Curated interview practice questions with answers. These are practice scenarios, not a verified list of past interview questions.
About TCS hiring for SQL roles
Tata Consultancy Services is a Tata Group company and one of the largest IT services and consulting firms in the world. It delivers application development, testing, cloud, data and analytics services to clients in banking, retail, manufacturing and other industries. In India it hires freshers and experienced staff for software testing, data engineering, BI reporting and analytics roles across its delivery centres.
For SQL roles, this TCS practice set focuses most on Joins (6), Window Functions (5), Subqueries and CTEs (4), Database Design (4). It has 12 easy, 18 medium questions, so you can start easy and work up to the harder ones.
Company type
IT services
Headquarters
Mumbai, India
Main India offices
Mumbai, Chennai, Pune, Bengaluru, Hyderabad
TCS SQL interview process
TCS NQT online assessment (aptitude + coding)
Technical interview
Managerial round
HR round
Typical pattern based on public candidate experiences. Rounds vary by role, team and year.
Solved for TCSStart practicing
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An index is a separate data structure, usually a B-tree, that stores column values in sorted order with pointers to rows, so the database can find matching rows without scanning the whole table, like a book index. Indexes speed up WHERE, JOIN, ORDER BY and GROUP BY, but use storage and slow down inserts, updates and deletes because the index must be maintained.
A join combines rows from two or more tables based on a related column. INNER JOIN returns matching rows only. LEFT JOIN returns all rows from the left table with matches or NULLs. RIGHT JOIN does the opposite. FULL OUTER JOIN returns all rows from both. CROSS JOIN returns every combination. A SELF JOIN joins a table to itself, such as employees and managers.
A subquery is a query nested inside another statement. It can appear in WHERE, such as salary > (SELECT AVG(salary) FROM employees), in FROM as a derived table, in SELECT as a scalar value, and in HAVING. It can return a single value, a list for IN, or a table. Subqueries break complex logic into smaller steps.
SELECT emp_id, name, department_id, salary,
DENSE_RANK() OVER (
PARTITION BY department_id
ORDER BY salary DESC
) AS dept_rank
FROM employees;
PARTITION BY restarts ranking for each department, and DENSE_RANK keeps equal salaries at the same rank.
05
Join three tables to get total sales by product category.
SELECT cat.category_name,
SUM(oi.quantity * oi.unit_price) AS revenue
FROM order_items oi
JOIN products p ON oi.product_id = p.product_id
JOIN categories cat ON p.category_id = cat.category_id
GROUP BY cat.category_name
ORDER BY revenue DESC;
Join from the most detailed table outward and aggregate at the end.
It depends on the database. SQL Server and MySQL usually inline CTEs like subqueries. PostgreSQL before version 12 always materialised CTEs, which could hurt performance; since version 12 non-recursive CTEs referenced once are inlined, and you can force behaviour with MATERIALIZED or NOT MATERIALIZED. Referencing a heavy CTE many times may cause repeated work, so check the plan.
A natural key is a real-world identifier like PAN number, email or product code. A surrogate key is a system-generated value, usually an integer or UUID, with no business meaning. Surrogate keys are stable, compact and good for joins, while natural keys can change or be long. A common design uses a surrogate primary key plus a unique constraint on the natural key.
SELECT city, COUNT(*) AS customers
FROM customers
GROUP BY city
HAVING COUNT(*) > 100
ORDER BY customers DESC;
HAVING is needed because the condition uses an aggregate. A WHERE clause could still filter rows first, for example only active customers.
09
What is the difference between MIN/MAX on numbers and on strings or dates?
MIN and MAX work on any comparable type. On numbers they return smallest and largest values, on dates the earliest and latest, and on strings the first and last in alphabetical order based on collation. MAX(order_date) gives the latest order, and MIN(name) gives the alphabetically first name, which is useful but not a meaningful business metric by itself.
10
Find departments where every employee earns above 40000.
SELECT d.department_id, d.name
FROM departments d
WHERE NOT EXISTS (
SELECT 1 FROM employees e
WHERE e.department_id = d.department_id
AND e.salary <= 40000
)
AND EXISTS (
SELECT 1 FROM employees e WHERE e.department_id = d.department_id
);
Double negation expresses for all; the EXISTS excludes empty departments.
11
What is the difference between DELETE, TRUNCATE and DROP?
DELETE removes selected rows using a WHERE clause, is logged row by row, fires triggers and can be rolled back inside a transaction. TRUNCATE removes all rows quickly by deallocating pages, usually resets identity values and cannot use WHERE. DROP removes the whole table including structure, indexes and permissions. Use DELETE for selective removal, TRUNCATE to empty staging tables and DROP to remove objects.
12
Tell me about a time you used advanced SQL to solve a business problem.
Situation: the product team wanted to know where users dropped off in the onboarding flow. Task: build a funnel from raw event logs. Action: I used window functions to order events per user, LEAD to find the next step and time taken, and grouped the results into a funnel with conversion rates. Result: we found a 40 percent drop at KYC upload, and the redesign improved completion by 15 percent.
13
After joining orders with payments, revenue doubled. What went wrong?
Some orders have multiple payment rows, such as partial payments or retries, so each order row repeats and SUM(order amount) counts it multiple times. This is join fan-out. Fix it by aggregating payments per order in a subquery or CTE before joining, or summing order amounts from the orders table alone. Always check row counts before and after a join.
Denormalisation intentionally adds redundancy, such as storing customer name in an orders reporting table, to reduce joins and speed up reads. It is common in data warehouses, reporting tables and high-read applications. The cost is extra storage and the need to keep copies consistent through ETL, triggers or application logic. Use it when read performance matters more than write simplicity.
A transaction is a group of SQL statements executed as a single unit of work that either fully succeeds or fully fails. It starts with BEGIN or START TRANSACTION and ends with COMMIT to save changes or ROLLBACK to undo them. For a fund transfer, debiting one account and crediting another must happen together, so both statements run in one transaction.
PARTITION BY divides rows into groups and the window function restarts for each group, like per department. ORDER BY defines the order of rows within each partition, needed for ranking, running totals and LAG or LEAD. Without PARTITION BY the whole result is one partition. Without ORDER BY, aggregate windows compute over the entire partition.
17
What is the difference between FULL OUTER JOIN and UNION of LEFT and RIGHT joins?
A FULL OUTER JOIN returns matched rows plus unmatched rows from both sides in one operation. MySQL does not support it, so it is emulated with a LEFT JOIN UNION a RIGHT JOIN, or LEFT JOIN UNION ALL a RIGHT JOIN filtered to unmatched rows. Using plain UNION removes duplicates but can also remove genuinely repeated rows, so the UNION ALL pattern is safer.
18
What is the difference between an index scan and an index seek?
An index seek navigates the B-tree directly to the matching keys, reading only the needed rows, which is efficient for selective filters. An index scan reads the whole index or a large range of it. A table scan reads every row. Scans are not always bad; for queries returning a large share of rows, a scan can be cheaper than many seeks.
19
What are dirty reads, non-repeatable reads and phantom reads?
A dirty read sees uncommitted changes from another transaction that may roll back. A non-repeatable read gets different values when reading the same row twice because another transaction updated and committed it. A phantom read gets a different set of rows for the same query because another transaction inserted or deleted matching rows. Higher isolation levels prevent more of these.
20
How do you get the top 3 rows in different databases?
MySQL and PostgreSQL:
SELECT * FROM products ORDER BY price DESC LIMIT 3;
SQL Server:
SELECT TOP 3 * FROM products ORDER BY price DESC;
Standard SQL:
SELECT * FROM products ORDER BY price DESC
FETCH FIRST 3 ROWS ONLY;
Always use ORDER BY, otherwise the top rows are not guaranteed.
Atomicity: all statements in a transaction succeed or none do. Consistency: a transaction moves the database from one valid state to another, respecting constraints. Isolation: concurrent transactions do not interfere and behave as if run in some order, depending on isolation level. Durability: once committed, changes survive crashes because they are written to logs. Banking and payment systems depend on ACID.
22
What is the difference between INNER JOIN and LEFT JOIN?
INNER JOIN keeps only rows where the join condition matches in both tables. LEFT JOIN keeps every row from the left table and fills columns from the right with NULL when there is no match. For customers and orders, INNER JOIN shows only customers who ordered, while LEFT JOIN shows all customers, including those with no orders, which is useful for finding inactive customers.
23
Why is NOT IN dangerous when the subquery contains NULL?
If the subquery returns any NULL, value NOT IN (list) becomes unknown for every row, because comparing with NULL is unknown, so the query returns no rows. For example, NOT IN (SELECT manager_id FROM employees) fails when a manager_id is NULL. Use NOT EXISTS or filter out NULLs in the subquery to get correct results.
Constraints enforce rules on data: PRIMARY KEY for unique row identity, FOREIGN KEY for valid references, UNIQUE for no duplicates, NOT NULL for required values, CHECK for conditions like age >= 18, and DEFAULT for automatic values. They protect data quality at the database level, so bad data is rejected even if an application has a bug.
25
What is GROUP BY and what is the rule for selected columns?
GROUP BY groups rows that share values in specified columns so aggregates can be computed for each group. Every column in SELECT must either be in GROUP BY or inside an aggregate function. SELECT city, COUNT(*) FROM customers GROUP BY city is valid, but adding name without aggregating it is an error in standard SQL, though older MySQL modes allowed it.
26
What are equi joins, non-equi joins and natural joins?
An equi join uses equality in the condition, such as a.id = b.id, the most common case. A non-equi join uses other operators like BETWEEN or greater than, for example matching salary to a grade range table. A natural join automatically joins on columns with the same names, which is risky because adding a column can silently change results, so explicit ON is preferred.
WITH spend AS (
SELECT customer_id, SUM(amount) AS total
FROM orders
GROUP BY customer_id
)
SELECT customer_id, total,
NTILE(4) OVER (ORDER BY total DESC) AS quartile
FROM spend;
Quartile 1 contains the highest spenders. Use this for targeted offers or loyalty tiers.
28
What is the difference between SUM with CASE and filtering with WHERE?
WHERE removes rows from the entire query, so all aggregates see only filtered data. SUM(CASE WHEN condition THEN amount END) applies the condition to one aggregate only, letting you calculate several conditional totals in one pass, such as paid and refunded amounts per customer. Some databases support the cleaner FILTER clause: SUM(amount) FILTER (WHERE status = 'Paid').
29
What is the difference between ROWS and RANGE frames?
ROWS counts physical rows, so 2 PRECEDING means exactly two rows before. RANGE works on values of the ORDER BY column, so rows with the same value are treated together, and value ranges like INTERVAL '7 days' PRECEDING are possible in some databases. With duplicate dates, a RANGE running total jumps for all ties at once, while ROWS increases row by row.
30
A table stores phone numbers as 'ph1, ph2, ph3' in one column. What is wrong and how do you fix it?
The column violates first normal form because it stores multiple values in one field, making search, validation and updates difficult. Create a separate customer_phones table with customer_id, phone and type columns, one row per number, with a foreign key to customers. Migrate data by splitting the strings, then drop the old column after applications are updated.
Tips to crack the TCS SQL interview
Practise TCS NQT style numerical, verbal and reasoning sections along with two basic coding problems under strict time limits.
Be ready to explain your final-year project, SQL basics and core concepts like OOP, DBMS and software testing life cycle.
Read about TCS service lines such as BFSI and its iON and Ignio platforms to answer why you want to join.
Keep a one-minute story ready for every project on your CV: problem, your part, tools, result.
Practise the hard questions on this page aloud; TCS interviewers usually ask follow-ups on your first answer.
Frequently asked questions
How many rounds are there in the TCS SQL interview?
Candidates usually go through 4 stages: TCS NQT online assessment (aptitude + coding); Technical interview; Managerial round; HR round. The exact rounds change with the role, team, location and your experience, so confirm the process with the recruiter.
Which topics should I prepare for the TCS SQL interview?
This practice set leans on Joins (6), Window Functions (5), Subqueries and CTEs (4), Database Design (4). Revise those first, then practise explaining one or two of your own projects end to end, because most TCS interviewers ask you to walk through real work.
Are these the exact questions asked at TCS?
No. These are curated practice questions that match the skills and difficulty usually tested in it services interviews like TCS's. They are not a leaked or verified list of past questions.
Where does TCS hire SQL professionals in India?
TCS has large teams in Mumbai, Chennai, Pune, Bengaluru, Hyderabad. Openings and locations change often, so check the TCS careers page and filter by city and experience.