SQL Interview Questions and Answers for experienced

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100 SQL Interview Questions and Answers

  1. What is SQL and why is it important?

    • Answer: SQL (Structured Query Language) is a domain-specific language used for managing and manipulating data held in a relational database management system (RDBMS). Its importance stems from its ability to efficiently retrieve, insert, update, and delete data, making it crucial for any application that relies on structured data storage and retrieval. It's the standard language for interacting with most relational databases like MySQL, PostgreSQL, Oracle, and SQL Server.

  2. Explain the difference between INNER JOIN and LEFT (OUTER) JOIN.

    • Answer: An `INNER JOIN` returns only the rows where the join condition is met in both tables. A `LEFT (OUTER) JOIN` returns all rows from the left table (the one specified before `LEFT JOIN`), even if there is no match in the right table. For rows in the left table without a match, the columns from the right table will have `NULL` values.

  3. What are the different types of JOINs in SQL?

    • Answer: SQL supports several types of JOINs: `INNER JOIN`, `LEFT (OUTER) JOIN`, `RIGHT (OUTER) JOIN`, `FULL (OUTER) JOIN`, and `CROSS JOIN`. Each type has a specific way of combining rows from two or more tables based on a related column.

  4. What is normalization in SQL? Why is it important?

    • Answer: Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves dividing larger tables into smaller ones and defining relationships between them. It's important because it minimizes data anomalies (insertion, update, and deletion anomalies), improves data consistency, and makes the database more efficient and easier to maintain.

  5. Explain ACID properties in the context of database transactions.

    • Answer: ACID stands for Atomicity, Consistency, Isolation, and Durability. These are properties that guarantee reliable database transactions. Atomicity ensures that a transaction is treated as a single unit of work; either all changes are made, or none are. Consistency ensures that a transaction maintains the database's integrity constraints. Isolation ensures that concurrent transactions do not interfere with each other. Durability ensures that once a transaction is committed, it remains persistent even in case of system failures.

  6. What are indexes in SQL? How do they improve query performance?

    • Answer: Indexes are special lookup tables that the database search engine can use to speed up data retrieval. Simply put, an index in SQL is a pointer to data in a table. They significantly improve query performance by allowing the database to quickly locate rows that match the search criteria without having to scan the entire table. However, indexes also add overhead to data modification operations (INSERT, UPDATE, DELETE).

  7. What is a stored procedure? What are its advantages?

    • Answer: A stored procedure is a pre-compiled SQL code block that can be stored in a database and executed repeatedly. Advantages include improved performance (due to pre-compilation), enhanced security (by controlling access), code reusability, and reduced network traffic (as the entire procedure is executed on the server).

  8. What is a trigger in SQL? Give an example of its use.

    • Answer: A trigger is a procedural code that is automatically executed in response to certain events on a particular table or view in a database. For example, a trigger could be used to automatically update an audit trail table whenever a row is inserted or updated in a main table, or to enforce business rules (e.g., preventing negative balances in an account).

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