Interview Q&A / SQL & DBMS

1. What are the ACID properties of a transaction?

   ACID stands for Atomicity, Consistency, Isolation and Durability. A transaction either fully happens or not at all, leaves the data valid according to its rules, behaves as if it ran without interference from other transactions, and stays saved once committed, even if the server crashes.

   BeginnerIn-depth3 min read
2. What is the difference between a primary key, a unique key and a foreign key?

   A primary key uniquely identifies each row and cannot be NULL; a table has only one. A unique key also prevents duplicates, but a table can have many and they usually allow NULL. A foreign key is a column that must match a primary or unique key in another table, which keeps related data consistent.

   BeginnerIn-depth3 min read
3. What is the difference between INNER JOIN and LEFT JOIN?

   INNER JOIN returns only rows that have a match in both tables. LEFT JOIN returns every row from the left table, and fills the right table's columns with NULL when there is no match.

   BeginnerIn-depth2 min read
4. What is the difference between WHERE and HAVING?

   WHERE filters individual rows before they are grouped, so it cannot use aggregate functions like SUM or COUNT. HAVING filters groups after GROUP BY has combined the rows, so it is where conditions on aggregates go.

   BeginnerIn-depth3 min read
5. Write an SQL query to find the second highest salary.

   Find the highest salary that is less than the maximum, with SELECT MAX(salary) FROM employees WHERE salary < (SELECT MAX(salary) FROM employees). For the Nth highest, use DENSE_RANK() so that duplicate salaries share a rank, or ORDER BY DISTINCT salary with LIMIT and OFFSET.

   BeginnerIn-depthCoding3 min read