50 Powerful Common SQL Interview Questions Every Candidate Must Know in 2024 Vaishnavi Tech Talks

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Common SQL Interview Questions

Below are some of the generally asked common SQL interview questions with their solutions. These common SQL interview questions are constructed to evaluate your grasp of basic SQL fundamentals, which include simple queries, complex database operations, and so on. Going through these common SQL interview questions is highly essential as it will allow you to tilt the interview in your favor and will also prove your technical expertise. With the help of the this common SQL interview questions, you will get a chance to enhance your thinking and obtain real examination of your knowledge of SQL, which you can get the full of headway in job interviews.

There is an extensive list of common SQL interview questions that cover many important topics such as joins, subqueries, and normalization. The command of these common SQL interview questions will enhance your proficiency in tackling different SQL issues that might arise during an interview. Going through the usual common SQL interview questions and their answers is an indispensable part of your journey in the IT department. Remember to practice your common SQL interview questions so you are ready for your next interview.Following are the common SQL interview questions

These common SQL interview questions are frequently used; they help to measure your knowledge with SQL as well as they are good indicators of your practical skills in SQL. The ability gained through the common SQL interview questions will grant you a much more efficient solution to the real problems people face.The below are the common SQL interview questions:

1. What is SQL?

SQL (Structured Query Language) is a programming language that is used in the management and manipulation of relational databases.
Networks can do several things such as make queries and insert, update, and delete data through SQL.
This is the concept of the definition, regulation, and the control of accessibility to data.

2. What are the different types of SQL commands?

SQL commands fall under several categories such as:

DML (Data Manipulation Language): This is the set of SELECT, INSERT, UPDATE and DELETE commands which are used to control the records in a database.
DDL (Data Definition Language): This is the group of CREATE, ALTER, and DROP statements which establish and modify database tables.
DCL (Data Control Language): This is the collection of the commands GRANT and REVOKE which we use to allot and take back the access rights of the users.
TCL (Transaction Control Language): This is the set of the commands COMMIT, ROLLBACK, and SAVEPOINT which are used to manage a transaction.

3. What is the difference between the INNER JOIN and the OUTER JOIN?

INNER JOIN: This returns only those rows that are common in both the tables.
OUTER JOIN: It will return both the rows from one table if matched and the one with the matched rows from the other one. If there is no match, NULLs are returned for the unmatched side. Here are three types of them:

- LEFT OUTER JOIN: It is the type of JOIN that gives all the records present in the left table and only those that are common in both tables.
- RIGHT OUTER JOIN: It is the type of JOIN that gives all the records present in the right table and only those that are common in both tables.
- FULL OUTER JOIN: It lists all the rows from both tables and adds NULLs for the non-matching rows.

4. What is a primary key?

A primary key is a unique column (or set of columns) in a database table that is used as a reference for each respective row in the table.
It makes it certain that no two rows have the same value for the primary key column(s).
A primary key is always the one to carry unique values and is not permitted to have NULL values.

5. What is a foreign key?

A foreign key is a column (or set of columns) in one table that refers to the primary key in another table.
The foreign key is the bridge connection between the tables.
It guarantees that the records with the foreign key are not utilised further in the children table, but they still receive no forward record action of the parent table.

6. What is a composite key?

Composite key is a collection of two or more columns combined to make a record in the table distinct.
Composite keys are not used when only one column can provide unique value to a record.

7. What is normalization?

Normalization is the process of rearranging data in a database to remove repetition and increase data accuracy.
This is accomplished by breaking down the large tables into smaller ones.
The ones that are dependent on each other must be logical.

8. What are the different normal forms?

The normal forms include:

1NF (First Normal Form): Makes Off each column to contain atomic value and to hold of the same type.
2NF (Second Normal Form): To eliminate partial dependencies is the way of 2NF accomplishment (when non-key attributes depend on part of the primary key).
3NF (Third Normal Form): To remove transitive dependencies facilitates 3NF achievement (when non-key attributes depend on other non-key attributes).
BCNF (Boyce-Codd Normal Form): 3NF, however, stronger is what BCNF is.
Other normal forms e.g. even 4NF are about multivalued dependencies.

9. What demoralization is?

Denormalization is the act of deliberately introducing redundancy into a database to improve query performance, particularly for complicated queries that involve multiple joins.
It can, obviously, cause some data anomalies, but it speeds up read operations by cutting down on the number of joins.

10. What is the difference between WHERE and HAVING?

WHERE: It helps to find rows before the grouping; the following are the conditions on individual rows.
HAVING: It serves to sort the rows and then deciding which the records to include are. It is used in the case of the functions like SUM(), AVG(), COUNT() and etc.

11. What is a subquery?

A subquery is a query that is nested in another one.
It can be used in WHERE, FROM, or SELECT clauses and a common application for it is doing operations with intermediate results.
A subquery yields a value or set of values that running the outer query require.

12. What is a correlated subquery?

A correlated subquery is a subquery that uses values from the outer query.
It is invoked once for every row in the outer query.

13. What is a view in SQL?

A logical view is a virtual table that is created in a SELECT statement.
This is a means of viewing the data similar to the actual data table.
Views are also the combination of the data or things that have been pulled through different tables and the partition of the data seen to be complex type of queries limited to the data were made easy through this encapsulation.

14. How do you create a view in SQL?

You create a view using a SQL command such as this one:

CREATE VIEW view_name AS

SELECT column1, column2

FROM table_name

WHERE condition;

15. What is an index?

An index is a database object which speeds up the process of data retrieval owing to data operations.
The mechanism is triggered by the generation of a data structure that speeds up the search process.
Columns which often appear in WHERE clauses or join conditions can be used to create indexes.

16. What are the types of indexes?

The following are examples of indexes that are found largely in the industry:

Single-column index: An index that is placed on one column.
Composite index: An index that is installed on multiple columns.
Unique index: Is the one that ensures that all column’s values in the indexed column are unique.
Full-text index: A type of an indexer that is used when a large amount of text is written in the column.

17. What is the difference between TRUNCATE and DELETE?

TRUNCATE: This operation is used for removing all the data records found in the database table. Unlike DELETE, it doesn’t register individual data rows being deleted. It is quicker than DELETE and cannot be undone.
DELETE: It is applied by getting rid of rows one by one, which are naturally written to the log file after they are gone and thus can be rolled back if wrapped in a transaction.

18. What is the difference between DELETE and DROP?

DELETE: It deletes rows from a table, but the system structure (the table, for example) is kept.
DROP: It totally removes an entire table (that is, both the structure and the data in it) from the database.

19. What is the difference between GROUP BY and ORDER BY?

GROUP BY: Takes row pairs and converts them into summary rows whose separating values are the same, frequently used in conjunction with aggregation functions like COUNT(), SUM(), etc.
ORDER BY: Homework help saw the result set by one or more columns as either ascending or descending order.

20. What are ACID properties in SQL?

The ACID and Base properties ensure reliable transaction processing:

Atomicity: Every operation within a transaction is done, or none is.
Consistency: The database transitions from one valid state to another one.
Isolation: Transactions are isolated from each other.
Durability: Changes made by committed transactions are permanent, even in case of a system crash

21. What is a stored procedure?

A stored procedure is a series of SQL commands that is stored in the database and then run as a whole.
It helps to hide the business logic, increase performance, and use a similar code.

22. What is a trigger in SQL?

A trigger in SQL is a stored procedure with specific characteristics that are executed automatically by certain events (such as INSERT, UPDATE, or DELETE) in the case of a specified table.

23. What is the difference between UNION and UNION ALL?

UNION: It combines the results of two queries and removes duplicate rows.
UNION ALL: It combines the results of two queries but includes duplicates.

24. What is a transaction in SQL?

A transaction refers to an ordered group of SQL commands that are executed as a single unit.
Here, a transaction means that either all operations are successfully completed or none of them are, thus the integrity of the database is maintained by the transaction.

**25. What is the difference between COUNT() and COUNT(*)?**

COUNT(column_name): It counts only those non-NULL values from that particular column.
COUNT(*): It is count of rows that include null rows too.

26. What is a cascading delete in SQL?

Cascading delete is the running of a trigger in case parent table row is deleted.
Then, the corresponding rows in the child table are removed automatically.
This effect is achieved by creating a foreign key along with a ON DELETE CASCADE constraint which is a behavior of this foreign key relationship.

27. How are the various types of relationships in SQL distinguished from one another?

The following are the different relationships that SQL tables can have:

One-to-one: A row in one table is connected with the row in another table.
One-to-many: A row in one table is connected with two or more rows in the other.
Many-to-many: Multiple rows in one table are related to multiple rows in another table, usually through a junction table.

28. What is a self-join?

A self-join is a kind of join where a table is connected to itself.
It is employed for the comparison of rows within the same table, which is often an aspect of hierarchical data like the employees reporting to the managers.

29. What is a database schema?

A schema is a logical aggregation of database objects such as tables, views, indexes, and relationships.
It is a component that defines the arrangement and the circulation of data within a database.

30. What is a temporary table in SQL?

A temporary table is a table that only lasts for the duration of a session or transaction.
It can be used to store intermediate results during query processing and will be dropped automatically when the session is over.

31. What is a BETWEEN operator in SQL?

The BETWEEN operator is utilized to limit the results which are in a given range.
It includes the boundary values.

SELECT * FROM table_name WHERE column_name BETWEEN 10 AND 20;

32. What is the IN operator in SQL?

The IN operator is used to match a value with any value in the list of values.

SELECT * FROM table_name WHERE column_name IN (1, 2, 3);

33. What is the LIKE operator in SQL?

LIKE operator is for searching for a specified pattern in a column.

SELECT * FROM table_name WHERE column_name LIKE ‘a%’;

34. What is a NULL value in SQL?

A NULL value is a value that is missing or that is not known.
It is not the same as an empty string or zero.

35. What is the difference between TRUNCATE and DELETE in terms of performance?

TRUNCATE is quicker because it doesn’t log individual row deletions and fire triggers.
DELETE is slower because it logs each row deletion and fires triggers, which can impact performance.

36. What is an alias in SQL?

An alias is a temporary name that is used to present a table or a column and this alias is usually implemented for achieving a better readability or convenience.

SELECT column_name AS alias_name FROM table_name;

37. What is a sequence in SQL?

A sequence is a database object that it generates unique numeric values, it is frequently used for auto-incrementing primary key columns.

38. What is the EXISTS operator in SQL?

The EXISTS operator is a SQL command used to test a subquery to find out if any result is returned.

SELECT * FROM table_name WHERE EXISTS (SELECT 1 FROM another_table WHERE condition);

39. What is a cross join?

A cross join is a Cartesian product of two tables, that is every row from the first table is combined with every row from the second table.

40. What are aggregate functions in SQL?

Aggregate functions are computing over all rows in a given group at once, and return a single value.
For example, one can find SUM (), AVG (), COUNT (), MAX (), MIN ().

41. What is the difference between CHAR and VARCHAR?

CHAR: A fixed-length string data type. If the string is shorter than the specified length, it is padded with spaces.
VARCHAR: A variable-length string data type. The storage space used will be only that which is needed.

42. What is the COALESCE function in SQL?

COALESCE the function that is used for handling NULL values.

SELECT COALESCE(column1, column2, ‘default’) FROM table_name;

43. What is a CASE statement in SQL?

The CASE statement is a way to implement conditional logic in SQL queries.

SELECT column_name,

CASE

WHEN condition THEN ‘Value1’

ELSE ‘Value2’

END

FROM table_name;

44. What is a JOIN condition?

A JOIN condition is a clause in the SELECT statement that is engineed to find matching rows between the two tables.
It is usually based on columns that match in both tables, typically the primary key of one table and the foreign key of another.

45. What is an OUTER APPLY in SQL?

OUTER APPLY joins the result of table-valued functions with the actual rows of the outer table where you can get back values from a function and still have all the rows of the outer table listed.

46. What is ROWNUM in SQL?

ROWNUM is a dummy column in Oracle that assigns a originated number to every line brought back as a consequence of a query, starting from 1.
It is that used for limiting a query which can be brought back by including rownums in the WHERE clause.

47. What is a WITH clause in SQL?

The WITH clause is employed for a brief set of records that can be used out of a SELECT, INSERT, UPDATE, or DELETE statement and refers the remaining conditions.

48. What is the difference between DISTINCT and GROUP BY?

DISTINCT only the vital information remains but the duplicate cells are removed.
GROUP BY groups rows with the same values and functions can be run on select subset of each group.

49. What is the RANK() function in SQL?

The RANK() is a function with usage like the usual integer and its task is to make every single row a separate entity, in a specific order that the user defines, thus, the equal value rows get the same rank among them.

50. What is the PARTITION BY clause in SQL?

By applying the PARTITION BY clause with window functions, we can break up the result set into partitions used for calculations such as ranking or cumulative totals among a certain group.