RDBMS is one of the most popular databases today, so most jobs require SQL skills. What are the most common SQL (Structured Query Language) interview questions? That’s what this article is all about. You can learn everything you need to know about SQL, Oracle, MS SQL Server, and MySQL databases from this article. This article, “Top 115 SQL Interview Questions,” has everything you need to improve your SQL interview prep.
Union is a powerful tool in SQL that allows you to combine the results of two or more queries into a single result set. This can be incredibly useful for a variety of tasks, such as merging data from different tables, creating complex reports, and performing data analysis.
Understanding the Basics of SQL Union
Before diving into interview questions and exercises, let’s first establish a solid understanding of the fundamental concepts behind SQL Union.
What is SQL Union?
Union is an SQL operator that combines the results of two or more SELECT statements into a single result set. The resulting set will contain all rows from the individual queries excluding duplicates.
Types of Union
There are two main types of union:
- UNION: This is the standard union operator. It removes duplicate rows from the combined result set.
- UNION ALL: This operator includes all rows from the individual queries, even duplicates.
Syntax
The syntax for using the union operator is as follows:
SELECT column1, column2, ...FROM table1UNIONSELECT column1, column2, ...FROM table2;Key Considerations
- The queries being unioned must have the same number of columns and the corresponding columns must have compatible data types.
- The order of the columns in the SELECT statements must be the same.
- The UNION ALL operator is generally faster than UNION, as it does not perform duplicate removal.
Interview Questions
Now that we have a grasp of the basics, let’s explore some common SQL Union interview questions:
1. What is the difference between UNION and UNION ALL?
As mentioned earlier, UNION removes duplicates while UNION ALL includes all rows.
2. When would you use UNION ALL instead of UNION?
If you need to include all rows from the individual queries, regardless of duplicates, then UNION ALL is the appropriate choice.
3. How can you use UNION to combine data from different tables?
You can use UNION to combine data from different tables as long as the tables have compatible column structures.
4. Write a query to find all employees who earn more than $50,000 or work in the sales department.
SELECT *FROM employeesWHERE salary > 50000UNIONSELECT *FROM employeesWHERE department = 'Sales';5. Write a query to find the top 10 highest-earning employees in each department.
SELECT department, employee_name, salaryFROM ( SELECT department, employee_name, salary, RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS rank FROM employees) AS ranked_employeesWHERE rank <= 10;Exercises
To solidify your understanding of SQL Union, try your hand at these exercises:
1. Combine the results of two queries that select customer names and order IDs.
2. Find all employees who have placed orders in the current month.
3. Create a report that shows the total sales for each product category.
4. Write a query to find the 10 most popular products based on the number of orders.
5. Combine the results of two queries that select customer names and contact information.
By mastering SQL Union, you can unlock its potential for combining data, creating complex reports, and performing advanced data analysis. The interview questions and exercises provided in this guide will help you prepare for technical interviews and enhance your SQL skills.
SQL Interview Questions for Data Analyst
Normalization in SQL is the process of organizing data to avoid duplication and redundancy. Some of the advantages are:
- Better Database organization
- More Tables with smaller rows
- Efficient data access
- Greater Flexibility for Queries
- Quickly find the information
- Easier to implement Security
- Allows easy modification
- Reduction of redundant and duplicate data
- More Compact Database
- Ensure Consistent data after modification
Besides this SQL Interview Questions Blog, you can also choose structured training from edureka if you want to learn about this technology from experts.
Q5 Explain different types of index in SQL.
There are three types of index in SQL namely:
This index does not allow the field to have duplicate values if the column is unique indexed. If a primary key is defined, a unique index can be applied automatically.
This index reorders the physical order of the table and searches based on the basis of key values. Each table can only have one clustered index.
While a non-clustered index doesn’t change the physical order of the table, it does keep the data in a logical order. Each table can have many nonclustered indexes.
8. union vs uninonall (Top 5 SQL Interview Questions)| GeeksforGeeks
FAQ
What is SQL UNION used for?
What is UNION and UNION all in SQL interview questions?
What interview questions are related to joins and subqueries in SQL?
Here are 50 interview questions related to Joins and Subqueries in SQL: 1. What is an SQL JOIN, and why is it important in database queries? Answer: An SQL JOIN is used to combine rows from two or more tables based on a related column between them. It’s crucial for retrieving data from multiple tables. 2.
How to prepare for a SQL interview?
You should also brush up on the tricky questions, as the interviewers like to use them to try and catch you off guard. If you like learning SQL using hands-on exercises, then you’ve got to try All Forever SQL Package. Some of the common tricky SQL interview questions for experienced users are presented below.
Which SQL interview question uses the table salaries?
Dataset: This SQL interview question uses the table salaries. You can find the script here. Solution: The tricky part here is to recognize that the query can be very short if you know how to use correlated subqueries. Explanation: So, the query first lists all the required columns from the table salaries.
What questions are asked in a SQL interview?
Advanced queries. You may be asked about subqueries, both nested and correlated, as well as how to perform specific tasks like finding the nth highest value in a column. To kick off, before asking you technical questions, your interviewer may ask you some general questions about your overall experience with SQL.
