SQL Questions

Karan Bhardwaj

Score: 🔥 123

Beginner

Took 2 months

Programming Language

SQL

100%

Questions Attempted

20 / 20

Correct Answers

Incorrect Answers

Points Earned

🔥 20

Question 1

What is the purpose of the COUNT() function in SQL?

Returns the number of rows in a table
Calculates the sum of values in a column
Returns the average value of a column
Finds the minimum value in a column

Explanation:

COUNT() is an aggregate function used to count the number of rows in a table or the number of rows that meet a specific condition in a query.

Question 2

How would you sort results in descending order by the 'date_created' column?

ORDER BY date_created DESC
ORDER BY date_created ASC
SORT BY date_created DESC
SORT BY date_created ASC

Explanation:

The 'DESC' keyword after the column name in the ORDER BY clause signals descending order sorting.

Question 3

What is the purpose of the 'LIMIT' clause in SQL?

It filters rows based on a condition.
It sorts the result set.
It restricts the number of rows returned by the query.
It groups rows with the same values.

Explanation:

LIMIT is used to control how many rows are included in the query result, helping manage the amount of data returned.

Question 4

What potential issue should be considered when using correlated subqueries?

They can lead to performance issues if not used carefully.
They cannot be used with aggregate functions.
They require the use of the JOIN clause.
They cannot return more than one column.

Explanation:

Correlated subqueries execute for each row of the outer query, potentially leading to performance bottlenecks, especially with large datasets. Proper indexing and optimization are crucial.

Question 5

What does SQL stand for?

Structured Query Language
Simple Query Language
Standard Query Logic
System Query Language

Explanation:

SQL stands for Structured Query Language. It's a standardized language for managing and interacting with relational databases.

Question 6

Which comparison operator means 'not equal to' in SQL?

Explanation:

Both '!=' and '<>' are used for 'not equal to' in SQL. They are interchangeable.

Question 7

Which SQL clause is used to sort the result set?

SORT
ORDER
ORDER BY
SORT BY

Explanation:

ORDER BY is the clause used in SQL to sort the result set based on a specified column.

Question 8

Which SQL statement is used to add new rows into a table?

ADD
INSERT
CREATE
APPEND

Explanation:

The INSERT statement is used to insert new rows with data into an existing table within a database.

Question 9

By default, how does the 'ORDER BY' clause sort data?

Randomly
Alphabetically
In ascending order
In descending order

Explanation:

If no specific order (ASC or DESC) is specified, ORDER BY defaults to sorting in ascending order.

Question 10

What is the difference between CURDATE() and NOW() in SQL?

CURDATE() returns the current date, NOW() returns the current time.
CURDATE() returns the current date, NOW() returns the current date and time.
CURDATE() returns the current date and time, NOW() returns the current date.
They are interchangeable.

Explanation:

CURDATE() returns only the current date in 'YYYY-MM-DD' format, while NOW() returns both the current date and time.

Question 11

Which SQL function would you use to combine the first and last names from two different columns into a single column?

COMBINE()
MERGE()
CONCAT()
JOIN()

Explanation:

CONCAT() is a string function that takes two or more strings as arguments and concatenates them into a single string.

Question 12

What is the result of SELECT NOW(); in SQL?

Current date
Current time
Current date and time
An error message

Explanation:

NOW() is a date function that returns the current date and time in the format 'YYYY-MM-DD HH:MM:SS'.

Question 13

How would you convert the text 'hello world' to uppercase in SQL?

TO_UPPER('hello world')
UPPER('hello world')
UCASE('hello world')
MAKE_UPPER('hello world')

Explanation:

UPPER() is a string function that takes a string as an argument and returns the string converted to uppercase.

Question 14

What type of join returns all rows from the left table, even if there are no matching rows in the right table?

INNER JOIN
LEFT JOIN
RIGHT JOIN
FULL JOIN

Explanation:

A LEFT JOIN returns all rows from the left table (the table mentioned before LEFT JOIN) and the matching rows from the right table. If there are no matches, it fills in NULL values for the columns from the right table.

Question 15

You want to find customers who have placed orders in the last month. You have two tables: 'Customers' and 'Orders'. Which subquery type would be most efficient to achieve this?

Correlated Subquery
IN Subquery
EXISTS Subquery
Any of the above would be equally efficient.

Explanation:

EXISTS subqueries are generally more efficient when checking for the existence of rows, especially in larger datasets, as it stops evaluation once a single matching row is found.

Question 16

In which scenario would a subquery be a more suitable choice than a JOIN operation?

When you need to combine data from multiple tables based on a common column.
When you need to filter data based on a condition that involves a separate query.
When you need to update data in one table based on data from another table.
When you need to sort data from multiple tables based on a common column.

Explanation:

Subqueries excel at providing dynamic filtering conditions for the main query. If the filtering relies on a separate query's results, using a subquery makes the logic clearer and more concise than attempting to achieve the same with JOINs.

Question 17

What SQL clause is used to group rows with the same value in one or more columns into a summary row?

WHERE
ORDER BY
GROUP BY
SELECT

Explanation:

The GROUP BY clause is specifically designed to group rows based on identical values in specified columns, allowing for aggregate functions to be applied to each group.

Question 18

Which keyword is used in conjunction with the UPDATE statement to specify the conditions for updating rows?

WHEN
IF
WHERE
CONDITION

Explanation:

The WHERE clause is crucial with UPDATE as it defines which rows should be modified. Without it, all rows would be updated.

Question 19

You are using a subquery to fetch a list of product IDs from an 'Orders' table. What is the main purpose of using the DISTINCT keyword within this subquery?

To sort the product IDs in ascending order.
To limit the number of product IDs returned.
To retrieve only unique product IDs, eliminating duplicates.
To filter product IDs based on a specific condition.

Explanation:

The DISTINCT keyword ensures that the subquery's result set contains only unique values, removing any duplicate product IDs, which is essential for accurate comparisons or joins in the main query.

Question 20

You want to select the department with the highest average salary. What SQL clause would you use to filter the grouped results based on the calculated average salary?

WHERE
HAVING
ORDER BY
LIMIT

Explanation:

HAVING is used to filter groups after the GROUP BY clause, making it suitable for conditions involving aggregate functions like AVG used for calculating the average salary.