What are the Most common SQ LI nterview Questions that involve Self-Join concept?
Use These Problems as a guide to Build up Confidence Before Your Interview with me
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SQL is a vital data source in the data domain. The job title could be data analyst, data engineer, or data scientist. In data-related interviews, a candidate’s SQL skills are subject to thorough testing by interviewers
The concept of self-join in SQL is not easily understood by many candidates, and interviewers are familiar with it because they have already analyzed it. Interviewers believe that individuals who understand self-joining are skilled at remaining active in the company
Self-joining questions are becoming a common question topic among Interviewers these days
The following is a list of SQL questions that are frequently asked and related to the self-join concept
It seems like there are only four questions. However, if you look closely, there are several questions I included in them. Each use case will be examined one at a time
Understanding self join should be your first step in solving these problems
Self Join
Self-join is a join that can be found in the list of joins in SQ L. B y joining a table multiple times, it can retrieve the necessary data
We have a single table, and we can retrieve diverse information by using the same table multiple times
Question — Solution In detail
The solution to this problem lies in the understanding of self join. We need to extract relative information from the single table provided. The possibility exists when a table is interconnected with one’s own
The sample data is presented below. Your data can be taken for your practice
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Observe that all employee information is provided. The solution to the problem is to return the manager’s information for each employee. By comparing the data of employees and their managers, we can obtain the necessary information
Let’s Do it!!
How can we link this table to its own database?
The context must have a shared quality to be able to merge with itself. Remember that every manager is just an employee. Managers will never be in demand
The table must be connected using this condition
All Managers = Employees
All Employees! = Managers
Employee IDs are the only form of identification for managers. Employee ID 2 is the manager’s identification number, as demonstrated by employee ID 1. Managerial ID 2 is identified by employee ID 20. Similarly for remaining all
Provided you have the identical copy of the table but with a different designation. The condition mentioned earlier can be utilized. Employees in the second table are equal to managers from the first table
The inclusion criteria will result in the extraction of all managers ‘data from the second table. The way to achieve this is by utilizing SQL queries
Select e1. *, e2. On e1, * is applied to both employees and their counterparts. Manager_id = e2. Employee_id
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The image above demonstrates that we have returned all data from both tables, as we consider one table to be two by joining itself. Moreover, it can be observed that we combined the two tables and transmitted all manager data to the correct inbox
The second table provides us with the ability to return any data we desire. The manager’s name, gender, and salary from the correct table will be returned exclusively
Select e1. *, e2. If the manager's name is used, it will be employee_name, E2. Salary as manager_salary, E2. The gender of employees e1 and a2 on c1 is the same gender as their manager_gender. Manager_id = e2. Employee_id
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The required dataset has been gathered in a single table. By filtering the data with specific conditions, we can now obtain our required information
Once we have identified the result data set as cte, we will specify all conditions and then write it down
The following are the details of employees whose pay is higher than that of their manager
With cte as (Select e1. *, e2. If the manager's name is used, it will be employee_name, E2. Salary as manager_salary, E2. The gender of employees e1 and a2 on c1 is the same gender as their manager_gender. Manager_id = e2. Employee_id) From the cte where the salary is, enter the employee_id, employee's name, age, department, salary, gender, and manager_ID> manager_salary
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The following are the details of employees whose pay is lower than that of their manager
With cte as (Select e1. *, e2. If the manager's name is used, it will be employee_name, E2. Salary as manager_salary, E2. The gender of employees e1 and a2 on c1 is the same gender as their manager_gender. Manager_id = e2. Employee_id) From the cte where the salary is, enter the employee_id, employee's name, age, department, salary, gender, and manager_ID< manager_salary
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Female managers will be among the employees now.
With cte as (Select e1. *, e2. If the manager's name is used, it will be employee_name, E2. Salary as manager_salary, E2. The gender of employees e1 and a2 on c1 is the same gender as their manager_gender. Manager_id = e2. Employee_id)If manager_id and employee_name are the same, then enter employee's name (employee only), age, department, salary, gender, and manager-item with cte =' F ')
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Observe that the output is identical when we associate a single table with itself. Subsequently, we are adjusting the conditions of the linked dataset. Our approach involved solving three problems with a single self-joint, as demonstrated in this example.
The employee's table can contain varying information depending on the circumstances of the interview.
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Additional SQL interview questions are included for your convenience.
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