✅ SQL Subquery Practice Questions with Answers — Part 2 🧠🗂️

🔎 Q1. Find employees earning more than the average salary of their department. 
🗂️ Table: "employees(emp_id, name, department_id, salary)"

✅ Answer: 

SELECT name, department_id, salary
FROM employees e1
WHERE salary > (
    SELECT AVG(salary)
    FROM employees e2
    WHERE e1.department_id = e2.department_id
);

🔎 Q2. Get customers who never placed any order. 
🗂️ Tables: "customers(customer_id, name)", "orders(order_id, customer_id)"

✅ Answer: 

SELECT customer_id, name
FROM customers
WHERE customer_id NOT IN (
    SELECT customer_id
    FROM orders
);

🔎 Q3. Find the second highest salary from employees. 
🗂️ Table: "employees(emp_id, name, salary)"

✅ Answer: 

SELECT MAX(salary) AS second_highest_salary
FROM employees
WHERE salary < (
    SELECT MAX(salary)
    FROM employees
);

🔎 Q4. List products priced higher than the average product price. 
🗂️ Table: "products(product_id, product_name, price)"

✅ Answer: 

SELECT product_name, price
FROM products
WHERE price > (
    SELECT AVG(price)
    FROM products
);

🔎 Q5. Find employees who work in the same department as 'John'. 
🗂️ Table: "employees(emp_id, name, department_id)"

✅ Answer: 

SELECT name, department_id
FROM employees
WHERE department_id = (
    SELECT department_id
    FROM employees
    WHERE name = 'John'
);

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✅ SQL Interview Roadmap – Step-by-Step Guide to Crack Any SQL Round 💼📊

Whether you're applying for Data Analyst, BI, or Data Engineer roles — SQL rounds are must-clear. Here's your focused roadmap:

1️⃣ Core SQL Concepts
🔹 Understand RDBMS, tables, keys, schemas
🔹 Data types, NULLs, constraints
🧠 Interview Tip: Be able to explain Primary vs Foreign Key.

2️⃣ Basic Queries
🔹 SELECT, FROM, WHERE, ORDER BY, LIMIT
🧠 Practice: Filter and sort data by multiple columns.

3️⃣ Joins – Very Frequently Asked!
🔹 INNER, LEFT, RIGHT, FULL OUTER JOIN
🧠 Interview Tip: Explain the difference with examples.
🧪 Practice: Write queries using joins across 2–3 tables.

4️⃣ Aggregations & GROUP BY
🔹 COUNT, SUM, AVG, MIN, MAX, HAVING
🧠 Common Question: Total sales per category where total > X.

5️⃣ Window Functions
🔹 ROW_NUMBER(), RANK(), DENSE_RANK(), LAG(), LEAD()
🧠 Interview Favorite: Top N per group, previous row comparison.

6️⃣ Subqueries & CTEs
🔹 Write queries inside WHERE, FROM, and using WITH
🧠 Use Case: Filtering on aggregated data, simplifying logic.

7️⃣ CASE Statements
🔹 Add logic directly in SELECT
🧠 Example: Categorize users based on spend or activity.

8️⃣ Data Cleaning & Transformation
🔹 Handle NULLs, format dates, string manipulation (TRIM, SUBSTRING)
🧠 Real-world Task: Clean user input data.

9️⃣ Query Optimization Basics
🔹 Understand indexing, query plan, performance tips
🧠 Interview Tip: Difference between WHERE and HAVING.

🔟 Real-World Scenarios
🧠 Must Practice:
• Sales funnel
• Retention cohort
• Churn rate
• Revenue by channel
• Daily active users

🧪 Practice Platforms
• LeetCode (Easy–Hard SQL)
• StrataScratch (Real business cases)
• Mode Analytics (SQL + Visualization)
• HackerRank SQL (MCQs + Coding)

💼 Final Tip:
Explain why your query works, not just what it does. Speak your logic clearly.

💬 Tap ❤️ for more!

✅ SQL Mistakes Beginners Should Avoid 🧠💻

1️⃣ Using SELECT *
• Pulls unused columns
• Slows queries
• Breaks when schema changes
• Use only required columns

2️⃣ Ignoring NULL Values
• NULL breaks calculations
• COUNT(column) skips NULL
• Use COALESCE or IS NULL checks

3️⃣ Wrong JOIN Type
• INNER instead of LEFT
• Data silently disappears
• Always ask: Do you need unmatched rows?

4️⃣ Missing JOIN Conditions
• Creates cartesian product
• Rows explode
• Always join on keys

5️⃣ Filtering After JOIN Instead of Before
• Processes more rows than needed
• Slower performance
• Filter early using WHERE or subqueries

6️⃣ Using WHERE Instead of HAVING
• WHERE filters rows
• HAVING filters groups
• Aggregates fail without HAVING

7️⃣ Not Using Indexes
• Full table scans
• Slow dashboards
• Index columns used in JOIN, WHERE, ORDER BY

8️⃣ Relying on ORDER BY in Subqueries
• Order not guaranteed
• Results change
• Use ORDER BY only in final query

9️⃣ Mixing Data Types
• Implicit conversions
• Index not used
• Match column data types

🔟 No Query Validation
• Results look right but are wrong
• Always cross-check counts and totals

🧠 Practice Task
• Rewrite one query
• Remove SELECT *
• Add proper JOIN
• Handle NULLs
• Compare result count

SQL Resources: https://whatsapp.com/channel/0029VanC5rODzgT6TiTGoa1v

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Best practices for writing SQL queries:

Join for more: https://t.me/learndataanalysis

1- Write SQL keywords in capital letters.

2- Use table aliases with columns when you are joining multiple tables.

3- Never use select *, always mention list of columns in select clause.

4- Add useful comments wherever you write complex logic. Avoid too many comments.

5- Use joins instead of subqueries when possible for better performance.

6- Create CTEs instead of multiple sub queries , it will make your query easy to read.

7- Join tables using JOIN keywords instead of writing join condition in where clause for better readability.

8- Never use order by in sub queries , It will unnecessary increase runtime.

9- If you know there are no duplicates in 2 tables, use UNION ALL instead of UNION for better performance.

SQL Interview Questions with Answers Part-1: ☑️

1. What is SQL? 
   SQL (Structured Query Language) is a standardized programming language designed to manage and manipulate relational databases. It allows you to query, insert, update, and delete data, as well as create and modify schema objects like tables and views.

2. Differentiate between SQL and NoSQL databases. 
   SQL databases are relational, table-based, and use structured query language with fixed schemas, ideal for complex queries and transactions. NoSQL databases are non-relational, can be document, key-value, graph, or column-oriented, and are schema-flexible, designed for scalability and handling unstructured data.

3. What are the different types of SQL commands?
⦁ DDL (Data Definition Language): CREATE, ALTER, DROP (define and modify structure)
⦁ DML (Data Manipulation Language): SELECT, INSERT, UPDATE, DELETE (data operations)
⦁ DCL (Data Control Language): GRANT, REVOKE (permission control)
⦁ TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT (transaction management)

4. Explain the difference between WHERE and HAVING clauses.
⦁ WHERE filters rows before grouping (used with SELECT, UPDATE).
⦁ HAVING filters groups after aggregation (used with GROUP BY), e.g., filtering aggregated results like sums or counts.

5. Write a SQL query to find the second highest salary in a table. 
   Using a subquery:

SELECT MAX(salary) FROM employees  
WHERE salary < (SELECT MAX(salary) FROM employees);

Or using DENSE_RANK():

SELECT salary FROM (  
  SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) as rnk  
  FROM employees) t  
WHERE rnk = 2;

6. What is a JOIN? Explain different types of JOINs. 
   A JOIN combines rows from two or more tables based on a related column:
⦁ INNER JOIN: returns matching rows from both tables.
⦁ LEFT JOIN (LEFT OUTER JOIN): all rows from the left table, matched rows from right.
⦁ RIGHT JOIN (RIGHT OUTER JOIN): all rows from right table, matched rows from left.
⦁ FULL JOIN (FULL OUTER JOIN): all rows when there’s a match in either table.
⦁ CROSS JOIN: Cartesian product of both tables.

7. How do you optimize slow-performing SQL queries?
⦁ Use indexes appropriately to speed up lookups.
⦁ Avoid SELECT *; only select necessary columns.
⦁ Use joins carefully; filter early with WHERE clauses.
⦁ Analyze execution plans to identify bottlenecks.
⦁ Avoid unnecessary subqueries; use EXISTS or JOINs.
⦁ Limit result sets with pagination if dealing with large datasets.

8. What is a primary key? What is a foreign key?
⦁ Primary Key: A unique identifier for records in a table; it cannot be NULL.
⦁ Foreign Key: A field that creates a link between two tables by referring to the primary key in another table, enforcing referential integrity.

9. What are indexes? Explain clustered and non-clustered indexes.
⦁ Indexes speed up data retrieval by providing quick lookups.
⦁ Clustered Index: Sorts and stores the actual data rows in the table based on the key; a table can have only one clustered index.
⦁ Non-Clustered Index: Creates a separate structure that points to the data rows; tables can have multiple non-clustered indexes.

10. Write a SQL query to fetch the top 5 records from a table. 
    In SQL Server and PostgreSQL:

SELECT * FROM table_name  
ORDER BY some_column DESC  
LIMIT 5;  

In SQL Server (older syntax):

SELECT TOP 5 * FROM table_name  
ORDER BY some_column DESC;  

React ♥️ for Part 2

✅ SQL Mistakes Beginners Should Avoid 🧠💻

1️⃣ Using SELECT *
• Pulls unused columns
• Slows queries
• Breaks when schema changes
• Use only required columns

2️⃣ Ignoring NULL Values
• NULL breaks calculations
• COUNT(column) skips NULL
• Use COALESCE or IS NULL checks

3️⃣ Wrong JOIN Type
• INNER instead of LEFT
• Data silently disappears
• Always ask: Do you need unmatched rows?

4️⃣ Missing JOIN Conditions
• Creates cartesian product
• Rows explode
• Always join on keys

5️⃣ Filtering After JOIN Instead of Before
• Processes more rows than needed
• Slower performance
• Filter early using WHERE or subqueries

6️⃣ Using WHERE Instead of HAVING
• WHERE filters rows
• HAVING filters groups
• Aggregates fail without HAVING

7️⃣ Not Using Indexes
• Full table scans
• Slow dashboards
• Index columns used in JOIN, WHERE, ORDER BY

8️⃣ Relying on ORDER BY in Subqueries
• Order not guaranteed
• Results change
• Use ORDER BY only in final query

9️⃣ Mixing Data Types
• Implicit conversions
• Index not used
• Match column data types

🔟 No Query Validation
• Results look right but are wrong
• Always cross-check counts and totals

🧠 Practice Task
• Rewrite one query
• Remove SELECT *
• Add proper JOIN
• Handle NULLs
• Compare result count

SQL Resources: https://whatsapp.com/channel/0029VanC5rODzgT6TiTGoa1v

❤️ Double Tap For More

✅ 🔤 A–Z of SQL Commands 🗄️💻⚡

A – ALTER
Modify an existing table structure (add/modify/drop columns).

B – BEGIN
Start a transaction block.

C – CREATE
Create database objects like tables, views, indexes.

D – DELETE
Remove records from a table.

E – EXISTS
Check if a subquery returns any rows.

F – FETCH
Retrieve rows from a cursor.

G – GRANT
Give privileges to users.

H – HAVING
Filter aggregated results (used with GROUP BY).

I – INSERT
Add new records into a table.

J – JOIN
Combine rows from two or more tables.

K – KEY (PRIMARY KEY / FOREIGN KEY)
Define constraints for uniqueness and relationships.

L – LIMIT
Restrict number of rows returned (MySQL/PostgreSQL).

M – MERGE
Insert/update data conditionally (mainly in SQL Server/Oracle).

N – NULL
Represents missing or unknown data.

O – ORDER BY
Sort query results.

P – PROCEDURE
Stored program in the database.

Q – QUERY
Request for data (general SQL statement).

R – ROLLBACK
Undo changes in a transaction.

S – SELECT
Retrieve data from tables.

T – TRUNCATE
Remove all records from a table quickly.

U – UPDATE
Modify existing records.

V – VIEW
Virtual table based on a query.

W – WHERE
Filter records based on conditions.

X – XML PATH
Generate XML output (mainly SQL Server).

Y – YEAR()
Extract year from a date.

Z – ZONE (AT TIME ZONE)
Convert datetime to specific time zone.

❤️ Double Tap for More

✅ Complete Roadmap to Learn SQL in 2026 🚀

💎 SQL powers 80% of data analytics jobs.

📚 🔹 SQL FOUNDATIONS

🎯 1️⃣ SELECT Basics (Week 1)
- SELECT \*, specific columns
- FROM tables
- WHERE filters
- ORDER BY, LIMIT

🟢 Practice: Query your first dataset today

🔍 2️⃣ Filtering Mastery
- Comparison operators (=, >, BETWEEN)
- Logical: AND, OR, IN
- Pattern matching: LIKE, %
- NULL handling

📊 3️⃣ Aggregate Power
- COUNT(\*), SUM, AVG, MIN/MAX
- GROUP BY essentials
- HAVING vs WHERE
- DISTINCT counts

🎓 🔥 SQL CORE SKILLS

🔗 4️⃣ JOINS (Most Important ⭐)
- INNER JOIN (must-know)
- LEFT, RIGHT, FULL JOIN
- Multi-table joins
- Self-joins

⚡ 5️⃣ Subqueries & CTEs
- Subqueries in WHERE/FROM
- WITH clause (CTEs)
- Multiple CTE chains
- EXISTS/NOT EXISTS

📈 6️⃣ Window Functions (Game-Changer ⭐)
- ROW_NUMBER(), RANK()
- PARTITION BY magic
- LAG/LEAD (trends)
- Running totals

🎨 🚀 ADVANCED SQL MASTERY

⏰ 7️⃣ Date & Time
- DATEADD, DATEDIFF
- DATE_TRUNC, EXTRACT
- Date filtering patterns
- Cohort analysis

🔤 8️⃣ String Functions
- CONCAT, SUBSTRING
- TRIM, UPPER/LOWER
- LENGTH, REPLACE

🤖 9️⃣ CASE Statements
- Simple vs searched CASE
- Nested logic
- Policy calculations

⚙️ 🔧 PERFORMANCE & JOBS

🚀 1️⃣0️⃣ Indexing Basics
- CREATE INDEX strategies
- EXPLAIN query plans
- Composite indexes

💻 1️⃣1️⃣ Practice Platforms
- LeetCode SQL (50 problems)
- HackerRank SQL
- StrataScratch (real cases)
- DDIA datasets

📱 1️⃣2️⃣ Modern SQL Tools
- pgAdmin (PostgreSQL)
- DBeaver (universal)
- BigQuery Sandbox (free)
- dbt + SQL

💼 ⚡ INTERVIEW READY

🎯 1️⃣3️⃣ Top Interview Questions
- Find 2nd highest salary
- Nth highest records
- Duplicate detection
- Window ranking

📊 1️⃣4️⃣ Real Projects
- Sales dashboard queries
- Customer segmentation
- Inventory optimization
- Build GitHub portfolio

🎨 ⭐ ESSENTIAL SQL TOOLS 2026
- PostgreSQL (free, powerful)
- MySQL Workbench
- BigQuery (cloud-native)
- Snowflake (trial)

1️⃣5️⃣ FREE RESOURCES
🌐 SQLBolt (interactive)
📚 Mode Analytics Tutorial
⚡ LeetCode SQL 50
🎥 DataCamp SQL (free tier)
🐙 W3schools

Double Tap ♥️ For Detailed Explanation

If I had to start learning data analyst all over again, I'd follow this:

1- Learn SQL:

---- Joins (Inner, Left, Full outer and Self)
---- Aggregate Functions (COUNT, SUM, AVG, MIN, MAX)
---- Group by and Having clause
---- CTE and Subquery
---- Windows Function (Rank, Dense Rank, Row number, Lead, Lag etc)

2- Learn Excel:

---- Mathematical (COUNT, SUM, AVG, MIN, MAX, etc)
---- Logical Functions (IF, AND, OR, NOT)
---- Lookup and Reference (VLookup, INDEX, MATCH etc)
---- Pivot Table, Filters, Slicers

3- Learn BI Tools:

---- Data Integration and ETL (Extract, Transform, Load)
---- Report Generation
---- Data Exploration and Ad-hoc Analysis
---- Dashboard Creation

4- Learn Python (Pandas) Optional:

---- Data Structures, Data Cleaning and Preparation
---- Data Manipulation
---- Merging and Joining Data (Merging and joining DataFrames -similar to SQL joins)
---- Data Visualization (Basic plotting using Matplotlib and Seaborn)

Hope this helps you 😊

🎯 SQL Fundamentals Part-1: SELECT Basics

SELECT is the most used SQL command, used to retrieve data from a database.

Think of SQL like asking questions to a database. SELECT = asking what data you want.

✅ What is SELECT in SQL?
SELECT statement retrieves data from one or more tables in a database.

👉 Basic Syntax

SELECT column_name 
FROM table_name;

How SQL executes:
1. Finds table (FROM)
2. Applies filter (WHERE)
3. Returns selected columns (SELECT)
4. Sorts results (ORDER BY)
5. Limits rows (LIMIT)

🔹 1. SELECT All Columns (SELECT *)
Used to retrieve every column from a table.

SELECT * 
FROM employees;

👉 Returns complete table data.

📌 When to use:
✔ Exploring new dataset
✔ Checking table structure
✔ Quick testing

⚠️ Avoid in production: Slow on large tables, fetches unnecessary data.

🔹 2. SELECT Specific Columns
Best practice — retrieve only required data.

SELECT name, salary 
FROM employees;

👉 Returns only selected columns.
💡 Why important:
✅ Faster queries
✅ Better performance
✅ Cleaner results

🔹 3. FROM Clause (Data Source)
Specifies where data comes from.

SELECT name 
FROM customers;

👉 SQL reads data from customers table.

🔹 4. WHERE Clause (Filtering Data)
Used to filter rows based on conditions.

SELECT column 
FROM table 
WHERE condition;

Examples:
- Filter by value: SELECT * FROM employees WHERE salary > 50000;
- Filter by text: SELECT * FROM employees WHERE city = 'Mumbai';

🔹 5. ORDER BY (Sorting Results)
Sorts query results.

SELECT column 
FROM table 
ORDER BY column ASC | DESC;

Examples:
- Ascending: SELECT name, salary FROM employees ORDER BY salary ASC;
- Descending: SELECT name, salary FROM employees ORDER BY salary DESC;

🔹 6. LIMIT (Control Output Rows)
Restricts number of returned rows.

SELECT * 
FROM employees 
LIMIT 5;

👉 Returns first 5 records.

⭐ SQL Query Execution Order
1. FROM
2. WHERE
3. SELECT
4. ORDER BY
5. LIMIT

🧠 Real-World Example
Business question: "Show top 10 highest paid employees."

SELECT name, salary 
FROM employees 
ORDER BY salary DESC 
LIMIT 10;

🚀 Mini Practice Tasks
✅ Task 1: Get all records from customers.
✅ Task 2: Show only customer name and city.
✅ Task 3: Find employees with salary > 40000.
✅ Task 4: Show top 3 highest priced products.

Double Tap ♥️ For Part-2

🔍 SQL Fundamentals Part-2: Filtering

After learning SELECT basics, the next step is learning how to filter data.

👉 In real-world data analysis, you rarely need full data — you filter specific rows.

Filtering = extracting only relevant data from a table.

✅ What is Filtering in SQL?
Filtering is done using the WHERE clause.

It allows you to:
✔ Get specific records
✔ Apply conditions
✔ Clean data
✔ Extract business insights

🔹 1. Comparison Operators
Used to compare values.
Operator Meaning
• = Equal
• > Greater than
• < Less than
• >= Greater than or equal
• <= Less than or equal
• != or <> Not equal

✅ Examples

• Equal to
SELECT * FROM employees WHERE city = 'Pune';

• Greater than
SELECT * FROM employees WHERE salary > 50000;

• Not equal
SELECT * FROM employees WHERE department != 'HR';

💡 Most commonly used in dashboards reporting.

🔹 2. Logical Operators (AND, OR, NOT)

Used to combine multiple conditions.

✅ AND — Both conditions must be true

SELECT * FROM employees WHERE salary > 50000 AND city = 'Mumbai';

👉 Returns employees with: salary > 50000 AND located in Mumbai

✅ OR — Any condition can be true

SELECT * FROM employees WHERE city = 'Delhi' OR city = 'Pune';

👉 Returns employees from either city.

✅ NOT — Reverse condition

SELECT * FROM employees WHERE NOT department = 'Sales';

👉 Excludes Sales department.

🔹 3. BETWEEN (Range Filtering)

Used to filter values within a range.

Syntax
SELECT * FROM table WHERE column BETWEEN value1 AND value2;

✅ Example
SELECT * FROM employees WHERE salary BETWEEN 30000 AND 70000;

👉 Includes boundary values.

🔹 4. IN Operator (Multiple Values Shortcut)

Better alternative to multiple OR conditions.

❌ Without IN
WHERE city = 'Pune' OR city = 'Delhi' OR city = 'Mumbai'

✅ With IN
SELECT * FROM employees WHERE city IN ('Pune','Delhi','Mumbai');

👉 Cleaner and faster.

🔹 5. LIKE — Pattern Matching
Used for searching text patterns.

⭐ Wildcards
Symbol Meaning
• % Any number of characters
• _ Single character

✅ Starts with "A"
SELECT * FROM customers WHERE name LIKE 'A%';

✅ Ends with "n"
WHERE name LIKE '%n';

✅ Contains "an"
WHERE name LIKE '%an%';

Used heavily in search features.

🔹 6. NULL Handling (Very Important ⭐)

NULL means:
👉 Missing / unknown value
👉 Not zero
👉 Not empty

❌ Wrong
WHERE salary = NULL

✅ Correct
SELECT * FROM employees WHERE salary IS NULL;

Check non-null values
WHERE salary IS NOT NULL;

💡 Very common interview question.

⭐ Order of Filtering Execution
SQL processes filtering after reading table:

FROM → WHERE → SELECT → ORDER BY → LIMIT

🧠 Real-World Data Analyst Examples

Q. Find customers from Pune
SELECT * FROM customers WHERE city = 'Pune';

Q. Find high-paying jobs in IT department
SELECT * FROM employees WHERE salary > 80000 AND department = 'IT';

Q. Find names starting with "R"
SELECT * FROM employees WHERE name LIKE 'R%';

Used daily in business analytics.

🚀 Mini Practice Tasks
✅ Q1
Find employees whose salary is greater than 60000.
✅ Q2
Find customers from Pune or Mumbai.
✅ Q3
Find products priced between 100 and 500.
✅ Q4
Find employees whose name starts with "S".
✅ Q5
Find records where email is missing (NULL).

✅ Double Tap ♥️ For More