SQL SELECT COUNT, SUM, and AVG

How are aggregate values calculated in SQL?

SELECT COUNT returns a count of the number of data values.
SELECT SUM returns the sum of the data values.
And SELECT AVG returns the average of the data values.

The SQL SELECT COUNT, SUM, and AVG syntax

The general COUNT syntax is:
SELECT COUNT(column-name)
  FROM table-name
The general SUM syntax is:
SELECT SUM(column-name)
  FROM table-name
The general AVG syntax is:
SELECT AVG(column-name)
  FROM table-name

CUSTOMER
Id
FirstName
LastName
City
Country
Phone

SQL SELECT COUNT, SUM,
and AVG Examples

Problem: Find the total number of customers.
SELECT COUNT(Id)
  FROM Customer
Result:
Count
91

ORDER
Id
OrderDate
OrderNumber
CustomerId
TotalAmount
Problem: Compute the total amount sold in 2013.
SELECT SUM(TotalAmount)
  FROM [Order]
 WHERE YEAR(OrderDate) = 2013
Result:
Sum
658388.75

ORDER
Id
OrderDate
OrderNumber
CustomerId
TotalAmount
Problem: Compute the average size of all orders.
SELECT AVG(TotalAmount)
  FROM [Order]
Result:
Average
1631.877819


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