Aggregate functions in SQL are built-in functions that perform calculations on a set of values to return a single summary value. They are particularly useful for analyzing data and summarizing information from multiple rows. Common aggregate functions include COUNT, SUM, AVG, MIN, and MAX. Each of these functions serves a different purpose: COUNT counts the number of rows in a dataset, SUM adds up numerical values, AVG computes the average of a set of values, MIN finds the smallest value, and MAX identifies the largest value.
To use aggregate functions, you typically employ them in queries with the SELECT statement, often coupled with the GROUP BY clause. This allows you to group rows that have the same values in specified columns before applying the aggregate function. For instance, if you have a table of sales with columns for the product type and revenue, you might want to find out the total revenue for each product type. In this scenario, you would write a query like SELECT product_type, SUM(revenue) FROM sales GROUP BY product_type;. This statement groups the sales data by product type and calculates the total revenue for each group.
It's important to note that aggregate functions can work alongside other SQL clauses, such as HAVING, which can filter results after the aggregation has been performed. This is different from the WHERE clause, which is applied before any aggregation occurs. For example, you might want to only display product types that generated more than $10,000 in revenue. In this case, the query would look like: SELECT product_type, SUM(revenue) FROM sales GROUP BY product_type HAVING SUM(revenue) > 10000;. This functionality enables developers to extract meaningful insights from databases effectively, allowing for better data-driven decision-making.