Interview Questions Answers in SQL PLSQL, STL

What is a primary key, and how is it different from a unique key?

Answer

• Primary Key:
o Ensures each row in a table is unique.
o Cannot contain NULL values.
o A table can have only one primary key.
• Unique Key:
o Ensures all values in a column are unique.
o Can contain a single NULL value.
o A table can have multiple unique keys.

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Question { 1165 }

How do you retrieve the second highest salary from a table?

Answer

Using a subquery:
SELECT MAX(salary)
FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees);
Using ROW_NUMBER():
SELECT salary
FROM (
SELECT salary, ROW_NUMBER() OVER (ORDER BY salary DESC) AS row_num
FROM employees
) ranked
WHERE row_num = 2;

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Question { 1404 }

What is the difference between DELETE, TRUNCATE, and DROP?

Answer

• DELETE:
o Deletes specific rows from a table.
o Can have a WHERE clause.
o DML command (can be rolled back).
• TRUNCATE:
o Deletes all rows from a table.
o Faster than DELETE (does not log individual row deletions).
o DDL command (cannot be rolled back).
• DROP:
o Removes a table entirely (schema + data).
o DDL command.

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Question { 1241 }

What are indexes, and what are the different types?

Answer

Indexes improve query performance by speeding up data retrieval.
Types of Indexes:
1. Clustered Index: Sorts and stores data physically in the table.
2. Non-clustered Index: Creates a separate structure to store pointers to the actual data.
3. Unique Index: Ensures all values in a column are unique.
4. Composite Index: Includes multiple columns in the index.

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Question { 1137 }

Explain the concept of normalization.

Answer

Normalization is the process of organizing data to minimize redundancy and improve data integrity.
Normal Forms:
• 1NF: Eliminate repeating groups, ensure atomicity.
• 2NF: Ensure 1NF and remove partial dependencies.
• 3NF: Ensure 2NF and remove transitive dependencies.
• BCNF: Ensure every determinant is a candidate key.

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Question { 1095 }

What is a CTE (Common Table Expression), and how is it different from a subquery?

Answer

• CTE: Temporary result set defined using WITH and reusable within the query. Improves readability for complex queries.
• Subquery: Nested query executed each time it is called. Example:
WITH SalesCTE AS (
SELECT employee_id, SUM(sales) AS total_sales
FROM sales
GROUP BY employee_id
)
SELECT * FROM SalesCTE WHERE total_sales > 5000;

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Question { 1314 }

What are triggers, and when would you use them?

Answer

A trigger is a block of PL/SQL code automatically executed in response to certain events (e.g., INSERT, UPDATE, DELETE) on a table.
Example Trigger:
CREATE OR REPLACE TRIGGER EmployeeAudit
AFTER INSERT ON employees
FOR EACH ROW
BEGIN
INSERT INTO audit_log(employee_id, action, action_date)
VALUES (:NEW.id, 'INSERT', SYSDATE);
END;

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Question { 1274 }

Explain References in C++

Answer

In this set of fresher-level C++ interview questions, one frequently encountered question is about references. Declaring a variable as a reference provides an alternate name for an existing variable.
A reference variable is created by placing an ‘&’ in the declaration. Essentially, a reference acts as an alias for another variable, allowing access to the associated variable through either its original name or the reference itself.
Syntax:Using references facilitates easier manipulation of variables without copying their values, making them particularly useful in function parameters and return types.

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Question { 1274 }

Explain References in C++

Answer

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Question { 1177 }

What Is Operator Overloading in C++?

Answer

In C++ interview questions, one frequently asked question is about operator overloading. Operator overloading is a form of compile-time polymorphism that allows operators to be given special meanings for user-defined data types.
This feature enables most operators to be redefined or overloaded to perform operations on these user-defined types. For instance,
C++ allows the addition of variables of user-defined data types in a manner similar to built-in data types.

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Question { 1177 }

What Is Operator Overloading in C++?

Answer

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Question { 794 }

Compare SQL and PL/SQL.

Answer

SQL is a domain-specific language to manage and manipulate relational databases. It primarily deals with querying, inserting, updating, and deleting data in a database. On the other hand, PL/SQL is a procedural language that extends SQL by adding programming constructs like variables, loops, and exception handling. PL/SQL is used for writing stored procedures, functions, and triggers, allowing for more complex and reusable database logic.

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Question { 1592 }

Do you know the basic structure of PL/SQL?

Answer

Yes, the basic structure of a PL/SQL block includes:
• Declaration section: Where you define variables, constants, and cursors.
• Execution section: Where you write the actual PL/SQL code, including SQL statements and procedural logic.
• Exception handling section: Where you handle errors and exceptions that may occur during execution.

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Question { 1467 }

What Is a Trigger? How Do You Use It?

Answer

A Triger is a database object in PL/SQL that automatically executes actions in response to specific events, such as insertions, updates, or data deletions in a table. Triggers are typically used to enforce business rules, maintain data integrity, and automate tasks. You create and define triggers using PL/SQL code and attach them to database tables.

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Question { 1284 }

What data types does pl/SQL have?

Answer

PL/SQL supports various data types, including:
• Scalar data types like VARCHAR2, NUMBER, DATE, and BOOLEAN.
• Composite data types like RECORD and TABLE.
• Reference data types like REF CURSOR and PL/SQL TABLE.

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