Curated interview practice questions with answers. These are practice scenarios, not a verified list of past interview questions.
About Infosys hiring for SQL roles
Infosys is an Indian multinational IT services and consulting company offering digital transformation, cloud, application management, testing and data services to global clients. It is known for its Mysuru training campus where new hires go through foundation programmes. In India it recruits for roles such as systems engineer, test engineer, data analyst and data engineer, often through campus and off-campus drives.
For SQL roles, this Infosys practice set focuses most on Transactions and Advanced SQL (6), Database Design (5), Joins (5), SQL Basics (4). It has 16 easy, 13 medium, 1 hard questions, so you can start easy and work up to the harder ones.
Typical pattern based on public candidate experiences. Rounds vary by role, team and year.
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01
Design tables for students, courses and enrolments.
CREATE TABLE students (
student_id INT PRIMARY KEY,
name VARCHAR(100) NOT NULL
);
CREATE TABLE courses (
course_id INT PRIMARY KEY,
title VARCHAR(100) NOT NULL
);
CREATE TABLE enrolments (
student_id INT REFERENCES students(student_id),
course_id INT REFERENCES courses(course_id),
enrolled_on DATE,
PRIMARY KEY (student_id, course_id)
);
The junction table resolves the many-to-many relationship.
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.
Common types are INT, BIGINT and SMALLINT for whole numbers, DECIMAL or NUMERIC for exact values like money, FLOAT for approximate values, CHAR for fixed-length and VARCHAR for variable-length text, DATE, TIME, DATETIME or TIMESTAMP for dates, and BOOLEAN where supported. Choosing correct types saves space, improves performance and prevents errors such as storing amounts in FLOAT and getting rounding issues.
Avoid indexes on small tables where scans are cheap, on columns with very low selectivity such as a boolean flag used alone, on tables with heavy writes and few reads, and duplicates of existing indexes. Every index slows inserts and updates and uses memory and storage. Review unused indexes periodically with database statistics and drop them after checking dependencies.
05
What is the logical order of execution of a SELECT query?
The logical order is FROM and JOIN, then WHERE, GROUP BY, HAVING, SELECT, DISTINCT, ORDER BY, and finally LIMIT or OFFSET (or TOP/FETCH). This explains why a column alias defined in SELECT cannot be used in WHERE but can be used in ORDER BY in most databases. The optimiser may execute differently physically, but results follow this logical order.
DDL (Data Definition Language) defines structure: CREATE, ALTER, DROP, TRUNCATE. DML (Data Manipulation Language) changes data: INSERT, UPDATE, DELETE, and SELECT is often grouped here or as DQL. DCL (Data Control Language) manages access: GRANT and REVOKE. TCL (Transaction Control Language) manages transactions: COMMIT, ROLLBACK and SAVEPOINT. Knowing the categories helps explain behaviour such as auto-commit of DDL in some databases.
07
Describe a time you reviewed someone else's SQL and suggested improvements.
Situation: a teammate's report query took eight minutes and had nested subqueries. Task: help without discouraging him. Action: I reviewed it with him, rewrote parts as CTEs, replaced a correlated subquery with a pre-aggregated join, and explained the execution plan differences. Result: the query ran in 40 seconds, and he later applied the same approach to two other reports.
08
Compare this month's and last month's sales per product using a full outer join.
SELECT COALESCE(c.product_id, p.product_id) AS product_id,
COALESCE(c.sales, 0) AS this_month,
COALESCE(p.sales, 0) AS last_month
FROM cur_month c
FULL OUTER JOIN prev_month p
ON c.product_id = p.product_id;
Products sold in only one of the months still appear, with zero for the other month.
A stored procedure is a named set of SQL statements saved in the database and executed with CALL or EXEC, optionally with input and output parameters. It encapsulates business logic, reduces network round trips, can be reused by many applications and allows permission to execute without direct table access. Overusing procedures can make logic harder to version and test compared with application code.
10
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.
A star schema has a central fact table with measures such as sales amount and foreign keys to denormalised dimension tables like date, product, store and customer. A snowflake schema normalises dimensions further, such as product to category to department. Star schemas are simpler and faster for BI queries, while snowflake schemas save space and reduce redundancy.
A view is a saved SELECT query that behaves like a virtual table. It simplifies complex joins for users, hides sensitive columns for security, and provides a stable interface when underlying tables change. A normal view does not store data, so it runs the query each time. Simple views can be updatable, while views with aggregates or joins usually are not.
13
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.
An execution plan shows how the database will run a query: which tables are scanned or seeked, which indexes are used, the join algorithms, sort and aggregate steps, and estimated row counts and costs. Use EXPLAIN in MySQL and PostgreSQL, EXPLAIN ANALYZE for actual timing, and the estimated or actual plan in SQL Server. Reading plans is the first step in tuning slow queries.
A primary key uniquely identifies each row in a table and cannot be NULL or duplicate, such as student_id. A foreign key is a column in one table that references the primary key of another, such as orders.customer_id referencing customers.customer_id. Foreign keys enforce referential integrity, preventing orders for customers that do not exist.
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 a window frame and what is the default frame?
A frame defines which rows around the current row are included, such as ROWS BETWEEN 2 PRECEDING AND CURRENT ROW for a 3-row moving average. When ORDER BY is present and no frame is specified, the default is RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, which produces a running total but includes all ties of the current value. Without ORDER BY, the frame is the whole partition.
18
Find the number of employees and average salary in each department.
SELECT department_id,
COUNT(*) AS emp_count,
ROUND(AVG(salary), 2) AS avg_salary
FROM employees
GROUP BY department_id
ORDER BY emp_count DESC;
Join with the departments table if department names are needed, and use LEFT JOIN from departments to include empty departments.
19
What happens when you join on a column that has NULL values?
NULL never equals NULL in a join condition, so rows with NULL in the join column do not match each other in an inner join. In a LEFT JOIN, left rows with NULL keys are kept but get NULLs for right columns. If you really need NULLs to match, use a condition such as a.key IS NOT DISTINCT FROM b.key, or COALESCE both sides carefully.
20
Two users book the last seat at the same time and both succeed. How do you fix it?
This is a race condition. Wrap the check and booking in one transaction and lock the seat row with SELECT ... FOR UPDATE so the second transaction waits and then sees it is taken. Alternatively use an atomic update such as UPDATE seats SET booked_by = :user WHERE seat_id = :id AND booked_by IS NULL and check affected rows, plus a unique constraint as a final safety net.
21
Describe your SQL project from college or internship.
Situation: during my internship the team tracked student attendance in spreadsheets. Task: build a database and reports. Action: I designed tables for students, courses and attendance, loaded data, and wrote queries with joins, group by and window functions to show monthly attendance and low-attendance alerts. Result: faculty got a weekly report automatically, and I learned how real data quality issues affect queries.
22
What is the difference between clustered and non-clustered indexes?
A clustered index defines the physical order of rows in the table, so there can be only one, and in SQL Server the primary key creates it by default. A non-clustered index is a separate structure with key values and pointers to rows, and a table can have many. Range queries on the clustered key are very fast, while non-clustered indexes suit selective lookups.
23
What are referential actions such as ON DELETE CASCADE?
Referential actions define what happens to child rows when a parent row is deleted or updated. CASCADE deletes or updates children automatically, SET NULL sets the foreign key to NULL, SET DEFAULT uses the default value, and RESTRICT or NO ACTION blocks the change. CASCADE is convenient for dependent data like order items but risky on important data, because one delete can remove many rows.
They produce multiple grouping levels in one query. ROLLUP(region, city) gives city totals, region subtotals and a grand total. CUBE(region, product) gives all combinations of subtotals. GROUPING SETS lets you list exact combinations, such as (region), (product) and (). The GROUPING function identifies subtotal rows, which is useful for labelling them in reports.
25
Why should we hire you for a SQL or data role as a fresher?
Situation: in college I built a library management project with a MySQL backend. Task: make queries fast and correct. Action: I designed normalised tables, wrote queries with joins and group by, and practised over 150 interview problems online. Result: I can write clean, tested SQL today. I learn quickly, ask good questions and want to grow into a strong data professional with your team.
26
A COUNT(DISTINCT) query on a huge events table is very slow. What options do you have?
Exact distinct counts need sorting or hashing all values, which is expensive. Options are filtering the date range tightly, pre-aggregating daily distinct users into a summary table, indexing the columns involved, or using approximate functions such as APPROX_COUNT_DISTINCT or HyperLogLog where small error is acceptable. Explain the accuracy trade-off to stakeholders before switching to approximations.
27
What is the difference between a join and a union?
A join combines columns from different tables side by side based on a matching condition, making rows wider. A union stacks results of queries with the same column structure vertically, making the result longer. Use a join to bring customer names next to orders, and a union to combine 2024 and 2025 order tables into one list.
28
What is the difference between a stored procedure and a function?
A function must return a value or table and can be used inside SELECT or WHERE, but in most databases cannot change data or manage transactions. A stored procedure is executed independently, can return multiple result sets or output parameters, can modify data and control transactions. Use functions for reusable calculations and procedures for workflows such as month-end processing.
29
Add a foreign key and a check constraint to an existing table.
ALTER TABLE orders
ADD CONSTRAINT fk_orders_customer
FOREIGN KEY (customer_id)
REFERENCES customers(customer_id);
ALTER TABLE orders
ADD CONSTRAINT chk_amount_positive
CHECK (amount > 0);
First clean existing rows that violate the rule, otherwise the ALTER fails. Naming constraints makes errors easier to read.
30
Your application logs frequent deadlock errors at peak hours. How do you investigate and reduce them?
Capture deadlock graphs or logs to see which statements and resources are involved. Common fixes are accessing tables and rows in the same order in all code paths, keeping transactions short, adding indexes so updates lock fewer rows, avoiding user interaction inside transactions, and using lower isolation where safe. Add retry logic for deadlock victims since some deadlocks cannot be fully eliminated.
Tips to crack the Infosys SQL interview
Practise pseudocode output questions and puzzle-style reasoning, which commonly appear in Infosys online tests and HackWithInfy style contests.
Revise SQL joins, normalisation and one programming language well enough to write and explain simple programs on paper.
Mention willingness to learn new technologies and relocate, since Infosys emphasises its training programme and flexible project allocation.
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; Infosys interviewers usually ask follow-ups on your first answer.
Frequently asked questions
How many rounds are there in the Infosys SQL interview?
Candidates usually go through 4 stages: Online assessment (aptitude, reasoning + pseudocode); Coding test; Technical interview; 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 Infosys SQL interview?
This practice set leans on Transactions and Advanced SQL (6), Database Design (5), Joins (5), SQL Basics (4). Revise those first, then practise explaining one or two of your own projects end to end, because most Infosys interviewers ask you to walk through real work.
Are these the exact questions asked at Infosys?
No. These are curated practice questions that match the skills and difficulty usually tested in it services interviews like Infosys's. They are not a leaked or verified list of past questions.
Where does Infosys hire SQL professionals in India?
Infosys has large teams in Bengaluru, Mysuru, Pune, Hyderabad, Chennai. Openings and locations change often, so check the Infosys careers page and filter by city and experience.