Question

I have table a and b, each have an average field in them. Data is inserted into only one of the tables at a time so either of the average fields can remain null at one given time. How can i retrieve the value in either of the fields in the two tables which is not null

Table a
id  average labref
1   325      123

Table b
id  average labref
2    null    123

If table a is the one with the average value, i pick that value and if next time table b is the a with the average value and table one is average is null, i pick the value of table a. They both have same id used called labref!

Answer 1

select average from (
select average from tablea
union
select average from tableb) a

where average is not null

OR

select CASE WHEN a.average is null then b.average else a.average end average from tablea a inner join table b 
on a.labref=b.labref