SQL, or Structured Query Language, is a standard language for accessing and managing data in databases. It was developed by IBM computer scientists in the 1970s and is used to create, update, delete, and retrieve data in databases like MySQL, Oracle, PostgreSQL, and more. This article provides a comprehensive list of 30+ SQL interview questions and answers, commonly asked in interviews for SQL developer roles at top companies like MAANG and other high-paying organizations. Whether you’re a fresher or an experienced professional with 2, 5, or even 10 years of experience, this guide equips you with the knowledge and confidence to excel in your next SQL interview.
1. What is SQL?
SQL (Structured Query Language) is a standardized language used for interacting with databases. It allows users to create databases, define tables, retrieve data, and perform operations such as updating and deleting records. SQL is an ANSI (American National Standards Institute) standard. It enables tasks like executing queries, inserting data, creating and deleting tables, and managing data efficiently.
2. What is a Database?
A database is an organized collection of data stored systematically on a computer, allowing easy access and manipulation. It includes elements like schemas, tables, queries, and views. Databases help store and manage data effectively, and a Database Management System (DBMS) facilitates user interaction with the database.
3. Does SQL Support Programming Language Features?
SQL, while a language, is not a programming language. It is a command-based language that lacks programming constructs like loops or conditional statements. Instead, SQL provides commands for querying, updating, and managing data in databases.
4. Difference Between CHAR and VARCHAR2 Data Types in SQL
CHAR: Used for fixed-length character strings. For instance, CHAR(5) can store only strings with exactly 5 characters.
VARCHAR2: Used for variable-length character strings. For example, VARCHAR2(5) can store strings of varying lengths up to 5 characters.
5. What is Data Definition Language (DDL)?
DDL refers to SQL commands that define or modify the database structure, such as CREATE, DROP, and ALTER. These commands manage the schema and structure of the database.
6. What is Data Manipulation Language (DML)?
DML includes commands to manipulate data within a database. Common DML operations are:
Insert: Adding new rows to a table.
Delete: Removing rows from a table.
Retrieve: Fetching data from a table.
Update: Modifying existing data.
7. What is a View in SQL?
A view is a virtual table that derives its data from one or more existing tables. It allows users to simplify complex queries by combining fields from multiple tables.
Syntax:
CREATE VIEW view_name AS
SELECT column1, column2
FROM table_name
WHERE condition;
view_name: The name of the view.
table_name: The base table(s).
condition: Filter criteria.
8. What is a Foreign Key?
A foreign key is a field in one table that uniquely identifies a row in another table by referencing its primary key. This creates a link between the two tables.
Example:
CREATE TABLE Orders (
O_ID INT NOT NULL,
ORDER_NO INT NOT NULL,
C_ID INT,
PRIMARY KEY (O_ID),
FOREIGN KEY (C_ID) REFERENCES Customers(C_ID)
);
9. What are Tables and Fields?
Table: A structured set of data organized in rows (records) and columns (fields).
Field: A single piece of information within a record.
10. What is a Primary Key?
A primary key is a unique identifier for a table, selected from candidate keys. It ensures that each record in the table is unique and cannot be null.
11. What is the Default Constraint?
The DEFAULT constraint automatically assigns a predefined value to a column when no value is specified during insertion.
12. What is Normalization?
Normalization is the process of organizing a database to reduce redundancy and improve data integrity by decomposing tables into smaller, logically related structures. It aims to:
Minimize redundancy.
Avoid anomalies during insertion, deletion, and updates.
13. What is Denormalization?
Denormalization is a performance optimization technique that involves intentionally adding redundancy to a database to reduce the need for joins, thus speeding up queries. It complements normalization and is applied after normalizing the database.
14. What is a Query?
A query is an SQL command used to retrieve data from a database. Efficient queries optimize resource use, improve performance, and ensure smooth database operations
15. What is a Subquery?
A subquery is a nested query embedded within another SQL query, often in the WHERE clause, to provide intermediate results for the main query.
16. What are the different operators available in SQL?
SQL supports three types of operators:
1. Arithmetic Operators
2. Logical Operators
3. Comparison Operators
17. What is a Constraint?
Constraints are rules applied to data in a table to ensure its validity and consistency. They define restrictions on the type of data that can be stored in specific columns, helping maintain data integrity.
18. What is Data Integrity?
Data integrity ensures that the data in a database is accurate and consistent. It involves enforcing rules and constraints to ensure the correctness of stored data. DBMS provides mechanisms to implement such rules, improving the overall reliability of the database.
19. What is Auto Increment?
The Auto Increment feature is used to automatically generate a unique identifier for records in a table. This is particularly useful when a unique key is required but not available. Auto Increment simplifies the process of assigning primary key values and is supported by most databases.
20. What is MySQL Collation?
MySQL collation is a set of rules that define how characters of a specific character set are compared and sorted. Each character set in MySQL can have one or more collations, with at least one default collation. Collations ensure proper encoding and comparison of characters in a database.
21. What are User-Defined Functions?
User-defined functions (UDFs) allow developers to create custom functionality in SQL using PL/SQL or Java. These functions can appear in various parts of SQL statements, such as:
The SELECT list
WHERE clause conditions
ORDER BY, GROUP BY, and CONNECT BY clauses
The VALUES clause of an INSERT statement
The SET clause of an UPDATE statement
22. Types of User-Defined Functions
User-defined functions in SQL are categorized as:
1. Scalar Functions: Return a single value of a scalar data type (e.g., integer, string).
2. Inline Table-Valued Functions: Return a table data type and act as parameterized, non-updateable views.
3. Multi-Statement Table-Valued Functions: Return a table and support multiple SQL statements to generate results, offering functionality beyond that of a simple view.
23. What is a Stored Procedure?
A stored procedure is a group of precompiled SQL statements that perform specific tasks. It can accept parameters, execute operations on the database, and optionally return results. Stored procedures are used to encapsulate complex logic and improve query efficiency.
24. What are Aggregate and Scalar Functions?
SQL provides built-in functions for data manipulation, categorized as:
1. Aggregate Functions: Perform calculations on a group of values and return a single result (e.g., SUM, AVG).
2. Scalar Functions: Operate on individual values and return a single output based on user input (e.g., UPPER, ROUND).
25. What is the ALIAS Command?
Aliases are temporary names assigned to tables or columns in a query to improve readability or simplify complex expressions. They are especially useful when dealing with lengthy or unclear names or when multiple tables are involved in a query.
26. What are UNION, MINUS, and INTERSECT Commands?
These set operations are used to combine or manipulate results from two or more queries:
1. UNION: Combines all unique rows from both queries. Use UNION ALL to include duplicates.
2. INTERSECT: Returns only the rows that are common to both queries. Use INTERSECT ALL to retain duplicates.
3. EXCEPT: Returns rows present in the first query but not in the second. Use EXCEPT ALL to retain duplicates.
27. What is T-SQL?
T-SQL (Transact-SQL) is Microsoft’s proprietary extension of SQL. It is designed for use with Microsoft SQL Server and supports additional features like procedural programming and error handling. T-SQL enables efficient database communication and transaction processing.
28. What is ETL in SQL?
ETL stands for Extract, Transform, and Load. It is a process used in data warehousing to extract data from various sources, transform it into a suitable format, and load it into a target system like a data warehouse. ETL tools streamline this process for better data management.
29. How to Copy Tables in SQL?
In SQL, you can create a duplicate of an existing table to use for testing or other purposes. The structure and data can be copied using the following syntax:
CREATE TABLE NewTable LIKE ExistingTable;
30. What is SQL Injection?
SQL injection is a type of attack where malicious SQL code is inserted into input fields to manipulate or exploit a database. It can compromise data security, corrupt databases, and lead to unauthorized access. Preventing SQL injection involves validating inputs and using secure coding practices.
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