SQL (Structured Query Language) : MCQs & PYQs with Explanation

Option B

In SQL, the GROUP BY clause is used to arrange rows into groups based on the same values in one or more columns. It is commonly used with aggregate functions such as COUNT(), SUM(), AVG(), MAX(), and MIN().
The GROUP BY clause helps summarize data.

Table : Student

SELECT Department, SUM(Students)
FROM Student
GROUP BY Department;

Option C

A Query Language allows users to specify what data they want rather than how the data should be retrieved. 
For example, in SQL, a user can write a query to fetch required records without worrying about the internal procedures used by the DBMS to access the data. This makes query languages Non-Procedural and easy to use.

Option A

SQL Injection is a malicious technique in which attackers provide crafted input to SQL queries, tricking the database into executing unintended commands. This can lead to unauthorized data access, modification, or deletion. It is a major security concern for databases connected to web applications.

Option B

In SQL, the REVOKE command is used to remove privileges that were previously granted to a user.

• The GRANT is used to give privileges.
• The DELETE is used to remove rows from a table.

Option A

In SQL, the DELETE statement removes rows that satisfy the WHERE condition. If the WHERE clause is omitted, there is no condition to restrict the deletion, so all rows in the table are deleted. The table structure remains intact.
The SQL command DELETE FROM Student; deletes every record from the Student table.

Option C

A Trigger in SQL is a set of actions automatically executed by the database in response to a specific event, such as an INSERT, UPDATE, or DELETE on a table. Triggers are often used for tasks like maintaining audit logs, enforcing business rules, or validating data automatically.

The action part of a trigger (the code that gets executed) can be performed in two ways:
Row-Level Trigger: The trigger is executed once for every row (tuple) that is modified by the operation.
Statement-Level Trigger: The trigger is executed only once, regardless of how many tuples are affected by the operation.

Option D

The error in this SQL statement occurs because the FROM clause is missing, which is required to specify the table from which data should be retrieved. A correct SELECT query must always include the FROM keyword followed by the table name. Additionally, if a column name contains spaces (for example, last name), it should be enclosed in quotes or brackets to avoid syntax errors.

Option C

A View is a virtual table created using a SELECT query. It allows users to see only specific columns or rows of a table while hiding the remaining data. This is commonly used for data security and access control.
E.g.:
CREATE VIEW Employee_View AS
SELECT Empid, Name
FROM Employee;
Here, users can see only Empid and Name, while other sensitive columns like salary,   designation, or address remain hidden.

Option C

In SQL, the GROUP BY clause is used to divide records into groups based on a column (rating). 
The query then calculates an aggregate value (average age) for each group. The HAVING clause is used to filter these groups based on a condition.

After grouping the records based on rating and applying the condition that only those groups should be considered where the number of sailors is greater than one, we find that four ratings (7, 8, 10, 3) satisfy the condition. 
Since the query returns one result (average age) for each valid group, the total number of rows returned is 4.

Option C

In SQL, the ORDER BY clause is used to sort the result set based on one or more columns. By default, the sorting order is ascending (ASC). To sort in descending order, the DESC keyword is used. When sorting by multiple columns, the correct syntax is:

• ORDER BY column1 [ASC | DESC], column2 [ASC | DESC], ...

The sorting keyword (ASC or DESC) must always be written after the column name, not before it.
E.g.:
SELECT *
FROM Customers
ORDER BY Country ASC, CustomerName DESC;