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SQL Interview Questions in Kolkata

36 practice questions with answers for SQL interviews in Kolkata.

SQL jobs in Kolkata

Kolkata has a steady IT-services and BFSI technology job market around Salt Lake Sector V and New Town. Many openings are with service companies and their delivery centres, with a focus on strong fundamentals.

Below are 36 questions that match the mix of service, product and domain employers hiring data analyst (sql)s in Kolkata.

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01

What are transaction isolation levels?

Transactions and Advanced SQLMedium
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Isolation levels balance consistency and concurrency. Read Uncommitted allows dirty reads. Read Committed, the default in PostgreSQL and SQL Server, prevents dirty reads but allows non-repeatable reads. Repeatable Read, the default in MySQL InnoDB, prevents non-repeatable reads, and InnoDB also prevents most phantoms with gap locks. Serializable prevents all anomalies but reduces concurrency. Choose based on business correctness needs.
02

How do you get the top 3 rows in different databases?

SQL BasicsMedium
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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.
03

What is a star schema and a snowflake schema?

Database DesignMedium
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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.
04

Tell me about a time you significantly improved query performance.

Indexes and PerformanceMedium
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Situation: a daily settlement report took 45 minutes and often missed its deadline. Task: bring it under ten minutes. Action: I studied the execution plan, replaced a non-SARGable date filter, added a composite covering index, pre-aggregated a large join in a temp table and updated statistics. Result: runtime dropped to six minutes, and the finance team received reports on time every day.
05

What is the CASE expression and where can it be used?

SQL BasicsMedium
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CASE adds conditional logic in SQL, returning a value based on conditions: CASE WHEN marks >= 60 THEN 'First' WHEN marks >= 45 THEN 'Second' ELSE 'Fail' END. It can be used in SELECT, WHERE, ORDER BY, GROUP BY and inside aggregates, for example SUM(CASE WHEN status = 'Paid' THEN amount ELSE 0 END) to pivot data. Conditions are checked in order and the first true one wins.
06

Assign each customer to a spending quartile.

Window FunctionsMedium
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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.
07

What are referential actions such as ON DELETE CASCADE?

Database DesignMedium
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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.
08

A search on name LIKE '%sharma%' is very slow on a 20 million row table. What can you do?

SQL BasicsMedium
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A leading wildcard cannot use a normal B-tree index, so the database scans the table. Options are full-text indexing, trigram indexes in PostgreSQL, a reversed or computed column if the pattern is a suffix, or a search engine for heavy text search. Also limit the result, filter on other indexed columns first, and check whether users really need contains search.
09

What is a SARGable query and why does it matter?

Indexes and PerformanceMedium
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A SARGable (Search ARGument able) predicate lets the optimiser use an index seek. Wrapping an indexed column in a function, such as WHERE YEAR(order_date) = 2025 or WHERE LOWER(email) = 'x', usually prevents index use. Rewrite as WHERE order_date >= '2025-01-01' AND order_date < '2026-01-01'. Leading wildcards and implicit type conversions also make predicates non-SARGable.
10

What are aggregate functions in SQL?

Aggregation and GroupingEasy
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Aggregate functions compute a single value from many rows: COUNT, SUM, AVG, MIN and MAX are standard, and databases add others such as STRING_AGG, GROUP_CONCAT, STDDEV and MEDIAN or PERCENTILE_CONT. Used with GROUP BY they return one value per group; without it they return one value for the whole table. Except COUNT(*), they ignore NULL values.
11

What is the difference between ROWS and RANGE frames?

Window FunctionsMedium
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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.
12

An interviewer asks for the Nth highest salary in a reusable way. How do you answer?

Subqueries and CTEsMedium
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Use DENSE_RANK so ties share a rank: SELECT salary FROM (SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) AS r FROM employees) t WHERE r = :n. Alternatively use ORDER BY salary DESC with OFFSET n-1 ROWS FETCH NEXT 1 ROW ONLY on distinct salaries. Mention how ties are handled and that per-department versions add PARTITION BY department_id.
13

What is a materialised view?

Transactions and Advanced SQLMedium
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A materialised view stores the result of a query physically, unlike a normal view. Reads are fast because data is precomputed, which suits heavy aggregations for dashboards. The trade-off is staleness, so it must be refreshed on schedule, on commit or incrementally depending on the database. PostgreSQL uses REFRESH MATERIALIZED VIEW, and SQL Server offers indexed views with similar benefits.
14

You need customers who bought both product A and product B. How would you write it?

JoinsMedium
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Group by customer and count distinct matching products: SELECT customer_id FROM order_items WHERE product_id IN ('A', 'B') GROUP BY customer_id HAVING COUNT(DISTINCT product_id) = 2. Alternatively use INTERSECT of two queries, or two EXISTS conditions. The HAVING approach is concise, scans the table once and is easy to extend to three or more products.
15

What is a window frame and what is the default frame?

Window FunctionsMedium
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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.
16

Write an upsert that inserts a row or updates it if it exists.

Transactions and Advanced SQLMedium
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PostgreSQL: INSERT INTO inventory (sku, qty) VALUES ('A100', 10) ON CONFLICT (sku) DO UPDATE SET qty = inventory.qty + EXCLUDED.qty; MySQL: INSERT INTO inventory (sku, qty) VALUES ('A100', 10) ON DUPLICATE KEY UPDATE qty = qty + VALUES(qty); SQL Server and others use MERGE. A unique key on sku is required.
17

Are CTEs materialised or inlined?

Subqueries and CTEsMedium
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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.
18

What are the common SQL data types?

SQL BasicsEasy
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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.
19

What is a subquery and where can it be used?

Subqueries and CTEsEasy
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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.
20

What is the difference between GROUP BY and window functions?

Window FunctionsEasy
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GROUP BY collapses rows into one row per group, so detail columns are lost. Window functions keep every row and add the computed value alongside, such as each employee with their department average. You can combine both: group first, then apply a window over the grouped result, for example percent of total using SUM(SUM(sales)) OVER ().
21

A business user needs a dashboard on a complex 6-table join that runs slowly. Would you use a view or a materialised view?

Transactions and Advanced SQLMedium
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A normal view only simplifies the query; it still runs the full join every time, so performance does not improve. If the data can be a few minutes or hours old, a materialised view or summary table refreshed on schedule gives fast reads. If real-time data is required, tune the underlying query with indexes or build an incrementally maintained table.
22

Average order value looks too low in a report. What could be wrong?

Aggregation and GroupingMedium
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Check the grain: if the query averages order items instead of orders, each line counts separately and the average drops. Joins can also duplicate rows. Verify whether cancelled or zero-value orders are included and how NULL amounts are treated. Compute SUM(amount) / COUNT(DISTINCT order_id) at order level and compare with business definitions before changing the report.
23

What are FIRST_VALUE, LAST_VALUE and NTH_VALUE?

Window FunctionsMedium
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FIRST_VALUE returns the first value in the window frame, LAST_VALUE the last, and NTH_VALUE the nth. A common trap is LAST_VALUE with the default frame, which ends at the current row, so it returns the current row's value. Specify ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING to get the true last value of the partition.
24

How do you handle a situation where business users dispute your numbers?

Aggregation and GroupingHard
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Situation: a regional manager said my sales report undercounted his region. Task: resolve it with facts. Action: I shared the query logic and definitions, compared a sample of his records with the database, and found his team counted orders on booking date while the report used invoice date. Result: we agreed on invoice date with finance, documented it, and added the definition to the report.
25

Rewrite a non-SARGable date filter to use an index.

Indexes and PerformanceMedium
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Slow: SELECT * FROM orders WHERE YEAR(order_date) = 2025 AND MONTH(order_date) = 3; Fast: SELECT * FROM orders WHERE order_date >= '2025-03-01' AND order_date < '2025-04-01'; The second form compares the raw column, so an index on order_date can be used with a range seek.
26

What are DDL, DML, DCL and TCL commands?

SQL BasicsEasy
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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.
27

How does AVG treat NULL values?

Aggregation and GroupingEasy
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AVG ignores NULLs, dividing the sum of non-null values by the count of non-null values. If three employees have bonuses 1000, 2000 and NULL, AVG(bonus) is 1500, not 1000. If NULL should mean zero, use AVG(COALESCE(bonus, 0)). Interviewers ask this to check whether you understand how missing data changes business metrics.
28

Write a query for monthly revenue for the current year.

Aggregation and GroupingMedium
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SELECT EXTRACT(MONTH FROM order_date) AS month, SUM(amount) AS revenue, COUNT(DISTINCT customer_id) AS buyers FROM orders WHERE order_date >= DATE '2025-01-01' AND order_date < DATE '2026-01-01' GROUP BY EXTRACT(MONTH FROM order_date) ORDER BY month; A date range in WHERE keeps an index on order_date usable.
29

What is the difference between CHAR and VARCHAR?

SQL BasicsEasy
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CHAR(n) stores fixed-length strings and pads shorter values with spaces, so every value uses n characters. VARCHAR(n) stores variable-length strings up to n and uses only the needed space plus a small length overhead. CHAR suits fixed codes such as country code or PIN format, while VARCHAR suits names, emails and addresses whose length varies.
30

How do you insert, update and delete a row?

SQL BasicsEasy
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INSERT INTO students (student_id, name, city) VALUES (101, 'Anita', 'Noida'); UPDATE students SET city = 'Gurugram' WHERE student_id = 101; DELETE FROM students WHERE student_id = 101; Always use WHERE with UPDATE and DELETE; without it every row changes.
31

What is the logical order of execution of a SELECT query?

SQL BasicsEasy
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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.
32

Your running total jumps by large amounts on some dates instead of row by row. Why?

Window FunctionsMedium
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With ORDER BY and no explicit frame, the default frame is RANGE UNBOUNDED PRECEDING, which treats all rows with the same order value as peers and includes them together. When several rows share the same date, the running total adds them all at once. Specify ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, or add a unique tiebreaker to ORDER BY.
33

Find departments where every employee earns above 40000.

Subqueries and CTEsMedium
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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.
34

Show all dates in a month with sales, including days with zero sales.

JoinsHard
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WITH days AS ( SELECT generate_series(DATE '2025-01-01', DATE '2025-01-31', INTERVAL '1 day')::date AS d ) SELECT d.d, COALESCE(SUM(s.amount), 0) AS sales FROM days d LEFT JOIN sales s ON s.sale_date = d.d GROUP BY d.d ORDER BY d.d; This PostgreSQL example uses a date series; other databases use a calendar table.
35

Your query WHERE manager_id <> 5 is not returning employees who have no manager. Why?

SQL BasicsMedium
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Employees with no manager have manager_id as NULL, and NULL <> 5 evaluates to unknown, not true, so those rows are excluded. Rewrite it as WHERE manager_id <> 5 OR manager_id IS NULL, or use COALESCE(manager_id, 0) <> 5. This three-valued logic with true, false and unknown causes many subtle bugs in reports and NOT IN subqueries.
36

What do PARTITION BY and ORDER BY do inside OVER?

Window FunctionsEasy
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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.

Companies with offices in Kolkata (2)

IT services

Wipro

SQL interview questions

  • 30 questions
  • 4 rounds

Bengaluru, Hyderabad, Pune +2

View questions
IT services

Cognizant

SQL interview questions

  • 30 questions
  • 4 rounds

Chennai, Bengaluru, Pune +2

View questions

Frequently asked questions

Which companies hire SQL professionals in Kolkata?

Companies with large offices in Kolkata include Wipro, Cognizant. Use their company pages below to practise employer-specific sets.

Are SQL interviews in Kolkata different from other cities?

The questions depend more on the company than the city. Kolkata's mix of employers decides the style: Many openings are with service companies and their delivery centres, with a focus on strong fundamentals

How do I find SQL jobs in Kolkata?

Shortlist companies with Kolkata offices, keep your CV focused on SQL projects, apply on company career pages and job portals with the Kolkata filter, and practise the questions on this page before each interview.

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