Which keyword is used to eliminate duplicate rows in SQL results?

The correct answer is B: DISTINCT. The DISTINCT keyword removes duplicate rows from the result set of a query.

What is the output of: SELECT COALESCE(NULL, NULL, 'SQL', 'Database');?

The correct answer is B: 'SQL'. COALESCE returns first non-NULL value. It skips two NULLs and returns 'SQL' as first non-NULL argument.

What will be the output of: SELECT MAX(salary) FROM employees GROUP BY department;

The correct answer is B: The maximum salary for each department. MAX() with GROUP BY returns the maximum salary value for each distinct department.

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SQL queries, DBMS, normalization, transactions

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Difficulty: All Easy Medium Hard 21–26 of 26

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All SQL Basics 57

Q.21 Medium SQL Basics

Which SQL function is used to count non-NULL values in a column?

A COUNT(column_name)

B SUM(column_name)

C MAX(column_name)

D AVG(column_name)

Correct Answer: A. COUNT(column_name)

EXPLANATION

COUNT(column_name) counts the number of non-NULL values in the specified column. COUNT(*) counts all rows.

Test

Q.22 Medium SQL Basics

What will be the result of this query: SELECT * FROM employees WHERE salary > 50000 AND department = 'IT';

A All employees with salary greater than 50000 or in IT department

B Only IT employees with salary greater than 50000

C All IT employees regardless of salary

D Employees with salary greater than 50000 in any department

Correct Answer: B. Only IT employees with salary greater than 50000

EXPLANATION

The AND operator requires both conditions to be true, so only employees in IT department with salary > 50000 are returned.

Test

Q.23 Medium SQL Basics

What is the difference between WHERE and HAVING clauses?

A WHERE filters rows before grouping; HAVING filters groups after grouping

B HAVING filters rows; WHERE filters groups

C They are identical and interchangeable

D WHERE is used with JOIN; HAVING is used with SELECT

Correct Answer: A. WHERE filters rows before grouping; HAVING filters groups after grouping

EXPLANATION

WHERE filters individual rows before GROUP BY is applied, while HAVING filters the grouped results after GROUP BY is applied.

Test

Q.24 Medium SQL Basics

Which SQL clause is used to sort the result set in ascending or descending order?

A WHERE

B ORDER BY

C GROUP BY

D HAVING

Correct Answer: B. ORDER BY

EXPLANATION

The ORDER BY clause sorts the result set in ascending (ASC) or descending (DESC) order. Default is ascending.

Test

Q.25 Medium SQL Basics

Which type of JOIN returns only matching rows from both tables?

A LEFT JOIN

B RIGHT JOIN

C INNER JOIN

D FULL OUTER JOIN

Correct Answer: C. INNER JOIN

EXPLANATION

INNER JOIN returns only the rows that have matching values in both tables being joined.

Test

Q.26 Medium SQL Basics

What is the purpose of the JOIN clause in SQL?

A To combine rows from two or more tables based on related columns

B To delete records from a table

C To update multiple rows at once

D To create a backup of a table

Correct Answer: A. To combine rows from two or more tables based on related columns

EXPLANATION

JOIN combines rows from two or more tables based on a relationship between the columns (usually foreign key relationship).

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