SQL Query Questions et reponses d'entretien

Question 11. Explain the difference between DELETE and TRUNCATE commands.

DELETE is used to remove rows from a table based on a condition. TRUNCATE removes all rows from a table without logging individual row deletions.

Example:

DELETE FROM table WHERE condition;

Question 12. What is a foreign key?

A foreign key is a column or a set of columns in a table that refers to the primary key of another table, establishing a link between the two tables.

Question 13. Write a SQL query to count the number of rows in a table.

SELECT COUNT(*) FROM table;

Question 14. Explain the concept of ACID properties in a database.

ACID (Atomicity, Consistency, Isolation, Durability) properties ensure the reliability of database transactions.

Question 15. Write a SQL query to find the third highest salary from an Employee table.

SELECT MAX(salary) FROM Employee WHERE salary < (SELECT MAX(salary) FROM Employee WHERE salary < (SELECT MAX(salary) FROM Employee));