If you have used SQL a bit, you are certainly familiar with so-called set functions or aggregate functions COUNT, SUM, AVG, described in the manual. For example, let’s say that I am the owner of a shop and I keep track of daily sales in this table:
create table sales (month int, day int, amount int);
The first column is the number of the month, between 1 and 12, the second column is the number of the day in the month, between 1 and 31, and the third column is how much we sold on that date.…
Source: mysqlserverteam.com
