Sql Practical Interview Questions
Mastering SQL: Practical Interview Questions
Sql Practical Interview Questions
SQL practical interview questions often focus on your ability to write complex queries, optimize database performance, and troubleshoot data-related issues. Employers may ask you to demonstrate your understanding of joins, subqueries, indexing, and normalization. Additionally, you may be asked to design databases, create tables, and write SQL scripts to extract specific data sets. It is important to be well-versed in SQL syntax, functions, and techniques to successfully navigate these types of interview questions. Practice coding SQL queries and reviewing common database concepts to prepare effectively for such interviews.
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1 - What are the different types of JOINs in SQL?
Answer: There are four main types of JOINs in SQL INNER JOIN, LEFT JOIN (or LEFT OUTER JOIN), RIGHT JOIN (or RIGHT OUTER JOIN), and FULL JOIN (or FULL OUTER JOIN). An INNER JOIN returns only the rows that have matching values in both tables. A LEFT JOIN returns all rows from the left table and the matched rows from the right table. A RIGHT JOIN does the opposite of a LEFT JOIN, returning all rows from the right table and the matched rows from the left table. A FULL JOIN returns all rows when there is a match in either the left or right table.
2) Explain the difference between WHERE and HAVING clause in SQL.
Answer: The WHERE clause is used to filter records before any groupings are made, while the HAVING clause is used to filter records after a GROUP BY clause has been applied. The WHERE clause is applied to individual rows before they are grouped while the HAVING clause is applied to grouped rows after the GROUP BY operation.
3) What is normalization in SQL and why is it important?
Answer: Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves breaking down a table into smaller tables and defining relationships between them. Normalization is important as it helps avoid data anomalies, such as insertion, update, and deletion anomalies, and ensures data consistency and accuracy.
4) Explain the difference between TRUNCATE and DELETE in SQL.
Answer: TRUNCATE is a DDL (Data Definition Language) command that removes all records from a table, but it does not log individual row deletions. DELETE is a DML (Data Manipulation Language) command that deletes specific records based on a condition, and it logs individual row deletions. TRUNCATE is faster than DELETE as it does not generate as much undo and redo log data.
5) What are aggregate functions in SQL?
Answer: Aggregate functions in SQL are functions that perform a calculation on a set of rows and return a single value. Common aggregate functions include SUM, AVG, COUNT, MAX, and MIN. These functions are often used with the GROUP BY clause to perform calculations on grouped data.
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