Top 10 Frequently asked SQL Query Interview Questions Answers

In this article, I am giving some examples of SQL queries which is frequently asked when you go for a programming interview, having one or two year experience in this field. Whether you go for a Java developer position, QA, BA, support professional, project manager, or any other technical position, may interviewer expects you to answer basic questions from Database and SQL. It's also obvious that if you are working for one or two years on any project there is a good chance that you come across to handle databases, writing SQL queries to insert, update, delete and select records.

Difference between Correlated and Non-Correlated Subquery in SQL? Examples

Hello guys, the difference between correlated and non-correlated subquery is one of the popular SQL interview question and an important concept for any programmer to learn. The correlated subquery is one of the tricky concepts of SQL. It's similar to recursion in programming which many programmers struggle to understand, but like recursion, it also offers the unique capability to solve many SQL query-based problems like the second-highest salary problem where you need to compare one row of the table to another row. It gives you a different kind of power. The main difference between a regular, non-correlated, and correlated subquery in SQL is in their working, a regular subquery just runs once and returns a value or a set of values that is used by the outer query.

Second Highest Salary in MySQL and SQL Server - LeetCode Solution

Write a SQL query to get the second highest salary from the Employee table.

+----+--------+
| Id | Salary |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
+----+--------+

For example, given the above Employee table, the second highest salary is 200. If there is no second highest salary, then the query should return NULL. You can write SQL query in any of your favorite databases e.g. MySQL, Microsoft SQL Server, or Oracle. You can also use database specific feature e.g. TOP, LIMIT or ROW_NUMBER to write SQL query, but you must also provide a generic solution which should work on all database.

Difference between ROW_NUMBER(), RANK() and DENSE_RANK() in SQL

The main difference between ROW_NUMBER() and RANK() in SQL server is that ROW_NUMBER doesn't handle ties while RANK() does. Though both ROW_NUMBER() and RANK() are window function used for ranking row, the way they handle duplicates differentiate them. ROW_NUMBER() function assigns a unique number to each row starting from 1 and arbitrarily assign values if two rows are equal, which means it's not guaranteed that which row will get 2nd or 3rd position if they are identical. In contrast, RANK() assigns an equivalent rank to similar rows, which creates gaps between RANK.

Difference between VARCHAR and CHAR data type in SQL Server? [Explained]

Hello all, today, I am going to share an interesting SQL Server interview question, which will not only help you in your interview but also in your day-to-day work. It explains one of the critical concepts of SQL Server, the difference between VARCHAR and CHAR data type. I am sure, you all have used both of them numerous times but because it's so common many of us ignore the difference between them and when asked to choose between VARCHAR and CHAR on interviews, they fail to give a convincing reason. The difference is not just significant from an interview point of view but also from a robust database design because an incorrect choice of data type not only limit what kind of data you can put on but also waste precious space and makes your query slower, particularly when you have to deal with the massive amount of data.

What is window function in SQL with Examples? How and when to use them?

Hello folks, if you are wondering what is window function and what problem does it solve then you have to the right place. In this article, I would be explaining the windows functions with examples, how it works, and when you need to use it. Firstly what is a Window Function in SQL? A window function performs a calculation across a set of table rows that are similarly related to the current row. The current row is the row in which function evaluation occurs. This is comparable to the type of calculation that can be done with an aggregate function. But unlike regular aggregate functions like sum, avg, max, and min, that perform operations on an entire table.

What is Normalization in SQL? 1NF, 2nd NF, 3rd NF and BCNF Example Tutorial

What is Normalization?

Normalization is one of the essential concept of relational database. It is the process or technique to remove duplicate data from tables and thus reduce the storage size. It also helps to maintain integrity of data. Normalization likewise assists with coordinating the information in the data set. It is a multi-step process that sets the information into even structure and eliminates the copied information from the relational tables. Normalization coordinates the segments and tables of a data set to guarantee that data set integrity constraints appropriately execute their conditions. It is an orderly method of deteriorating tables to take out information overt repetitiveness (redundant) and unfortunate qualities like Insertion, Update, and Deletion anomalies.

What is temporary table in Database and SQL? Example Tutorial

Hello folks, if you have heard the term temporary table online or by your colleague in a call and wondering what is temporary table, what is the difference between a normal table and a temporary table, and when and how to use it then you have come to the right place. In this blog, we shall learn how to use a temporary table in SQL. But before we go into that we need an overview of SQL. What is SQL? SQL stands for Structured query language. it is a programming language used in communicating or relating to the relational database. And this programming language performs various forms of operation in the data. It was created in the 1970s, SQL is used by database administrators, and developers writing data integration scripts, etc.

7 Reasons of NOT using SELECT * in a Production SQL Query? Best Practices

Hello guys, if you are doing a code review and see a SELECT * in production code, would you allow it? Well, its not a simple question as it looks like but let's find out pros and cons about using SELECT * in a production SQL query and then decide. I have read many articles on the internet where people suggest that using SELECT * in SQL queries is a bad practice and you should always avoid that, but they never care to explain why? Some of them will say you should always use an explicit list of columns in your SQL query, which is a good suggestion and one of the SQL best practices I teach to junior programmers, but many of them don't explain the reason behind it.

Difference between ISNULL() and COALESCE() function in SQL? Example

Even though both ISNULL() and COALESCE() function provides alternate values to NULL in T-SQL and Microsoft SQL Server e.g. replacing NULL values with empty String, there are some key differences between them, which is often the topic of SQL Server interview. In this article, you will not only learn the answer to this question but also learn how to use COALESCE and ISNULL function properly. One of the main differences between them is that COALESCE() is a standard SQL function but ISNULL() is Microsoft SQL Server-specific, which means it's not guaranteed to be supported by other database vendors like Oracle, MySQL, or PostgreSQL.

Difference between table scan, index scan, and index seek in SQL Server Database? Example

Hello guys, a good understanding of how the index works and how to use them to improve your SQL query performance is very important while working in a database and SQL and that's why you will find many questions based upon indexes on Programming Job interviews. One of such frequently asked SQL questions is the real difference between table scan, index scan, and index seek? which one is faster and why? How does the database chooses which scan or seek to use? and How you can optimize the performance of your SQL SELECT queries by using this knowledge. In general, there are only two ways in which your query engine retrieves the data, using a table scan or by using an index.

How to find Nth Highest Salary in MySQL and SQL Server? Example LeetCode Solution

Nth Highest Salary in MySQL and SQL Server - LeetCode Solution
---------------------------------------------------------------
Write a SQL query to get the nth highest salary from the Employee table.

+----+--------+
| Id | Salary |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
+----+--------+

For example, given the above Employee table, the nth highest salary where n = 2 is 200. If there is no nth highest salary, then the query should return null.

Difference between Self and Equi Join in SQL - INNER Join example MySQL

The main difference between Self Join and Equi Join is that In Self Join we join one table to itself rather than joining two tables. Both Self Join and Equi Join are types of INNER Join in SQL, but there is a subtle difference between the two. Any INNER Join with equal as join predicate is known as Equi Join. SQL Joins are the fundamental concept of SQL similar to correlated and noncorrelated subqueries or using group by clause and a good understanding of various types of SQL join is a must for any programmer.

Difference between Primary and Foreign keys in SQL [Answer]

The database is a collection of tables and a table is the collection of rows or records. A primary key is the data to uniquely identify a row in the table. For example, if an Employee table has millions of rows and emp_id is a primary key then it can uniquely identify that row. You can use this primary key to retrieve (SELECT) the row, update the row, and delete the row. In SQL, a table can have only one primary key but it can contain more than one column. There are two types of primary key, a simple primary key, and a composite primary key.

When to use TRUNCATE vs DELETE command in SQL?

While working with the database we often need to delete data, sometimes to create more space, sometimes just remove all data to make the table ready for the next day's transaction, or sometimes just selectively remove stale data. SQL provides a couple of handy commands to remove data e.g. truncate, delete and drop. The last one is a bit different because instead of removing data it just deletes the table. What is the difference between truncate and delete command in SQL or when to use truncate vs delete is one of the most frequently asked SQL interview questions?

Difference between View and Materialized View in Database or SQL? [Answer]

The difference between View and Materialized view is one of the popular SQL interview questions, much like truncate vs delete, correlated vs noncorrelated subquery, or primary key vs unique key. This is one of the classic questions which keeps appearing in SQL interviews now and then and you simply can’t afford to learn about them. Doesn’t matter if you are a programmer, developer, or DBA, these SQL questions are common to all. Views are a concept which not every programmer is familiar with, it is simply not in the category of CRUD operation or database transactions or SELECT query, its little advanced concept for the average programmer.

10 Examples of ALTER Table Command in SQL

In this SQL tutorial, you will learn how to use ALTER command in the table on the database. ALTER command is mainly used to add, modify and drop columns, indexes, and constraints on the table in relational databases e.g. MySQL, Oracle, Sybase, and SQL Server. Though ALTER is not part of classical CRUD operation but it’s one of the important DDL commands. One of the most frequent uses of ALTER command in SQL is adding and removing indexes to improve the performance of SQL SELECT queries.

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