Skip to content
SQL
All coursesPlacement Report
‹ Back to previous sectionCourse homeAll page URLs

SQL Rapid-Fire Questions & Answers

50 questions for the rapid-fire stage — 34 easy, 16 medium.

About the rapid-fire round

Rapid-fire questions are quick definitions and one-line answers that interviewers fire at the start or end of a round to check your SQL basics. Aim to answer each in under 20 seconds.

Tips for the rapid-fire round

  • Answer in one or two sentences, then stop.
  • If you don't know, say so quickly and move on; don't guess at length.
  • Revise these the morning of the interview.
  • Practise with a friend who reads them in random order.
Solved — rapid-fireStart practicing
3 QUESTIONS FREESign up to unlock the full question bank and track your progress.Unlock every question and answer, then continue practising company-wise and package-wise.
3 questions free Unlock the rest in seconds
01

What is the difference between a unique index and a normal index?

Indexes and PerformanceEasy
Save to ▾
Show answer
A unique index enforces that no two rows have the same value in the indexed columns, and it is created automatically for primary keys and unique constraints. A normal index only speeds up lookups and allows duplicates. Both speed up searches, but a unique index also protects data integrity, for example preventing two users from registering with the same email.
02

How does AVG treat NULL values?

Aggregation and GroupingEasy
Save to ▾
Show answer
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.
03

What is the difference between WHERE and HAVING?

SQL BasicsEasy
Save to ▾
Show answer
WHERE filters individual rows before grouping and cannot use aggregate functions. HAVING filters groups after GROUP BY and can use aggregates such as COUNT or SUM. For example, WHERE city = 'Pune' keeps Pune rows, and HAVING COUNT(*) > 5 keeps only groups with more than five rows. Filtering early with WHERE is also more efficient.
04

What are equi joins, non-equi joins and natural joins?

JoinsMedium
Save to ▾
Show answer
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.
05

What is the difference between DELETE, TRUNCATE and DROP?

SQL BasicsEasy
Save to ▾
Show answer
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.
06

What is the difference between INNER JOIN and LEFT JOIN?

JoinsEasy
Save to ▾
Show answer
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.
07

What is a stored procedure?

Transactions and Advanced SQLEasy
Save to ▾
Show answer
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.
08

What are the common SQL data types?

SQL BasicsEasy
Save to ▾
Show answer
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.
09

What is a CROSS JOIN and when is it useful?

JoinsEasy
Save to ▾
Show answer
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.
10

What is the difference between a correlated and a non-correlated subquery?

Subqueries and CTEsEasy
Save to ▾
Show answer
A non-correlated subquery runs independently and returns a result once, such as the overall average salary. A correlated subquery references columns from the outer query and logically runs for each outer row, such as comparing each employee's salary with the average of their own department. Correlated subqueries are powerful but may be slow without good indexes, although optimisers often rewrite them as joins.
11

What is a composite index and why does column order matter?

Indexes and PerformanceEasy
Save to ▾
Show answer
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.
12

What is a join and what types of joins exist?

JoinsEasy
Save to ▾
Show answer
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.
13

What is a composite key?

Database DesignEasy
Save to ▾
Show answer
A composite key is a primary key made of two or more columns that together uniquely identify a row. For example, in an enrolments table, student_id and course_id together form the key because a student can join many courses and a course has many students, but each pair appears once. Column order in a composite key matters for index usage.
14

What are aggregate functions in SQL?

Aggregation and GroupingEasy
Save to ▾
Show answer
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.
15

What is a self join? Give an example.

JoinsEasy
Save to ▾
Show answer
A self join joins a table to itself using aliases, used for hierarchical or comparative data in one table. Example: SELECT e.name AS employee, m.name AS manager FROM employees e LEFT JOIN employees m ON e.manager_id = m.emp_id. Each employee row is matched with the row of its manager. LEFT JOIN keeps the top manager whose manager_id is NULL.
16

What is a transaction in SQL?

Transactions and Advanced SQLEasy
Save to ▾
Show answer
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.
17

What is SQL and what is it used for?

SQL BasicsEasy
Save to ▾
Show answer
SQL, or Structured Query Language, is the standard language for working with relational databases. It is used to create tables, insert, update and delete data, query data with filters, joins and aggregations, and control access. Almost every application, analytics team and reporting tool relies on SQL, which is why it is tested in interviews for developers, analysts, testers and data engineers.
18

What is the difference between a join and a union?

JoinsMedium
Save to ▾
Show answer
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.
19

What is the difference between a primary key and a unique key?

Database DesignEasy
Save to ▾
Show answer
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.
20

What is the difference between ROW_NUMBER, RANK and DENSE_RANK?

Window FunctionsEasy
Save to ▾
Show answer
For salaries 900, 800, 800, 700 ordered descending: ROW_NUMBER gives 1, 2, 3, 4 with unique numbers even for ties; RANK gives 1, 2, 2, 4, skipping after ties; DENSE_RANK gives 1, 2, 2, 3 without gaps. Use ROW_NUMBER for deduplication or picking one row, and DENSE_RANK for Nth highest value questions where ties should share a position.
21

What are ACID properties?

Transactions and Advanced SQLEasy
Save to ▾
Show answer
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 clustered and non-clustered indexes?

Indexes and PerformanceEasy
Save to ▾
Show answer
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 is the difference between a CTE, a subquery, a temp table and a view?

Subqueries and CTEsMedium
Save to ▾
Show answer
A subquery and a CTE exist only for one statement; a CTE is more readable and can be recursive. A temporary table stores data physically for the session, can be indexed and reused across statements, useful for large intermediate results. A view is a saved query stored in the database that anyone with access can reuse. Choose by reuse, size and performance needs.
24

What are window functions in SQL?

Window FunctionsEasy
Save to ▾
Show answer
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.
25

What is an index in a database?

Indexes and PerformanceEasy
Save to ▾
Show answer
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.
26

What is the difference between FULL OUTER JOIN and UNION of LEFT and RIGHT joins?

JoinsMedium
Save to ▾
Show answer
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.
27

What are DDL, DML, DCL and TCL commands?

SQL BasicsEasy
Save to ▾
Show answer
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.
28

What is the difference between CHAR and VARCHAR?

SQL BasicsEasy
Save to ▾
Show answer
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.
29

What is the difference between a stored procedure and a function?

Transactions and Advanced SQLMedium
Save to ▾
Show answer
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.
30

What are ROLLUP, CUBE and GROUPING SETS?

Aggregation and GroupingMedium
Save to ▾
Show answer
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.
31

What is a view and why use it?

Transactions and Advanced SQLEasy
Save to ▾
Show answer
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.
32

Can you use window functions with aggregate functions in the same query?

Window FunctionsMedium
Save to ▾
Show answer
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.
33

What is a subquery and where can it be used?

Subqueries and CTEsEasy
Save to ▾
Show answer
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.
34

What are referential actions such as ON DELETE CASCADE?

Database DesignMedium
Save to ▾
Show answer
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.
35

What is the difference between IN and EXISTS?

Subqueries and CTEsMedium
Save to ▾
Show answer
IN checks whether a value matches any value in a list or subquery result. EXISTS checks whether a correlated subquery returns at least one row and stops at the first match. Modern optimisers often treat them similarly, but EXISTS is clearer for correlated checks and handles NULLs safely. For large subquery results, EXISTS with an index on the join column is usually efficient.
36

What is an execution plan?

Indexes and PerformanceEasy
Save to ▾
Show answer
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.
37

What is the difference between MIN/MAX on numbers and on strings or dates?

Aggregation and GroupingEasy
Save to ▾
Show answer
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.
38

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

SQL BasicsEasy
Save to ▾
Show answer
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.
39

What is a deadlock and how do databases handle it?

Transactions and Advanced SQLMedium
Save to ▾
Show answer
A deadlock occurs when two transactions each hold a lock the other needs, so both wait forever. For example, T1 locks account A and wants B, while T2 locks B and wants A. The database detects the cycle and kills one transaction as the victim, rolling it back with an error. Applications should retry the victim and access resources in a consistent order.
40

What is NULL in SQL and how do you compare it?

SQL BasicsEasy
Save to ▾
Show answer
NULL means a missing or unknown value, not zero or an empty string. Any comparison with NULL using = or <> returns unknown, so rows are not returned. Use IS NULL or IS NOT NULL to test it, and COALESCE or IFNULL to replace it. Aggregates like SUM and AVG ignore NULLs, while COUNT(*) counts all rows and COUNT(column) skips NULLs.
41

Can you use a column alias in GROUP BY or HAVING?

Aggregation and GroupingMedium
Save to ▾
Show answer
In standard SQL, GROUP BY and HAVING are evaluated before SELECT, so aliases are not available there. MySQL and PostgreSQL allow aliases in GROUP BY, and MySQL allows them in HAVING, but SQL Server does not. For portable code, repeat the expression or wrap the query in a subquery or CTE and filter on the alias in the outer query.
42

What do NTILE, PERCENT_RANK and CUME_DIST do?

Window FunctionsMedium
Save to ▾
Show answer
NTILE(n) splits ordered rows into n roughly equal buckets, useful for quartiles or deciles. PERCENT_RANK returns (rank - 1) / (rows - 1), a relative position between 0 and 1. CUME_DIST returns the fraction of rows with values less than or equal to the current row. They are used for customer segmentation, percentile reports and scoring.
43

Explain 1NF, 2NF and 3NF with simple examples.

Database DesignEasy
Save to ▾
Show answer
1NF: each column holds atomic values and there are no repeating groups, so no comma-separated phone numbers. 2NF: in 1NF and every non-key column depends on the whole composite key, so product name should not sit in an order_items table keyed by order and product. 3NF: in 2NF with no transitive dependency, so city should not determine state inside the employees table.
44

What is the difference between UNION and UNION ALL?

SQL BasicsMedium
Save to ▾
Show answer
UNION combines the results of two queries and removes duplicate rows, which requires sorting or hashing. UNION ALL combines results and keeps all rows, so it is faster. Both need the same number of columns with compatible types in the same order. Use UNION ALL when duplicates are impossible or acceptable, such as stacking monthly tables, and UNION only when deduplication is required.
45

What is a star schema and a snowflake schema?

Database DesignMedium
Save to ▾
Show answer
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.
46

What are constraints in SQL?

Database DesignMedium
Save to ▾
Show answer
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.
47

What is GROUP BY and what is the rule for selected columns?

Aggregation and GroupingEasy
Save to ▾
Show answer
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.
48

What does DISTINCT do and how is it different from GROUP BY?

SQL BasicsMedium
Save to ▾
Show answer
DISTINCT removes duplicate rows from the result based on all selected columns. GROUP BY groups rows so you can apply aggregates per group. SELECT DISTINCT city FROM customers and SELECT city FROM customers GROUP BY city give the same output, but GROUP BY is needed when you also want counts or sums. Overusing DISTINCT to hide duplicate join rows is a common mistake.
49

What is normalisation and why is it needed?

Database DesignEasy
Save to ▾
Show answer
Normalisation organises data into tables to reduce redundancy and avoid insert, update and delete anomalies. For example, storing customer address in every order row means updating it in many places. By separating customers and orders and linking with keys, each fact is stored once. Common forms are 1NF, 2NF, 3NF and BCNF, and most transactional systems target 3NF.
50

What do PARTITION BY and ORDER BY do inside OVER?

Window FunctionsEasy
Save to ▾
Show answer
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.

Frequently asked questions

What happens in the SQL rapid-fire round?

Rapid-fire questions are quick definitions and one-line answers that interviewers fire at the start or end of a round to check your SQL basics. Aim to answer each in under 20 seconds.

How do I clear the rapid-fire round?

Answer in one or two sentences, then stop. If you don't know, say so quickly and move on; don't guess at length. Revise these the morning of the interview.

Which round comes next?

A typical order is online assessment, telephonic screening, one or two technical rounds, a managerial round and HR. Use the other round pages to practise each stage.

Practise every interview round

Online Assessment questionsTelephonic Interview questionsTechnical Round questionsManagerial Round questionsHR & behavioural questionsFull mock interview

More ways to practise

Company-wise SQL questionsExperience-wise SQL questionsRole-wise SQL questionsCity-wise SQL questionsMost asked SQL questionsScenario-based questionsPractical / hands-on questionsHR & behavioural questionsSQL mock interviewOnline assessment questionsTechnical round questionsManagerial round questionsSQL topic sheets