Question

I have a table item_table like this:

item   age
--------------    
1      1 
1      6 
2      2    

I have the other table price_table like this:

item    pricetype    price
--------------------------    
1       O             5
1       P             6
1       V             7
2       O             8
2       P             9
2       V             10

So, I want to inner join above two tables.

select *
from item_table i
inner join price_table p
on ...

There are some conditions about the on:

1. if the average of age of an item is bigger than 3, then I do: inner join price_table on pricetype = 'O' or pricetype = 'P'
2. If not, then I do: inner join price_table on pricetype = 'O' or pricetype = 'P' or pricetype = 'V'

So there are conditions for on conditions.

I then write the query like this:

select i.item, i.type, p.pricetype, p.price
from item_table i
inner join price_table p on i.item = p.item 
    and (avg(i.age) >= 3 and p.pricetype in ('O', 'P'))
        or (avg(i.age) < 3 and p.pricetype in ('O', 'P', 'V'))

The error is given: An aggregate cannot appear in an ON clause unless it is in a subquery contained in a HAVING clause or select list, and the column being aggregated is an outer reference.

I can’t move the avg to Having because other conditions are depending on the avg.

How can I write the select query?

Answer 1

select *
from (
    select item, avg(age) as AvgAge
    from item_table
    group by item
) ia
inner join price_table p on ia.item = p.item 
    and ((ia.AvgAge >= 3 and p.pricetype in ('O', 'P'))
        or (ia.AvgAge < 3 and p.pricetype in ('O', 'P', 'V')))

SQL Fiddle Example 1

This can be simplified to:

select *
from (
    select item, avg(age) as AvgAge
    from item_table
    group by item
) ia
inner join price_table p on ia.item = p.item 
    and (p.pricetype in ('O', 'P')
        or (ia.AvgAge < 3 and p.pricetype = 'V'))

SQL Fiddle Example 2