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Interview Questions and Answers
1. What is a trigger in SQL?
A trigger is an automatic action executed in response to a specific event on a table, such as INSERT, UPDATE, or DELETE.
2. What is the difference between EXISTS and IN?
EXISTS checks for the existence of rows, while IN checks if a value exists within a given set.
3. What is a self-join in SQL?
A self-join is a join in which a table is joined with itself.
4. What is a subquery in SQL?
A subquery is a query inside another SQL query that returns a single value or a set of rows.
5. What are SQL views?
Views are virtual tables based on SQL queries that allow data abstraction and security.
6. What is the difference between CHAR and VARCHAR?
CHAR stores fixed-length strings, while VARCHAR stores variable-length strings.
7. What is the difference between DELETE and TRUNCATE?
DELETE removes specific rows and logs the operation, whereas TRUNCATE removes all rows without logging each row deletion.
8. What is the purpose of the GROUP BY clause?
GROUP BY groups rows that have the same values in specified columns and applies aggregate functions.
9. What is ACID compliance in SQL?
ACID stands for Atomicity, Consistency, Isolation, and Durability, ensuring reliable transactions in a database.
10. What is a stored procedure in SQL?
A stored procedure is a precompiled collection of SQL statements stored in the database for reuse.
11. What is the difference between UNION and UNION ALL?
UNION removes duplicate rows, whereas UNION ALL includes all rows, including duplicates.
12. What is a foreign key in SQL?
A foreign key is a column that establishes a relationship between two tables by referencing the primary key of another table.
13. What is normalization in SQL?
Normalization is the process of structuring a relational database to reduce redundancy and improve data integrity.
14. What are indexes in SQL?
Indexes improve the speed of data retrieval operations in a database by reducing the number of required scans.
15. What is the difference between INNER JOIN and LEFT JOIN?
INNER JOIN returns only matching rows from both tables, whereas LEFT JOIN returns all rows from the left table and matching rows from the right table.
16. What is the difference between WHERE and HAVING in SQL?
WHERE filters rows before aggregation, while HAVING filters groups after aggregation.
17. What are the different types of SQL commands?
SQL commands are categorized into DDL (Data Definition Language), DML (Data Manipulation Language), DCL (Data Control Language), and TCL (Transaction Control Language).
18. What is the difference between SQL and MySQL?
SQL is a query language, whereas MySQL is a relational database management system (RDBMS) that uses SQL to manage databases.
19. What is a primary key in SQL?
A primary key is a column (or a set of columns) that uniquely identifies each row in a table.
20. What is SQL?
SQL (Structured Query Language) is a standard language for managing and manipulating relational databases.