AVG to Average Values

1. Introduction to the AVG Function

The AVG function is used to calculate the average of a set of values. This is one of the most commonly used aggregate functions. If we wanted to find the average total from the invoice table, we could run something like this:

SELECT AVG(total) 
FROM invoice;

This would return a value like:

Since the value we're interested in is a price, it may make more sense to limit the result to two decimal places. Unique to PostgreSQL, we can cast the result as follows:

SELECT AVG(total)::numeric(10,2) 
FROM invoice;

This will cast it to 10 digits, with two digits after the decimal:

In other databases, you may see the use of functions like ROUND, to round the value, or TRUNC, to truncate the value to a certain position without rounding.

There are times when you may want to find averages of values with criteria being applied. For example, if we were interested in all of the orders that were set by customer_id equal to 2:

SELECT * 
FROM invoice
WHERE customer_id = 2;

We should find seven rows with a total value in each:

We can calculate the average to two decimal places, like this:

SELECT AVG(total)::numeric(10,2) 
FROM invoice
WHERE customer_id = 2;

2. Dealing with 0 and NULL

It is important to note that the average only consists of values that are not NULL. For example, consider what happens if we set one of the seven invoices to have the total as NULL:

UPDATE invoice 
SET total = NULL 
WHERE invoice_id = 1;

If we queried for the customer_id = 2, we would see that the total has no value. This is different than having a value of 0.

Calculating the average would result in the following:

SELECT AVG(total)::numeric(10,2) 
FROM invoice
WHERE customer_id = 2;

However, if we updated that same invoice to set the total to 0, like this:

UPDATE invoice
SET total = 0 
WHERE invoice_id = 1;

Running the same calculation:

SELECT AVG(total)::numeric(10,2) 
FROM invoice
WHERE customer_id = 2;

In the first case, even though there are seven rows, the null value does not count. The query runs the following calculation: (14 + 9 + 2 + 4 + 6 + 1)/6 = 6.00. In the second case, since the 0 is added instead of the null value, it counts it as part of the calculation: (14 + 9 + 2 + 4 + 6 + 1 + 0)/7 = 5.14

Similar to the SUM function, if we only have NULL values, using the AVG would return NULL.

SELECT AVG(total) 
FROM invoice
WHERE invoice_id < 1;

To have a 0 returned, you would need to use the COALESCE function that would return the second argument if the first argument was NULL.

SELECT COALESCE(AVG(total),0) 
FROM invoice
WHERE invoice_id < 1;

try it

Your turn! Open the SQL tool by clicking on the LAUNCH DATABASE button below. Then enter in one of the examples above and see how it works. Next, try your own choices for which columns you want the query to provide.