Calculate AVG in SQL Select statement when some values are NULL - sql

Question

I have struggled to find a solution to this problem, so any help would be hugely appreciated.
I have two tables, lets call one table_a and one table_b, representing two possible sources of customer orders. They both contain a field called "customer_number" which I am using to outer join table_b to table_a. I am then calculating an average customer order size given the two tables.
Where i struggle is where a customer has an entry in either table, but not in both. Then my average calc returns NULL.
Table_a:
Customer Number
Total Online Orders
123456789
1350
987654321
650
Table_b:
Customer Number
Total InStore Orders
123456789
350
So basically the second customer does not have an entry in table_b.
My code as follows:
select distinct
a.[customer number],
a.[Total Online Orders],
b.[Total InStore Orders],
coalesce(a.[Total Online Orders] + b.[Total InStore Orders]) / 2 as [Average Order Size]
from table_a a
full outer join table_b b on a.[customer number]=b.[customer number]
Results table:
Customer Number
Total Online Orders
Total InStore Orders
Average Order Size
123456789
1350
350
850
987654321
650
NULL
NULL
I basically want the results table to show 650 for customer 987654321. Any ideas what I am doing wrong?
thanks!

Answer 1

You can use the brute force method:
( coalesce(a.[Total Online Orders], 0) + coalesce(b.[Total InStore Orders], 0)) /
nullif(case when a.[Total Online Orders] is not null then 1 else 0 end +
case when b.[Total InStore Orders] is not null then 1 else 0 end, 0)
)
) as [Average Order Size]

Answer 2

Use COALESCE() for the values of the 2 columns twice:
(
coalesce(a.[Total Online Orders], b.[Total InStore Orders]) +
coalesce(b.[Total InStore Orders], a.[Total Online Orders])
) / 2 as [Average Order Size]

Answer 3

Consider UNION as well, which handles a few other cases too, especially where customer number isn't necessarily unique in each table:
WITH cte (xtype, [customer number], order_amt) AS (
select 1 AS xtype, [customer number], [Total Online Orders] FROM table_a UNION ALL
select 2 AS xtype, [customer number], [Total InStore Orders] FROM table_b
)
SELECT [customer number]
, SUM(CASE WHEN xtype = 1 THEN order_amt ELSE 0 END) AS [Total Online Orders]
, SUM(CASE WHEN xtype = 2 THEN order_amt ELSE 0 END) AS [Total InStore Orders]
, AVG(order_amt) AS [Average Order Size]
FROM cte
GROUP BY [customer number]
ORDER BY [customer number]
;