SQL Query - SUM of data in transaction (for each transaction)

I have this table :

Trans_ID    Name    Value    开发者_运维技巧Total_Item  
100          I1     0.33333333        3  
100          I2     0.33333333        3  
400          I1     0.33333333        3  
400          I2     0.33333333        3  
800          I1     0.25              4  
800          I2     0.25              4    
900          I1     0.33333333        3  
900          I2     0.33333333        3    
1000         I1     0.2               5  
1000         I2     0.2               5  

i need to make it into :

ITEM      VALUE        
I1,I2     0.28999998      

Value is calculated from sum of each 2 item in all transaction / total transaction

EX: item I1 & I2 at trans 100

(0.33333333 + 0.33333333) = 0.666666666

trans 400

(0.33333333 + 0.33333333) = 0.666666666

trans 800
(0.25+0.25) = 0.5

trans 900

(0.33333333 + 0.33333333) = 0.666666666

trans 1000

(0.2+0.2) = 0.4

So Value will be:

(0.666666666+0.666666666+0.5+0.666666666+0.4)/10= 0.28999998

*since total transaction in this table is example table 10. there's aprox 50k transaction in my real table

please note that Total_item value is fixed for each transaction and there's no mistake (note that trans 100 only have 2 item and i put 3 in total item)

i'm working with ms access (but general sql query is fine)

If you are correct about your process, then all the grouping you are using is not necessary - the value is the same without the grouping. That is:

((T100_I1 + T100_I2) + (T400_I1 + T400_I2)) / 4 = (T100_I1 + T100_I2 + T400_I1 + T400_I2) / 4 =

In other words, to get the value you described you want, you just need to sum all the values and divide by their count.

select sum(value)/count(*) 
from table