# Aggregate Functions

Aggregate or GROUP BY functions are use to perform calculation on a multiples rows of a single column. Some common aggregate functions include: COUNT(), SUM(), and AVG().

The GROUP BY clause is use with Aggregate Functions to group the result by one or more columns. The effect of the using the GROUP BY clause on a results of a Aggregate Functions will include a separate row for each group with the aggregate calculation occurring only for the data related to that group.

# COUNT() Function

The COUNT() function is use to count the number of non-NULL rows for a specify column. If the * is used in place of the column it will count every row returned.

SELECT COUNT(*) FROM books;

# AVG() Function

The AVG() function will return the mean average of the values of the given column.

SELECT AVG(book_pages) FROM books;

# MIN() and MAX() Functions

The MIN() and MAX() functions are use to minimum and maximum values of the provide column.

SELECT MIN(book_year) FROM books;

# SUM() Function

The SUM() function return the sum of all the matched rows of the provide column.

SELECT SUM(book_pages) FROM books;

# GROUP BY Clause

The GROUP BY clause is use with Aggregate Functions to group the result by one or more columns. The effect of the using the GROUP BY clause on a results of a Aggregate Functions will include a separate row for each group with the aggregate calculation occurring only for the data related to that group.

SELECT category_id, COUNT(*) FROM books GROUP BY category_id;

# HAVING Clause

The WHERE clause is used to filter results BEFORE any grouping occurs. As such, the WHERE clause cannot be use with aggregate functions. Instead, the HAVING clause should be used. The HAVING clause also filter results, but does AFTER grouping occurs and can be use with aggregate functions.

SELECT category_id, COUNT(*)
FROM books
GROUP BY category_id 
HAVING COUNT(*) > 15;