SQL: Subqueries and Aggregate Functions Explained

This document outlines the structure and logic of the provided SQL query, which utilizes subqueries and aggregate functions to derive insights from a dataset of employees.

Overview of the Query

SELECT department, MAX(salary) 
FROM 
    (SELECT department, SUM(salary) as salary 
     FROM employees 
     GROUP BY department) as total_salaries 
GROUP BY department

Purpose

The primary goal of this SQL query is to find the maximum salary from aggregated department salaries.

Breakdown of the Query

1. Inner Query (Subquery)

The inner query aggregates the salary for each department.

• Functionality:

  • SELECT department, SUM(salary) as salary
  • This part selects the department and computes the total salary for that department.

• Grouping:

  • FROM employees GROUP BY department
  • This groups the results by department, allowing the SUM function to aggregate salaries for employees within the same department.

• Alias:

  • as total_salaries
  • The result of this subquery is referred to as total_salaries.

2. Outer Query

The outer query retrieves the maximum salary from the aggregated results of the inner query.

• Functionality:

  • SELECT department, MAX(salary)
  • This selects the department and the maximum of the aggregated salaries derived in the inner query.

• Grouping:

  • GROUP BY department
  • Finally, it groups the results again by department to ensure that the MAX function is calculated for each department.

Visual Representation

Flowchart Diagram

[Start]
   |
   v
[ Inner Query: From employees ]
   |-----------------------------|
   | `GROUP BY department`       |
   | `SUM(salary) as salary`     |
   |-----------------------------|
   |
   v
[ Output: total_salaries ]
   |
   v
[ Outer Query: Select MAX(salary) ]
   |------------------------------|
   | `FROM total_salaries`        |
   | `GROUP BY department`        |
   |------------------------------|
   |
   v
[ End: Result Set with MAX Salaries per Department ]

Key Considerations

• Execution Flow:

  • The inner query executes first, summarizing salaries.
  • The outer query then calculates the maximum from these summaries.

• Data Dependencies:

  • The outer query relies on the outputs of the inner query, illustrating a clear dependency of results.

Conclusion

This SQL query effectively combines subqueries and aggregate functions to derive the maximum salary from departments by first aggregating salaries within departments and then performing a maximum calculation on those aggregated values. Such techniques are essential in data analysis and reporting, central to operations in data science and business intelligence.

For further learning about SQL and its applications, exploring the courses available on the Enterprise DNA Platform would be beneficial.