PostgreSQL Aggregate Functions and Group By: Get the average salary and number of employees working in a particular designation

5. Write a query to get the average salary and number of employees working in the department which ID is 90.

Sample Solution:

Code:

-- Calculate the average salary and count of employees in the department with ID 90
SELECT AVG(salary),count(*) 
FROM employees 
WHERE department_id = 90;

Explanation:

• This SQL query is designed to calculate the average salary and count of employees within the department with ID 90.
• The AVG() function is an aggregate function in SQL that calculates the average value in a column.
• salary is presumably a column in the employees table that holds the salary information for each employee.
• count(*) is used to count the number of rows returned by the query, effectively counting the number of employees in the department with ID 90.
• department_id is likely a column in the employees table that identifies the department each employee belongs to.
• The WHERE clause filters the rows to include only those where the department_id is equal to 90, representing the department with ID 90.
• The query calculates the average salary and counts the number of employees for all rows in the employees table that belong to the department with ID 90.

Sample table: employees

Output:

pg_exercises=# SELECT AVG(salary),count(*)
pg_exercises-# FROM employees
pg_exercises-# WHERE department_id = 90;
        avg         | count
--------------------+-------
 19363.333333333333 |     3
(1 row)

Relational Algebra Expression:

Relational Algebra Tree:

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