Aggregation Functions in SQL

Aggregation Functions in SQL are used to perform calculations on a set of values and return a single summary value. They are essential for generating summary reports and insights from large datasets.

Common Aggregation Functions

• COUNT(): Returns the number of rows that match a specified condition.

  Example: To count the number of employees in a company:

  SELECT COUNT(*) FROM employees;
  

• SUM(): Adds up all the values in a specified column.

  Example: To calculate the total sales amount:

  SELECT SUM(sales_amount) FROM sales;
  

• AVG(): Calculates the average value of a specified column.

  Example: To find the average score of students:

  SELECT AVG(score) FROM exam_results;
  

• MIN(): Returns the smallest value in a specified column.

  Example: To find the minimum price of products:

  SELECT MIN(price) FROM products;
  

• MAX(): Returns the largest value in a specified column.

  Example: To find the highest salary in a department:

  SELECT MAX(salary) FROM employees WHERE department_id = 10;
  

Understanding Levels of Data

Data in SQL can be understood at different levels, which helps in organizing and analyzing it effectively.

Types of Data Levels

• Row Level: Each row in a table represents a single record or entry. For example, in a customers table, each row contains data for one customer.
• Column Level: Each column in a table represents a specific attribute or property of the data. For instance, a column named customer_name would contain the names of all customers.
• Table Level: A table is a collection of rows and columns, representing a specific type of data. For example, an orders table might include all orders made by customers.

Understanding these levels helps in structuring SQL queries and data retrieval.