Top 99 SQL Interview Questions and Answers for 2026

SQL Interview Questions at a Glance

SQL interviews tend to mirror the way people actually learn the language.

Early on, it's about vocabulary and grammar, like knowing what a join is, how SELECT works, and the difference between DDL and DML.

As you become more familiar, the focus shifts from "can you write a query" to "can you write the right query." You start to understand when an index helps, why normalization matters, and how subtly different commands like DELETE and TRUNCATE behave under the hood.

At the senior end, syntax is assumed, and the real test is problem-solving: handling duplicates, NULLs, ties, and ranking logic with window functions and CTEs, and reasoning about transactions, concurrency, and performance. 

For intermediate practitioners

1. Functions in SQL. You should know about aggregate and scalar functions, as well as built-in and user-defined functions.

2. Advanced commands. Questions may cover topics like joins, primary and foreign keys, indexes, and SQL relationships.

3. Database design. Expect questions on normalization, denormalization, and the differences between various SQL statements like DELETE, TRUNCATE, and DROP.

4. Advanced queries. You may be asked about subqueries, both nested and correlated, as well as how to perform specific tasks like finding the nth highest value in a column.

General SQL Interview Questions

Before asking you technical questions, your interviewer may ask you some general questions about your overall experience with SQL.\

Don't worry if your experience in SQL is limited: this is something your interviewer, most probably, already knows from your resume. Since they are interested in talking to you anyway, your profile was considered a good fit for their company. Also, it's perfectly fine if you have only worked with one SQL dialect. Remember that all SQL dialects are fairly similar. Therefore, being familiar with only one of them is a solid basis for you to learn any others.

1. What is SQL?

It stands for Structured Query Language, and it's a programming language used for interaction with relational database management systems (RDBMS). This includes fetching, updating, inserting, and removing data from tables.

2. What are SQL dialects? Give some examples.

The various versions of SQL, both free and paid, are also called SQL dialects. All the dialects of SQL have a very similar syntax and vary insignificantly only in additional functionality. Some examples are Microsoft SQL Server, PostgreSQL, MySQL, SQLite, T-SQL, and Oracle.

3. What are the main applications of SQL?

Using SQL, we can:

• create, delete, and update tables in a database
• access, manipulate, and modify data in a table
• retrieve and summarize the necessary information from a table or several tables
• add or remove certain rows or columns from a table

SQL Interview Questions for Beginners 

Your interviewer may get the ball rolling by starting with the easier questions:

4. What is an SQL statement?

Also known as an SQL command. It's a string of characters interpreted by the SQL engine as a legal command and executed accordingly. Some examples of SQL statements are SELECT, CREATE, DELETE, DROP, REVOKE, and so on.

5. What is an SQL query?

A query is a piece of code written in SQL to access or modify data from a database.

There are two types of SQL queries: queries for data retrieval and data modification. The first ones are used to retrieve the necessary data (this also includes limiting, grouping, ordering the data, extracting the data from multiple tables, etc.), while the second ones are used to create, add, delete, update, and rename the data.

6. What is an SQL subquery?

Also called an inner query, a query placed inside another query, or an outer query. A subquery may occur in the clauses such as SELECT, FROM, WHERE, UPDATE, etc. It's also possible to have a subquery inside another subquery. The innermost subquery is run first, and its result is passed to the containing query (or subquery).

7. What is an SQL join?

A clause used to combine and retrieve records from two or multiple tables. SQL tables can be joined based on the relationship between the columns of those tables. Check out our SQL joins tutorial for more context, plus our dedicated guide to SQL joins interview questions. 

8. What is an SQL comment?

A human-readable clarification of what a particular piece of code does. SQL code comments can be single-line (preceded by a double dash --) or span over multiple lines (as follows: /*comment_text*/). When the SQL engine runs, it ignores code comments. The purpose of adding SQL code comments is to make the code more comprehensive for those people who will read it in the future.

9. What is an SQL alias?

A temporary name given to a table (or a column in a table) while executing a certain SQL query. Aliases are used to improve the code readability and make the code more compact. An alias is introduced with the AS keyword:

SELECT col_1 AS column
FROM table_name;

Technical SQL Interview Questions

Now, let's move on to the technical SQL interview questions and some potential answers to them.

When answering technical questions, the best strategy is to give as precise answers as possible. It may look like an attempt to deviate from the main topic. In addition, it may provoke additional questions about which you can feel less confident.

All in all, SQL allows querying a database in multiple ways. In addition, SQL easily integrates with other programming languages, such as Python or R, so we can use their combined power.

10. What types of SQL commands do you know?

• Data Definition Language (DDL) – to define and modify the structure of a database.
• Data Manipulation Language (DML) – to access, manipulate, and modify data in a database.
• Data Control Language (DCL) – to control user access to the data in the database and give or revoke privileges to a specific user or a group of users.
• Transaction Control Language (TCL) – to control transactions in a database.
• Data Query Language (DQL) – to perform queries on the data in a database to retrieve the necessary information from it.

11. Give some examples of common SQL commands.

• DDL: CREATE, ALTER TABLE, DROP, TRUNCATE, and ADD COLUMN

• DML: UPDATE, DELETE, and INSERT

• DCL: GRANT and REVOKE

• TCL: COMMIT, SET TRANSACTION, ROLLBACK, and SAVEPOINT

• DQL: – SELECT

12. What is DBMS, and what types of DBMS do you know?

It stands for Database Management System, a software package used to perform various operations on the data stored in a database, such as accessing, updating, wrangling, inserting, and removing data. There are various types of DBMS, such as relational, hierarchical, network, graph, or object-oriented. These types are based on the way the data is organized, structured, and stored in the system.

13. What is RDBMS? Give some examples of RDBMS.

It stands for Relational Database Management System. It's the most common type of DBMS used for working with data stored in multiple tables related to each other by means of shared keys. The SQL programming language is designed to interact with RDBMS. Some examples of RDBMS are MySQL, PostgreSQL, Oracle, MariaDB, etc.

14. What are tables and fields in SQL?

A table is an organized set of related data stored in a tabular form, i.e., in rows and columns. A field is another term for a column of a table.

15. What types of SQL subqueries do you know?

• Single-row – returns at most one row.
• Multi-row – returns at least two rows.
• Multi-column – returns at least two columns.
• Correlated – a subquery related to the information from the outer query.
• Nested – a subquery inside another subquery.

16. What is a constraint, and why use constraints?

A set of conditions defining the type of data that can be input into each column of a table. Constraints ensure data integrity in a table and block undesired actions.

17. What SQL constraints do you know?

• DEFAULT – provides a default value for a column.

• UNIQUE – allows only unique values.

• NOT NULL – allows only non-null values.

• PRIMARY KEY – allows only unique and strictly non-null values (NOT NULL and UNIQUE).

• FOREIGN KEY – provides shared keys between two or more tables.

18. What types of joins do you know?

• (INNER) JOIN – returns only those records that satisfy a defined join condition in both (or all) tables. It's a default SQL join.

• LEFT (OUTER) JOIN – returns all records from the left table and those records from the right table that satisfy a defined join condition.

• RIGHT (OUTER) JOIN – returns all records from the right table and those records from the left table that satisfy a defined join condition.

• FULL (OUTER) JOIN – returns all records from both (or all) tables. It can be considered as a combination of left and right joins.

Note: FULL OUTER JOIN is supported by PostgreSQL, SQL Server, Oracle, and MySQL 8.0 and above, but MySQL only allows it through UNION patterns; meanwhile, SQLite does not support RIGHT JOIN, which can be emulated using LEFT JOIN in combination with UNION.

19. What is a primary key in SQL?

A column (or multiple columns) of a table to which the PRIMARY KEY constraint was imposed to ensure unique and non-null values in that column. In other words, a primary key is a combination of the NOT NULL and UNIQUE constraints.The primary key uniquely identifies each record of the table.

Related concept — Surrogate key: A surrogate key is an artificially generated identifier (typically a sequential integer or UUID) assigned to each record, independent of the actual data values. Unlike natural keys (e.g., an email address), surrogate keys remain stable when business data changes, making them a popular choice for primary keys in production databases.

 Each table can define at most one PRIMARY KEY (which may be composite). A PRIMARY KEY  is strongly recommended but not strictly required by all engines.

20. What is a unique key in SQL?

A column (or multiple columns) of a table to which the UNIQUE constraint was imposed to ensure unique values in that column, including a possible NULL value (the only one).

Note: 

• SQL Server: Only one NULL allowed unless you use a filtered index.
• PostgreSQL / Oracle / MySQL: Multiple NULLs allowed because NULL <> NULL.

21. What is a foreign key in SQL? 

A column (or multiple columns) of a table to which the FOREIGN KEY (or UNIQUE key) constraint was imposed to link this column to the primary key in another table (or several tables).The purpose of foreign keys is to keep connected various tables of a database.

22. What is an SQL index?

A special data structure related to a database table and used for storing its important parts and enabling faster data search and retrieval. Indexes are especially efficient for large databases, where they significantly enhance query performance.

23. What types of indexes do you know?

• Unique index – doesn't allow duplicates in a table column and hence helps maintain data integrity.
• Clustered index – defines the physical order of records of a database table and performs data searching based on the key values. A table can have only one clustered index.
• Non-clustered index – keeps the order of the table records that don't match the physical order of the actual data on the disk. It means that the data is stored in one place and a non-clustered index – in another one. A table can have multiple non-clustered indexes.

24. What is a schema?

A collection of database structural elements such as tables, stored procedures, indexes, functions, and triggers. It shows the overall database architecture, specifies the relationships between various objects of a database, and defines different access permissions for them. Read our database schema guide for a deeper understanding.

25. What is a SQL operator?

A reserved character, a combination of characters, or a keyword used in SQL queries to perform a specific operation. SQL operators are commonly used with the WHERE clause to set a condition (or conditions) for filtering the data.

26. What types of SQL operators do you know?

• Arithmetic (+, -, *, /, etc.)

• Comparison (>, <, =, >=, etc.)

• Compound (+=, -=, *=, /=, etc.)

• Logical (AND, OR, NOT, BETWEEN, etc.)

• String (%, _, +, ^, etc.)

• Set (UNION, UNION ALL, INTERSECT, and MINUS (or EXCEPT))

27. What is a clause?

A condition imposed on a SQL query to filter the data to obtain the desired result. Some examples are WHERE, LIMIT, HAVING, LIKE, AND, OR, ORDER BY, etc.

28. What are some common statements used with the SELECT query?

The most common ones are FROM, GROUP BY, JOIN, WHERE, ORDER BY, LIMIT, and HAVING.

29. How do you create a table in SQL?

Using theCREATE TABLE statement. For example, to create a table with three columns of predefined datatypes, we apply the following syntax:

CREATE TABLE table_name (col_1 datatype,
                         col_2 datatype,
                         col_3 datatype);

30. How to update a table?

Using the UPDATE statement. The syntax is:

UPDATE table_name
SET col_1 = value_1, col_2 = value_2
WHERE condition;

31. How to delete a table from a database?

Using the DROP TABLE statement. The syntax is: DROP TABLE table_name;.

32. How to get the count of records in a table?

Using the COUNT() aggregate function with the asterisk passed as its argument: SELECT COUNT(*) FROM table_name;.

33. How to sort records in a table?

Using the ORDER BY statement:

SELECT * FROM table_name
ORDER BY col_1;

We can specify that we need a descending order using the DESC keyword; otherwise, the order will be ascending by default. Also, we can sort by more than one column and specify for each one, ascending or descending order separately. For example:

SELECT * FROM table_name
ORDER BY col_1 DESC, col_3, col_6 DESC;

34. How to select all columns from a table?

Using the asterisk * with the SELECT statement. The syntax is: SELECT * FROM table_name;.

35. How to select common records from two tables?

Using the INTERSECT statement:

SELECT col1, col2 FROM table_1
INTERSECT
SELECT col1, col2 FROM table_2;

Note: INTERSECT requires the same number of columns and compatible types.

36. What is the DISTINCT statement, and how do you use it?

This statement is used with the SELECT statement to filter out duplicates and return only unique values from a column of a table. The syntax is:

SELECT DISTINCT col_1
FROM table_name;

37. What are relationships? Give some examples.

Relationships are the connections and correlations between entities, basically meaning how two or more tables of a database are related to one another. For example, we can find an ID of the same client in a table on sales data and in a customer table.

38. What is a NULL value? How is it different from zero or a blank space?

A NULL value indicates the absence of data for a certain cell of a table. Instead, zero is a valid numeric value, and an empty string is a legal string of zero length.

39. What is the difference between SQL and NoSQL?

SQL databases are relational, structured, and use tables with predefined schemas, while NoSQL databases are non-relational, schema-less, and designed to handle unstructured or semi-structured data.

40. What are some common challenges when working with SQL databases?

Challenges include performance tuning for large datasets, managing indexing strategies, ensuring data integrity with constraints, handling concurrent transactions, and optimizing query execution.

Intermediate SQL Interview Questions

41. What is a Common Table Expression (CTE)?

A Common Table Expression (CTE) is a temporary named result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. CTEs improve query readability and allow you to break complex queries into simpler, reusable parts. The syntax uses the WITH keyword:

WITH sales_summary AS (
  SELECT product_id, SUM(amount) AS total_sales
  FROM sales
  GROUP BY product_id
)
SELECT p.product_name, s.total_sales
FROM products p
JOIN sales_summary s ON p.id = s.product_id
WHERE s.total_sales > 10000;

CTEs are particularly useful for recursive queries (hierarchical data like org charts) and when you need to reference the same subquery multiple times.

42. What are window functions, and how do they differ from aggregate functions?

Window functions perform calculations across a set of rows related to the current row, without collapsing the result into a single output row like aggregate functions do. They use the OVER() clause to define the window of rows.

Key window functions include:

• ROW_NUMBER() – assigns a unique sequential number to each row
• RANK() – assigns a rank with gaps for ties
• DENSE_RANK() – assigns a rank without gaps for ties
• LAG() / LEAD() – access data from previous/next rows
• SUM() OVER(), AVG() OVER() – running or cumulative calculations

Example: Calculate running total of sales

SELECT 
  order_date,
  amount,
  SUM(amount) OVER (ORDER BY order_date) AS running_total
FROM orders;

43. What is the difference between RANK(), DENSE_RANK(), and ROW_NUMBER()?

All three are ranking window functions, but they handle ties differently:

• ROW_NUMBER() – always assigns unique sequential numbers (1, 2, 3, 4...), even for ties
• RANK() – assigns the same rank to ties, then skips numbers (1, 2, 2, 4...)
• DENSE_RANK() – assigns the same rank to ties, without skipping (1, 2, 2, 3...)

SELECT 
  name, 
  score,
  ROW_NUMBER() OVER (ORDER BY score DESC) AS row_num,
  RANK() OVER (ORDER BY score DESC) AS rank,
  DENSE_RANK() OVER (ORDER BY score DESC) AS dense_rank
FROM students;

Use DENSE_RANK() when finding the "nth highest" value where ties should share the same position.

In this section, we take a look at the most popular intermediate SQL questions and answers so that you'll know what to expect from your interviewer.

44. What is a function in SQL?

A database object representing a set of SQL statements frequently used for a certain task. A function takes in some input parameters, performs calculations or other manipulations on them, and returns the result. Functions help improve code readability and avoid repetition of the same code snippets.

45. What types of SQL functions do you know?

• Aggregate functions – work on multiple, usually grouped records for the provided columns of a table, and return a single value (usually by group).
• Scalar functions – work on each individual value and return a single value.

On the other hand, SQL functions can be built-in (defined by the system) or user-defined (created by the user for their specific needs).

46. What SQL aggregate functions do you know?

• AVG() – returns the average value

• SUM() – returns the sum of values

• MIN() – returns the minimum value

• MAX() – returns the maximum value

• COUNT() – returns the number of rows, including those with null values

47. What SQL scalar functions do you know?

• LEN() (in other SQL dialects – LENGTH()) – returns the length of a string, including the blank spaces

• UCASE() (in other SQL dialects – UPPER()) – returns a string converted to the upper case

• LCASE() (in other SQL dialects – LOWER()) – returns a string converted to the lower case

• INITCAP() – returns a string converted to the title case (i.e., each word of the string starts from a capital letter)

• MID() (in other SQL dialects – SUBSTR()) – extracts a substring from a string

• ROUND() – returns the numerical value rounded to a specified number of decimals

• NOW() – returns the current date and time

48. What are case manipulation functions in SQL?

Case manipulation functions represent a subset of character functions, and they're used to change the case of the text data. With these functions, we can convert the data into the upper, lower, or title case.

• UCASE() (in other SQL dialects – UPPER()) – returns a string converted to the upper case

• LCASE() (in other SQL dialects – LOWER()) – returns a string converted to the lower case

• INITCAP() – returns a string converted to the title case (i.e., each word of the string starts from a capital letter)

49. What are character manipulation functions in SQL?

Character manipulation functions represent a subset of character functions, and they're used to modify the text data.

• CONCAT() – joins two or more string values appending the second string to the end of the first one

• SUBSTRING()/SUBSTR() – returns a part of a string satisfying the provided start and end points

• LENGTH() (in other SQL dialects – LEN()) – returns the length of a string, including the blank spaces

• REPLACE() – replaces all occurrences of a defined substring in a provided string with another substring

• INSTR() – returns the numeric position of a defined substring in a provided string

• LPAD() and RPAD() – return the padding of the left-side/right-side character for right-justified/left-justified value

• TRIM() – removes all the defined characters, as well as white spaces, from the left, right, or both ends of a provided string

50. What is the difference between local and global variables?

Local variables can be accessed only inside the function in which they were declared. Instead, global variables, being declared outside any function, are stored in fixed memory structures and can be used throughout the entire program.

51. What is the difference between SQL and PL/SQL?

There is no difference between LEFT JOIN and LEFT OUTER JOIN. They are interchangeable. SQL allows the OUTER keyword to be optional, so LEFT JOIN is simply a shorthand for LEFT OUTER JOIN. Both return all records from the left table and the matching records from the right table.

53. What is indexing in SQL, and how does it improve performance?

Indexing creates a special data structure that speeds up data retrieval by allowing the database to find rows more efficiently. It works like an optimized lookup table, reducing the need for full table scans. However, excessive indexing can slow down insert, update, and delete operations due to the need for index maintenance.

54. What is a stored procedure, and how is it different from a function?

A stored procedure is a precompiled set of SQL statements executed as a unit to perform a task. Procedures can modify data or schema objects, manage transactions, and return zero or more result sets. Functions, on the other hand, are typically used in SQL expressions, must return a value (scalar or table-valued), and in many databases are restricted from side effects. Exact behavior differs by DB (e.g., T-SQL has scalar and table-valued functions; PostgreSQL distinguishes between functions and procedures).

55. What is the default data ordering with the ORDER BY statement, and how do you change it?

Default is ascending (NULLS FIRST/LAST varies by DB). Use ASC/DESC per column keyword as follows:

SELECT * FROM table_name 
ORDER BY col_1 DESC;

56. What are SQL set operators?

• UNION – returns the records obtained by at least one of two queries (excluding duplicates)

• UNION ALL – returns the records obtained by at least one of two queries (including duplicates)

• INTERSECT – returns the records obtained by both queries

• EXCEPT (called MINUS in MySQL and Oracle) – returns only the records obtained by the first query but not the second one

57. What operator is used in the query for pattern matching?

The LIKE operator in combination with the % and _ wildcards. The % wildcard represents any number of characters including zero, while _ – strictly one character.

58. What is the difference between a primary key and a unique key in SQL?

While both types of keys ensure unique values in a column of a table, the first one uniquely identifies each record of the table, and the second one prevents duplicates in that column.

59. What is a SQL composite primary key?

The primary key of a table, based on multiple columns.

60. What is the typical order of SQL clauses in a SELECT statement?

SELECT – FROM – JOIN – ON – WHERE – GROUP BY – HAVING – ORDER BY – LIMIT

61. In which order does the interpreter execute the common statements in the SELECT query?

Here is the SQL order of execution: 

FROM → ON → JOIN → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMIT/OFFSET (FETCH)

62. What is a view in SQL?

A virtual table containing a subset of data retrieved from one or more database tables (or other views). Standard views store no data; materialized views do.Views can simplify queries, encapsulate logic, and, combined with privileges, restrict columns/rows. They can join/aggregate data from multiple tables.

63. Can we create a view based on another view in SQL?

Yes. This is also known as nested views. However, we should avoid nesting multiple views since the code becomes difficult to read and debug.

64. Can we still use a view if the original table is deleted?

No. Any views based on that table will become invalid after deleting the base table. If we try to use such a view anyway, we'll receive an error message.

65. What types of SQL relationships do you know?

• One-to-one – each record in one table corresponds to only one record in another table
• One-to-many – each record in one table corresponds to several records in another table
• Many-to-many – each record in both tables corresponds to several records in another table

66. What are the possible values of a BOOLEAN data field?

In some SQL dialects, such as PostgreSQL, the BOOLEAN data type exists explicitly and takes values TRUE, FALSE, or NULL. In other dialects, such as Microsoft SQL Server, the BIT datatype is used to store Boolean values as integers 1 (true) or 0 (false).

67. What is normalization in SQL?

Normalization is a process of database design that includes organizing and restructuring data in a way to reduce data redundancy, dependency, duplication, and inconsistency. This leads to enhanced data integrity, more tables within the database, more efficient data access and security control, and greater query flexibility.

68. What is denormalization in SQL?

Denormalization is the process opposite of normalization: it introduces data redundancy and combines data from multiple tables. Denormalization optimizes the performance of the database infrastructure in situations when read operations are more important than write operations since it helps avoid complex joins and reduces the time of query running.

69. What is the difference between renaming a column and giving an alias to it?

Renaming a column means permanently changing its actual name in the original table. Giving an alias to a column means giving it a temporary name while executing an SQL query, with the purpose to make the code more readable and compact.

70. What is the difference between nested and correlated subqueries?

A correlated subquery is an inner query nested in a bigger (outer) query that refers to the values from the outer query for its execution, meaning that a correlated subquery depends on its outer query.Instead, a non-correlated subquery doesn't rely on the data from the outer query and can be run independently of it.

71. What is the difference between clustered and non-clustered indexes?

While a clustered index defines the physical order of records of a table and performs data searching based on the key values, a non-clustered index keeps the order of records that do not match the physical order of the actual data on the disk. A table can have only one clustered index but many non-clustered ones.

72. What is the CASE() function?

The way to implement the if-then-else logic in SQL. This function sequentially checks the provided conditions in the WHEN clauses and returns the value from the corresponding THEN clause when the first condition is satisfied. If none of the conditions is satisfied, the function returns the value from the ELSE clause in case it's provided, otherwise, it returns NULL. The syntax is:

CASE
    WHEN condition_1 THEN value_1
    WHEN condition_2 THEN value_2
    WHEN condition_3 THEN value_3
    ...
    ELSE value
END;

73. What is the difference between the DELETE and TRUNCATE statements?

DELETE is a DML (Data Manipulation Language) command used to remove one or more rows from a table based on a condition in the WHERE clause. It is reversible only if wrapped in a transaction (via ROLLBACK).

TRUNCATE is a DDL (Data Definition Language) command that removes all rows from a table by deallocating pages. It is faster but generally irreversible, and cannot be used on tables that are referenced by a foreign key.

74. What is the difference between the DROP and TRUNCATE statements?

DROP deletes a table from the database completely, including the table structure and all the associated constraints, relationships with other tables, and access privileges. TRUNCATE deletes all rows from a table without affecting the table structure and constraints.  Both are DDL. DROP removes the table and its metadata; TRUNCATE deletes all rows but keeps the table definition. Performance and transactional behavior depend on the DB engine.

75. What is the difference between the HAVING and WHERE statements?

The first one works on aggregated data after they are grouped, while the second one checks each row individually. If both statements are present in a query, they appear in the following order:WHERE – GROUP BY – HAVING. The SQL engine interprets them also in the same order.

76. How do you add a record to a table?

Using the INSERT INTO statement in combination with VALUES. The syntax is:

INSERT INTO table_name
VALUES (value_1, value_2, ...);

77. How do you delete a record from a table?

Using the DELETE statement. The syntax is:

DELETE FROM table_name
WHERE condition;

In this way, we can also delete multiple records if they satisfy the provided condition.

78. How do you add a column to a table?

Using the ALTER TABLE statement in combination with ADD. The syntax is:

ALTER TABLE table_name
ADD column_name datatype;

79. How do you rename a column of a table?

Using the ALTER TABLE statement in combination with RENAME COLUMN ... TO ... The syntax is:

ALTER TABLE table_name
RENAME COLUMN old_column_name TO new_column_name;

80. How do you delete a column from a table?

Using the ALTER TABLE statement in combination with DROP COLUMN. The syntax is:

ALTER TABLE table_name
DROP COLUMN column_name;

81. How do you select all even or all odd records in a table?

By checking the remainder of the division by 2. In some SQL versions (e.g., PostgreSQL and MySQL), we use the MOD function, in the others (Microsoft SQL Server and SQLite) – the modulo operator (%). To select all even records using MOD:

SELECT * FROM table_name
WHERE MOD(ID_column, 2) = 0;

To select all even records using %:

SELECT * FROM table_name 
WHERE ID_column % 2 = 0;

To select all odd records, the syntax is identical in both cases, only that we would use the inequality operator <> instead of =.

82. How to prevent duplicate records when making a query?

Using the DISTINCT statement in combination with SELECT or creating a unique key for that table.

83. How do you insert many rows in a table?

Using the INSERT INTO statement in combination with VALUES. The syntax is:

INSERT INTO table_name
VALUES (value_1, value_2, ...),
      (value_3, value_4, ...),
      (value_5, value_6, ...),
      ...;

84. How do you find the nth highest value in a column of a table?

Using window functions to handle ties correctly:

SELECT column_name
FROM (
  SELECT column_name, DENSE_RANK() OVER (ORDER BY column_name DESC) AS rnk
  FROM table_name
) t
WHERE rnk = :n;

For the nth row by order (ignoring ties): ORDER BY column_name DESC OFFSET n-1 ROWS FETCH NEXT 1 ROW ONLY.

85. How do you find the values in a text column of a table that start with a certain letter?

Using the LIKE operator in combination with the % and _ wildcards. For example, we need to find all surnames in a table that start with "A". The query is:

SELECT * FROM table_name
WHERE surname LIKE 'A_';

Here, we assume that a surname must contain at least two letters. Without this assumption (meaning that a surname can be just A), the query is as follows:

SELECT * FROM table_name
WHERE surname LIKE 'A%';

86. How do you find the last id in a table?

The simplest method is to use the aggregate function MAX().

SELECT MAX(id) AS highest_id
FROM table_name;

Using ORDER BY with LIMIT or TOP

SELECT id
FROM table_name
ORDER BY id DESC
LIMIT 1;

87. How to select random rows from a table?

Using the RAND() function in combination with ORDER BY and LIMIT. In some SQL dialects, such as PostgreSQL, it's called RANDOM(). For example, the following code will return five random rows from a table in MySQL:

SELECT * FROM table_name
ORDER BY RAND()
LIMIT 5;

Scenario-Based SQL Interview Questions

Scenario-based questions test your ability to solve real-world business problems with SQL. Interviewers use these to assess not just your syntax knowledge, but your problem-solving approach and ability to handle edge cases like duplicates, NULL values, and ties.

88. How do you find and remove duplicate records from a table?

Finding duplicates:

SELECT email, COUNT(*) AS duplicate_count
FROM users
GROUP BY email
HAVING COUNT(*) > 1;

Removing duplicates (keeping one record):

WITH duplicates 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 duplicates WHERE rn > 1);

This approach uses a CTE with ROW_NUMBER() to identify duplicates, keeping the record with the lowest ID.

89. How do you calculate a running total (cumulative sum)?

Use the SUM() window function with an ORDER BY clause:

SELECT 
  transaction_date,
  amount,
  SUM(amount) OVER (ORDER BY transaction_date) AS running_total
FROM transactions;

For a running total per category (e.g., per customer):

SELECT 
  customer_id,
  transaction_date,
  amount,
  SUM(amount) OVER (
    PARTITION BY customer_id 
    ORDER BY transaction_date
  ) AS customer_running_total
FROM transactions;

90. How do you find employees who earn more than the average salary in their department?

This is a classic correlated subquery scenario:

SELECT e.name, e.department, e.salary
FROM employees e
WHERE e.salary > (
  SELECT AVG(salary) 
  FROM employees 
  WHERE department = e.department
);

Alternatively, using a window function:

SELECT name, department, salary
FROM (
  SELECT 
    name, 
    department, 
    salary,
    AVG(salary) OVER (PARTITION BY department) AS dept_avg
  FROM employees
) t
WHERE salary > dept_avg;

91. How do you find gaps in a sequence of numbers (e.g., missing invoice numbers)?

Use LEAD() to compare each value with the next:

SELECT 
  invoice_number,
  LEAD(invoice_number) OVER (ORDER BY invoice_number) AS next_invoice,
  LEAD(invoice_number) OVER (ORDER BY invoice_number) - invoice_number - 1 AS gap_size
FROM invoices
WHERE LEAD(invoice_number) OVER (ORDER BY invoice_number) - invoice_number > 1;

This identifies where the gap between consecutive invoice numbers is greater than 1.

92. How do you find customers who made purchases in consecutive months?

Use LAG() to compare with the previous row:

WITH monthly_purchases AS (
  SELECT 
    customer_id,
    DATE_TRUNC('month', purchase_date) AS purchase_month,
    LAG(DATE_TRUNC('month', purchase_date)) OVER (
      PARTITION BY customer_id 
      ORDER BY DATE_TRUNC('month', purchase_date)
    ) AS prev_month
  FROM purchases
  GROUP BY customer_id, DATE_TRUNC('month', purchase_date)
)
SELECT DISTINCT customer_id
FROM monthly_purchases
WHERE purchase_month = prev_month + INTERVAL '1 month';

93. How do you pivot data from rows to columns?

Use conditional aggregation with CASE statements:

SELECT 
  product_name,
  SUM(CASE WHEN EXTRACT(MONTH FROM sale_date) = 1 THEN amount ELSE 0 END) AS jan_sales,
  SUM(CASE WHEN EXTRACT(MONTH FROM sale_date) = 2 THEN amount ELSE 0 END) AS feb_sales,
  SUM(CASE WHEN EXTRACT(MONTH FROM sale_date) = 3 THEN amount ELSE 0 END) AS mar_sales
FROM sales
GROUP BY product_name;

Some databases (SQL Server, Oracle) have native PIVOT syntax for this operation.

94. How do you find the top 3 products by sales in each category?

Use DENSE_RANK() with PARTITION BY:

WITH ranked_products AS (
  SELECT 
    category,
    product_name,
    SUM(sales_amount) AS total_sales,
    DENSE_RANK() OVER (
      PARTITION BY category 
      ORDER BY SUM(sales_amount) DESC
    ) AS sales_rank
  FROM sales
  GROUP BY category, product_name
)
SELECT category, product_name, total_sales
FROM ranked_products
WHERE sales_rank <= 3
ORDER BY category, sales_rank;

95. What are ACID properties in database transactions?

ACID is an acronym for the four key properties that guarantee reliable database transactions:

• Atomicity: A transaction is "all or nothing" – either all operations complete successfully, or none do
• Consistency: A transaction brings the database from one valid state to another, maintaining all defined rules and constraints
• Isolation: Concurrent transactions don't interfere with each other; each transaction sees a consistent snapshot of the data
• Durability: Once a transaction is committed, the changes persist even if the system crashes

Example transaction:

BEGIN TRANSACTION;

UPDATE accounts SET balance = balance - 100 WHERE account_id = 1;
UPDATE accounts SET balance = balance + 100 WHERE account_id = 2;

COMMIT;

96. What is a deadlock, and how do you prevent it?

A deadlock occurs when two or more transactions are waiting for each other to release locks, creating a circular dependency where neither can proceed.

Example scenario: Transaction A locks Table X and waits for Table Y, while Transaction B locks Table Y and waits for Table X.

Prevention strategies:

• Consistent lock ordering: Always acquire locks in the same order across all transactions
• Keep transactions short: Commit or rollback as quickly as possible
• Use appropriate isolation levels: Lower isolation levels reduce lock contention
• Access only required data: Avoid locking unnecessary rows
• Handle deadlock retries: Most databases automatically detect and resolve deadlocks by rolling back one transaction

97. How do you optimize a slow-running SQL query?

Key optimization strategies:

• Add appropriate indexes: Create indexes on columns used in WHERE, JOIN, and ORDER BY clauses

• Analyze execution plans: Use EXPLAIN (PostgreSQL/MySQL) or EXPLAIN PLAN (Oracle) to identify bottlenecks

• Avoid SELECT *: Only retrieve the columns you need

• Use JOINs instead of subqueries: JOINs are often more efficient than correlated subqueries

• Limit result sets: Use LIMIT/TOP when you don't need all rows

• Avoid functions on indexed columns: WHERE YEAR(date_col) = 2026 can't use an index; use WHERE date_col >= '2026-01-01' instead

• Consider partitioning: For very large tables, partition by date or category

98. How do you handle NULL values in calculations and comparisons?

NULL represents missing or unknown data and requires special handling:

• Comparisons: Use IS NULL or IS NOT NULL instead of = NULL

• COALESCE(): Returns the first non-NULL value: COALESCE(column, 'default')

• NULLIF(): Returns NULL if two values are equal: NULLIF(a, b)

• ISNULL() / NVL(): Database-specific functions to replace NULL values

SELECT 
  name,
  COALESCE(phone, email, 'No contact') AS contact_info,
  CASE WHEN status IS NULL THEN 'Unknown' ELSE status END AS status
FROM customers;

99. How do you find the longest streak of consecutive login days for a user?

This is an advanced "islands and gaps" problem:

WITH login_groups AS (
  SELECT 
    user_id,
    login_date,
    login_date - ROW_NUMBER() OVER (
      PARTITION BY user_id 
      ORDER BY login_date
    ) * INTERVAL '1 day' AS group_id
  FROM user_logins
)
SELECT 
  user_id,
  MIN(login_date) AS streak_start,
  MAX(login_date) AS streak_end,
  COUNT(*) AS streak_length
FROM login_groups
GROUP BY user_id, group_id
ORDER BY streak_length DESC
LIMIT 1;

The trick is subtracting a row number from each date – consecutive dates will produce the same "group_id".

Upskilling Your Team with SQL

While SQL interview preparation is crucial for job seekers and hiring managers, it’s equally important for businesses to invest in continuous SQL training for their teams. Being able to work with data is more important than ever, so ensuring that your employees have strong SQL skills can be a game-changer for your company's success.

If you’re a team leader or business owner looking to ensure your whole team is proficient in SQL, DataCamp for Business offers tailored training programs that can help your employees master SQL skills, from the basics to advanced concepts. We can provide:

• Targeted learning paths: Customizable to your team’s current skill level and business needs.
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• Progress tracking: Tools to monitor and assess your team’s progress, ensuring they achieve their learning goals.

Investing in SQL upskilling through platforms like DataCamp not only enhances your team’s capabilities but also provides your business with a strategic advantage, enabling you to stay competitive and deliver results. Talk to one of our team and request a demo today. 

Conclusion

To sum up, we discussed the 99 essential beginner, intermediate, and scenario-based SQL interview questions and the right answers to them. Hopefully, this information will help you to get ready for the interview and feel more confident, whether you're looking for a job in SQL or hiring candidates for an intermediate SQL position.

If you feel that you need more training to better prepare for an interview, consider browsing our full list of SQL courses:

• Introduction to SQL
• Associate Data Analyst in SQL
• Exploratory Data Analysis in SQL
• SQL for Business Analysts
• Analyzing Business Data in SQL
• Intermediate SQL
• Intermediate SQL Server
• Applying SQL to Real-World Problems
• SQL Server for Database Administrators