Top 20 SQL interview questions

1. Top 20 SQL interview questions & Answers for Testers Here are the top 20 SQL interview questions and answers for testers: 1) What is SQL and its significance in software testing? SQL (Structured Query Language) is a programming language used to manage and manipulate relational databases. Testers often use SQL to query databases, validate data, and perform data-related tests. 2) What are the different types of SQL statements? SQL statements can be classified into four main types: Data Manipulation Language (DML): Used to manipulate data in the database (e.g., SELECT, INSERT, UPDATE, DELETE). Data Definition Language (DDL): Used to define and modify database structures (e.g., CREATE, ALTER, DROP). Data Control Language (DCL): Used to control access and permissions on the database (e.g., GRANT, REVOKE). Transaction Control Language (TCL): Used to manage transactions in the database (e.g., COMMIT, ROLLBACK).     3) What is a primary key?

2. A primary key is a unique identifier for a row in a table. It ensures that each row can be uniquely identified and helps maintain data integrity. 4) What is a foreign key? A foreign key is a field in a table that refers to the primary key of another table. It establishes a relationship between two tables, ensuring referential integrity. 5) What is normalization? Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves breaking down tables into smaller, more manageable structures. 6) What is a stored procedure? A stored procedure is a set of SQL statements that are stored and executed on the database server. It can be called and executed multiple times, reducing network traffic and improving performance. 7) What is a view in SQL? A view is a virtual table derived from one or more tables. It does not store data but presents the data in a predefined manner, simplifying complex queries. 8) What is the difference between UNION and UNION ALL? UNION combines the result sets of two or more SELECT statements, removing duplicate rows. UNION ALL also combines result sets but includes all rows, including duplicates. 9) What is an index? An index is a database structure that improves the speed of data retrieval operations. It allows faster searching, sorting, and filtering of data. 10) What is the difference between a clustered and a non-clustered index? A clustered index determines the physical order of data rows in a table, while a non- clustered index is a separate structure that stores a copy of the indexed columns along with a pointer to the actual row. 11) What is the purpose of the GROUP BY clause?

3. The GROUP BY clause is used to group rows based on one or more columns. It is often used with aggregate functions like COUNT, SUM, AVG, etc., to perform calculations on groups of data. 12) What is a self-join? A self-join is a join operation where a table is joined with itself. It is useful when there is a need to compare rows within the same table. 13) Explain the difference between the WHERE and HAVING clauses. The WHERE clause is used to filter rows before grouping, while the HAVING clause is used to filter groups after grouping. 14) What is a subquery? A subquery is a query nested within another query. It is used to retrieve data based on the results of another query. 15) What is the purpose of the EXISTS operator? The EXISTS operator is used to check the existence of rows returned by a subquery. It returns true if the subquery returns any rows; otherwise, it returns false. 16) What is the ACID property in database systems? ACID stands for Atomicity, Consistency, Isolation, and Durability. It defines the properties that ensure reliable processing of database transactions. 17) Explain the difference between a full backup and an incremental backup. A full backup copies all data in a database, while an incremental backup only copies the changes made since the last backup. Incremental backups are faster but require more effort to restore. 18) What is a deadlock? A deadlock is a situation where two or more transactions are waiting for each other to release resources, resulting in a permanent state of inactivity. It can lead to a system freeze or deadlock resolution by a database management system. 19) How can you optimize a SQL query?

4. SQL query optimization can be achieved through various techniques like creating indexes, minimizing the use of subqueries, using efficient join methods, and retrieving only the necessary columns. 20) What is data integrity? Data integrity ensures the accuracy, consistency, and reliability of data. It can be enforced through primary key constraints, foreign key constraints, and data validation rules. Remember, these questions and answers serve as a guide for interview preparation, but it’s always beneficial to have a solid understanding of SQL concepts and be able to explain them in your own words. Good luck with your interviews!