
HAVING COUNT(**) > 1;

"After identifying duplicates, I investigate whether they are genuine duplicate records or legitimate repeated transactions before removing anything."

8️⃣ What is an outlier? How would you handle it?

Sample Answer:

"An outlier is a value that is significantly different from the typical observations in a dataset.

I wouldn't automatically remove an outlier. First, I would investigate whether it represents a data-quality issue or a genuine business event.

For example, a transaction worth ₹10 million might initially look like an outlier, but it could be a legitimate high-value transaction. If it is a data-entry error, I would correct or exclude it according to the business rules."

9️⃣ What is the difference between a dimension and a measure?

Sample Answer:

"A dimension is generally used to categorize or describe data, while a measure is a numerical value that can usually be aggregated.

For example, in a sales dataset:

Dimensions: Customer, Product, Region, Date

Measures: Sales Amount, Quantity, Profit, Discount

In a dashboard, dimensions are commonly used to slice or group the data, while measures are used to calculate KPIs and metrics."

🔟 What steps do you follow when solving a data analysis problem?

Sample Answer:

"I generally follow a structured approach:

1. Understand the business problem.

2. Define the required metrics and success criteria.

3. Identify the relevant data sources.

4. Extract and validate the data.

5. Clean and transform the data.

6. Perform exploratory analysis.

7. Identify trends, patterns, and anomalies.

8. Validate the results.

9. Communicate the insights using appropriate visualizations.

10. Recommend actions based on the findings.

The most important step is understanding the business question first, because technically correct analysis can still be useless if it doesn't answer the actual business problem."

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📊 Data Analyst Interview Series — Part 2

Guys, let's continue our Data Analyst Interview Series.

In Part 2, let's move into some important SQL and data-related interview questions that are frequently tested in Data Analyst interviews. 👇

1️⃣ What is SQL and why is it important for a Data Analyst?

Sample Answer:

"SQL stands for Structured Query Language. It is used to interact with relational databases. As a Data Analyst, I use SQL to retrieve, filter, join, aggregate, and analyze data. It is important because a large amount of business data is stored in databases, and SQL allows analysts to efficiently extract the data required for analysis."

2️⃣ What is the difference between WHERE and HAVING?

Sample Answer:

"WHERE filters individual rows before aggregation, whereas HAVING filters groups after aggregation.

For example, if I want to find customers whose total sales exceed ₹1 lakh, I would use HAVING because the condition is applied to an aggregated result."

SELECT Customer_ID, SUM(Sales) AS Total_Sales
FROM Sales
GROUP BY Customer_ID
HAVING SUM(Sales) > 100000;

3️⃣ What is the difference between INNER JOIN and LEFT JOIN?

Sample Answer:

"An INNER JOIN returns only the records that have matching values in both tables.

A LEFT JOIN returns all records from the left table and the matching records from the right table. If there is no match, the columns from the right table contain NULL."

For example, if I want all customers, including customers who haven't placed any orders, I would use a LEFT JOIN.

4️⃣ What is a primary key?

Sample Answer:

"A primary key is a column or combination of columns that uniquely identifies each record in a table. It must contain unique values and cannot contain NULL values.

For example, Customer_ID can be a primary key in a Customer table if every customer has a unique ID."

5️⃣ What is a foreign key?

Sample Answer:

"A foreign key is a column that references a primary key or another unique key in another table. It establishes a relationship between tables.

For example, Customer_ID in an Orders table can reference Customer_ID in the Customers table."

6️⃣ What is the difference between UNION and UNION ALL?

Sample Answer:

"Both are used to combine the results of two or more SELECT statements.

UNION removes duplicate records from the combined result, while UNION ALL retains duplicates.

Because UNION performs duplicate elimination, UNION ALL can generally be faster when duplicate removal isn't required."

7️⃣ What is a NULL value in SQL?

Sample Answer:

"NULL represents a missing, unknown, or unavailable value. It is different from zero, an empty string, or a blank value.

We should use IS NULL or IS NOT NULL to check for NULL values rather than using an equals operator."

SELECT *
FROM Customers
WHERE Email IS NULL;

8️⃣ What is GROUP BY used for?

Sample Answer:

"GROUP BY is used to group rows that have the same values in one or more columns so that aggregate functions can be applied to each group.

For example, to calculate total sales by region:"

SELECT Region, SUM(Sales) AS Total_Sales
FROM Sales
GROUP BY Region;

9️⃣ What are aggregate functions in SQL?

Sample Answer:

"Aggregate functions perform calculations on multiple rows and return a single result for each group.

Common aggregate functions include:"

• COUNT() — counts records

• SUM() — calculates the total

• AVG() — calculates the average

• MIN() — finds the minimum value

• MAX() — finds the maximum value

For example:

SELECT
    COUNT(*) AS Total_Orders,
    SUM(Sales) AS Total_Sales,
    AVG(Sales) AS Average_Sales
FROM Sales;

🔟 How would you find duplicate records in SQL?

Sample Answer:

"I would first identify the column or combination of columns that should uniquely identify a record. Then I would use GROUP BY and HAVING COUNT(*) > 1."

SELECT Customer_ID, COUNT(*) AS Count_Records
FROM Customers
GROUP BY Customer_ID
HAVING COUNT(*) > 1;

"This identifies Customer_ID values that appear more than once. I would then investigate whether those records are genuine duplicates before taking any corrective action."

📌 Double Tap ❤️ For Part-3

📊 Data Analyst Interview Series — Part 3

Guys, let's continue our Data Analyst Interview Series.

Today, let's cover 10 important SQL interview questions that test your practical SQL knowledge. 👇

1️⃣ What is a subquery in SQL?

Sample Answer:

“A subquery is a query written inside another SQL query. It can be used to retrieve intermediate results that are then used by the outer query.

For example, to find employees whose salary is greater than the average salary:”

SELECT Employee_ID, Salary
FROM Employees
WHERE Salary > (
    SELECT AVG(Salary)
    FROM Employees
);

2️⃣ What is a CTE?

Sample Answer:

“CTE stands for Common Table Expression. It allows us to define a temporary named result set using the WITH clause, which can then be referenced within the main query.

CTEs make complex queries easier to read, maintain, and debug.”

WITH CustomerSales AS (
    SELECT Customer_ID,
           SUM(Sales) AS Total_Sales
    FROM Sales
    GROUP BY Customer_ID
)
SELECT *
FROM CustomerSales
WHERE Total_Sales > 100000;

3️⃣ What is a window function?

Sample Answer:

“A window function performs a calculation across a set of related rows while still retaining the individual rows in the result.

Unlike GROUP BY, it does not collapse multiple rows into a single row.

Common window functions include ROW_NUMBER(), RANK(), DENSE_RANK(), LAG(), and LEAD().”

4️⃣ What is the difference between RANK(), DENSE_RANK(), and ROW_NUMBER()?

Sample Answer:

“ROW_NUMBER() assigns a unique sequential number to every row.

RANK() assigns the same rank to tied values but leaves gaps after a tie.

DENSE_RANK() also assigns the same rank to tied values but does not leave gaps.”

Example:

Values: 100, 100, 90

ROW_NUMBER: 1, 2, 3

RANK: 1, 1, 3

DENSE_RANK: 1, 1, 2

5️⃣ How would you find the second-highest salary?

Sample Answer:

“One approach is to use DENSE_RANK(). This also handles duplicate salaries correctly.”

WITH RankedEmployees AS (
    SELECT Employee_ID,
           Salary,
           DENSE_RANK() OVER (ORDER BY Salary DESC) AS Salary_Rank
    FROM Employees
)
SELECT Employee_ID, Salary
FROM RankedEmployees
WHERE Salary_Rank = 2;

6️⃣ How would you find the top 3 salaries in each department?

Sample Answer:

“I would use a window function to rank employees within each department.”

WITH RankedEmployees AS (
    SELECT Employee_ID,
           Department,
           Salary,
           DENSE_RANK() OVER (
               PARTITION BY Department
               ORDER BY Salary DESC
           ) AS Salary_Rank
    FROM Employees
)
SELECT *
FROM RankedEmployees
WHERE Salary_Rank <= 3;

“The PARTITION BY ensures that ranking starts separately for each department.”

7️⃣ What is PARTITION BY in SQL?

Sample Answer:

“PARTITION BY divides the result set into groups for a window function without collapsing the rows.

For example, if I want to rank employees separately within each department, I can use PARTITION BY Department.”

SELECT Employee_ID,
       Department,
       Salary,
       RANK() OVER (
           PARTITION BY Department
           ORDER BY Salary DESC
       ) AS Salary_Rank
FROM Employees;

8️⃣ What are LAG() and LEAD() functions?

Sample Answer:

“LAG() allows me to access a value from a previous row, while LEAD() allows me to access a value from a following row.

They are particularly useful for comparing current values with previous or future values, such as month-over-month sales.”

SELECT Month,
       Sales,
       LAG(Sales) OVER (ORDER BY Month) AS Previous_Month_Sales
FROM Monthly_Sales;

9️⃣ How would you calculate month-over-month growth?

Sample Answer:

“I would first retrieve the previous month's sales using LAG(), then calculate the percentage change between the current month and previous month.”

SELECT Month,
       Sales,
       LAG(Sales) OVER (ORDER BY Month) AS Previous_Sales,
       (Sales - LAG(Sales) OVER (ORDER BY Month))
       * 100.0 /
       LAG(Sales) OVER (ORDER BY Month) AS MoM_Growth
FROM Monthly_Sales;

“I would also handle cases where the previous month's value is zero or NULL to avoid incorrect calculations.”

🔟 What is the difference between DELETE, TRUNCATE, and DROP?

Sample Answer:

“DELETE removes selected rows from a table and can be used with a WHERE condition.

TRUNCATE removes all rows from a table while keeping the table structure.

DROP removes the entire table, including its structure and data.

So, the key difference is whether I'm removing specific records, all records, or the entire table itself.”

📌 Double Tap ❤️ For Part-4