In concurrent transaction scenarios, which isolation level allows dirty reads?

The correct answer is C: READ UNCOMMITTED. READ UNCOMMITTED is lowest isolation level allowing dirty reads (reading uncommitted data). SERIALIZABLE prevents all anomalies but reduces concurrency.

Which SQL aggregate function returns the total sum of a numeric column?

The correct answer is C: SUM(). The SUM() function calculates the total sum of all values in a numeric column.

Which SQL command is used to retrieve data from a database?

The correct answer is A: SELECT. SELECT is the standard SQL command used to retrieve or query data from one or more tables in a database.

A company table has 500K employee records. To find employees with salary > 50,000 efficiently, which strategy is optimal?

The correct answer is C: Create non-clustered index on salary for faster filtering. Non-clustered index on salary column enables fast filtering. Clustered index affects primary key ordering. Filtering in application is inefficient.

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SQL Basics

SQL queries, DBMS, normalization, transactions

19 Q 1 Topics Take Mock Test

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All SQL Basics 57

Q.1 Easy SQL Basics

Which feature of SQL allows preventing duplicate values in a column?

A PRIMARY KEY constraint

B UNIQUE constraint

C NOT NULL constraint

D FOREIGN KEY constraint

Correct Answer: B. UNIQUE constraint

EXPLANATION

UNIQUE constraint prevents duplicate values. PRIMARY KEY is unique + NOT NULL. NOT NULL only prevents nulls. FOREIGN KEY maintains referential integrity.

Test

Q.2 Easy SQL Basics

Which SQL statement is used to add a new column to existing table?

A ADD COLUMN

B ALTER TABLE ... ADD

C INSERT COLUMN

D UPDATE TABLE ... ADD

Correct Answer: B. ALTER TABLE ... ADD

EXPLANATION

ALTER TABLE ... ADD COLUMN syntax is correct for adding columns. Other options are invalid SQL syntax.

Test

Q.3 Easy SQL Basics

Which aggregate function ignores NULL values in SQL?

A COUNT(*)

B COUNT(column_name)

C SUM()

D Both B and C

Correct Answer: D. Both B and C

EXPLANATION

COUNT(column_name), SUM(), AVG(), MAX(), and MIN() all ignore NULL values. COUNT(*) counts all rows including NULLs.

Test

Q.4 Easy SQL Basics

What is the default sort order for ORDER BY clause in SQL?

A Ascending

B Descending

C Random

D Alphabetical

Correct Answer: A. Ascending

EXPLANATION

ORDER BY sorts in ascending order by default. Use DESC keyword for descending order.

Test

Q.5 Easy SQL Basics

In a student enrollment database, you need to find students enrolled in multiple courses. Which JOIN type should be used to match course_id from enrollment table with course table?

A INNER JOIN

B LEFT JOIN

C RIGHT JOIN

D CROSS JOIN

Correct Answer: A. INNER JOIN

EXPLANATION

INNER JOIN returns only matching records from both tables, perfect for finding students with matching course enrollments.

Test

Q.6 Easy SQL Basics

Which SQL command is used to remove all records from a table without removing the table structure?

A DELETE

B TRUNCATE

C DROP

D REMOVE

Correct Answer: B. TRUNCATE

EXPLANATION

TRUNCATE removes all rows but keeps the table structure. DELETE removes rows one by one and can be rolled back. DROP removes the entire table.

Test

Q.7 Easy SQL Basics

What is the result of: SELECT CASE WHEN 1=1 THEN 'A' WHEN 2=2 THEN 'B' ELSE 'C' END;?

A 'B'

B 'A'

C 'C'

D NULL

Correct Answer: B. 'A'

EXPLANATION

CASE statement evaluates conditions sequentially and returns value for first TRUE condition. 1=1 is TRUE, so 'A' is returned.

Test

Q.8 Easy SQL Basics

What will be the result of: SELECT 10 % 3; in SQL?

A 3

B 1

C 3.33

D Error

Correct Answer: B. 1

EXPLANATION

The modulus operator (%) returns remainder after division. 10 divided by 3 gives remainder 1.

Test

Q.9 Easy SQL Basics

Which SQL 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 rows where join condition is satisfied in both tables. Other JOINs include non-matching rows.

Test

Q.10 Easy SQL Basics

What is the purpose of the DISTINCT keyword in SQL?

A To sort results in ascending order

B To remove duplicate rows from results

C To limit the number of rows returned

D To group rows by column values

Correct Answer: B. To remove duplicate rows from results

EXPLANATION

DISTINCT eliminates duplicate rows from query results, ensuring each unique combination appears only once.

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