36 practice questions with answers for SQL interviews in Mumbai.
SQL jobs in Mumbai
Mumbai is India's financial capital, so banks, fintechs, insurers, media and consulting firms drive much of the hiring. Data and QA interviews often include domain questions on payments, risk, trading or customer analytics.
Below are 36 questions that match the mix of service, product and domain employers hiring data analyst (sql)s in Mumbai.
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01
Rewrite a non-SARGable date filter to use an index.
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.
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.
03
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').
SELECT sale_date, amount,
SUM(amount) OVER (
ORDER BY sale_date
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
) AS running_total
FROM daily_sales
ORDER BY sale_date;
Add PARTITION BY store_id to restart the running total for each store.
05
What is the difference between a primary key and a unique key?
Both enforce uniqueness. A table can have only one primary key, and its columns cannot contain NULL. A table can have many unique constraints, and in most databases a unique column can hold NULL, often more than one, though SQL Server allows only one NULL by default. Primary keys often create a clustered index in SQL Server; unique keys create non-clustered indexes.
06
How do you read an EXPLAIN ANALYZE output in PostgreSQL?
EXPLAIN ANALYZE
SELECT * FROM orders WHERE customer_id = 101;
Read the tree from the innermost node outward. Check node types such as Seq Scan, Index Scan or Hash Join, compare estimated rows with actual rows, and look at actual time per node. Large estimate errors suggest stale statistics, and Seq Scan on a selective filter suggests a missing index.
07
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.
08
A recursive CTE on a category tree runs forever. What is the likely cause and fix?
The data probably has a cycle, such as category A as parent of B and B as parent of A, so recursion never ends. Add a depth column and stop at a maximum level, track a path string or array and exclude nodes already visited, and use database limits such as MAXRECURSION in SQL Server. Then fix the cyclic data at the source with constraints or validation.
09
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.
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.
11
Can you use window functions with aggregate functions in the same query?
Yes. Window functions are evaluated after GROUP BY, so they can operate on aggregated values. For example, SELECT region, SUM(sales) AS s, RANK() OVER (ORDER BY SUM(sales) DESC) AS rnk FROM orders GROUP BY region ranks regions by total sales. The aggregate is computed per group first, and then the window function runs over the grouped rows.
12
An interviewer asks how you would find why a query is slow. What steps do you describe?
First reproduce the issue and measure time. Then check the execution plan for scans, expensive sorts, row estimate errors and join types. Look at indexes on filter and join columns, whether predicates are SARGable, and statistics freshness. Check for blocking or locks from other sessions. Change one thing at a time, measure again, and document the improvement.
A CROSS JOIN returns the Cartesian product, every row of the first table combined with every row of the second, so 10 rows by 5 rows gives 50 rows. It is useful for generating combinations, such as all stores by all dates to build a calendar grid and then left join actual sales to show zero-sales days. Accidental cross joins from missing conditions produce huge results.
14
A business user needs a dashboard on a complex 6-table join that runs slowly. Would you use a view or a materialised view?
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.
15
An interviewer asks to show each employee's salary next to their department's average. How do you do it?
Use a window aggregate: SELECT name, department_id, salary, AVG(salary) OVER (PARTITION BY department_id) AS dept_avg, salary - AVG(salary) OVER (PARTITION BY department_id) AS diff FROM employees. Unlike GROUP BY, every employee row is kept. Mention that a join with a grouped subquery produces the same result but is longer.
16
What is the difference between putting a condition in ON versus WHERE in a LEFT JOIN?
In a LEFT JOIN, a condition in ON filters the right table before matching, so all left rows are still kept. A condition on the right table in WHERE runs after the join and removes rows where the right side is NULL, effectively turning it into an inner join. To get all customers with only their 2025 orders, put the date condition in ON.
17
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.
18
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.
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.
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.
21
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.
22
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.
23
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.
24
You need to delete duplicate rows but keep one copy of each. How do you do it?
Use a CTE with ROW_NUMBER: WITH d AS (SELECT id, ROW_NUMBER() OVER (PARTITION BY email ORDER BY id) AS rn FROM users) DELETE FROM users WHERE id IN (SELECT id FROM d WHERE rn > 1). SQL Server allows DELETE FROM d WHERE rn > 1 directly. Back up the table first, run a SELECT to verify, then add a unique constraint to prevent new duplicates.
25
Match each employee's salary to a grade using a range table.
SELECT e.name, e.salary, g.grade
FROM employees e
JOIN salary_grades g
ON e.salary BETWEEN g.min_salary AND g.max_salary;
This is a non-equi join. Ensure grade ranges do not overlap, otherwise an employee matches more than one grade and appears twice.
26
What is parameter sniffing and how do you handle it?
In databases that cache plans, such as SQL Server, the plan for a parameterised query is compiled using the first parameter values. If those values are unusual, the cached plan may be poor for typical values, causing sudden slowness. Fixes include OPTION (RECOMPILE) for that query, OPTIMIZE FOR hints, plan guides, splitting procedures for different data shapes, or updating statistics.
27
Tell me about a time you had to deliver a report under a tight deadline.
Situation: the sales head needed region-wise quarterly numbers for a client meeting the same afternoon. Task: deliver accurate figures within three hours. Action: I confirmed metric definitions, reused tested queries, wrote one grouped query with ROLLUP for subtotals and cross-checked totals with the finance summary. Result: the report was delivered in two hours, and it later became a scheduled weekly report.
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.
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.
30
Create a simple stored procedure that returns orders for a customer.
MySQL:
DELIMITER //
CREATE PROCEDURE get_orders(IN p_customer INT)
BEGIN
SELECT order_id, order_date, amount
FROM orders
WHERE customer_id = p_customer
ORDER BY order_date DESC;
END //
DELIMITER ;
CALL get_orders(101);
Parameters prevent SQL injection compared with building query strings in code.
31
An interviewer asks for the Nth highest salary in a reusable way. How do you answer?
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.
32
You need each customer's latest address from a history table. How do you query it?
Use ROW_NUMBER partitioned by customer and ordered by updated date descending, then keep rn = 1: WITH r AS (SELECT *, ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY updated_at DESC) AS rn FROM address_history) SELECT * FROM r WHERE rn = 1. Add a tiebreaker such as id DESC in case two updates share the same timestamp.
Window functions perform calculations across a set of rows related to the current row without collapsing them into one row like GROUP BY. They use the OVER clause with optional PARTITION BY, ORDER BY and a frame. Examples are ROW_NUMBER, RANK, running SUM, LAG and LEAD. You can show each order alongside the customer's total, which GROUP BY alone cannot do.
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.
35
What is a composite index and why does column order matter?
A composite index covers multiple columns, such as (customer_id, order_date). It is sorted by the first column, then the second within it. It helps queries filtering on customer_id alone or on both columns, but not queries filtering only on order_date, because of the leftmost prefix rule. Put the most commonly filtered equality column first, followed by range columns.
36
A reporting team wants to query the production OLTP database directly. What are the risks and alternatives?
Heavy analytical queries can lock rows, use CPU and slow customer transactions, and normalised schemas make reporting complex. Alternatives are a read replica for reporting, a data warehouse loaded by ETL or CDC with a star schema, or materialised summary tables. Grant read-only access with limits if direct access is unavoidable, and schedule heavy jobs off-peak.
Companies with large offices in Mumbai include TCS, Accenture, Capgemini, Paytm, Razorpay, JP Morgan Chase, Goldman Sachs, Deloitte. Use their company pages below to practise employer-specific sets.
Are SQL interviews in Mumbai different from other cities?
The questions depend more on the company than the city. Mumbai's mix of employers decides the style: Data and QA interviews often include domain questions on payments, risk, trading or customer analytics
How do I find SQL jobs in Mumbai?
Shortlist companies with Mumbai offices, keep your CV focused on SQL projects, apply on company career pages and job portals with the Mumbai filter, and practise the questions on this page before each interview.