SQL - using alias in Group By

Question

Just curious about SQL syntax. So if I have

SELECT 
 itemName as ItemName,
 substring(itemName, 1,1) as FirstLetter,
 Count(itemName)
FROM table1
GROUP BY it         


        

Answer 1

Caution that using alias in the Group By (for services that support it, such as postgres) can have unintended results. For example, if you create an alias that already exists in the inner statement, the Group By will chose the inner field name.

-- Working example in postgres
select col1 as col1_1, avg(col3) as col2_1
from
    (select gender as col1, maritalstatus as col2, 
    yearlyincome as col3 from customer) as layer_1
group by col1_1;

-- Failing example in postgres
select col2 as col1, avg(col3)
from
    (select gender as col1, maritalstatus as col2,
    yearlyincome as col3 from customer) as layer_1
group by col1;